15 Scalability and Performance Best Practices

These are from Laura Thomson of OmniTi:

  1. Profile early, profile often. Pick a profiling tool and learn it in and out. 
  2. Dev-ops cooperation is essential. The most critical difference in organizations that handles crises well.
  3. Test on production data. Code behavior (especially performance) is often data driven.
  4. Track and trend. Understanding your historical performance characteristics is essential for spotting emerging problems.
  5. Assumptions will burn you. Systems are complex and often break in unexpected ways.
  6. Decouple. Isolate performance failures.
  7. Cache. Caching is the core of most optimizations.
  8. Federate. Data federation is taking a single data set and spreading it across multiple database/application servers.
  9. Replicate. Replication is making synchronized copies of data available in more than one place.
  10. Avoid straining hard-to-scale resources. Some resources are inherently hard to scale: Uncacheable’ data, Data with a very high read+write rate, Non-federatable data, Data in a black-box
  11. Use a compiler cache. A compiler cache sits inside the engine and caches the parsed optrees.
  12. Be mindful of using external data sources. External data (RDBMS, App Server, 3rd Party data feeds) are the number one cause of application bottlenecks.
  13. Avoid recursive or heavy looping code. Deeply recursive code is expensive in PHP.
  14. Don’t Outsmart Yourself . Don’t try to work around perceived inefficiencies in PHP (at least not in userspace code!)
  15. Build with caching in mind. Caching is the most important tool in your tool box.


NSFW: Hilarious Fault-Tolerance Cartoon 

Another fun one... Should I be Worried About Scaling?

What happens when the comfortable SQL world of yesterday changes out from under you? You feel like this (it uses some language picante so be advised):

Here's a link to the source:

A great way to communicate the dislocation one feels when technology changes. It also gets you to wonder if those changes at the user level are completely necessary for carrying out the task?

Lest you think this just an old guard reactionary sentiment, Boxed Ice chose MongoDB over CouchDB partly because "CouchDB requires map/reduce style queries which is much more complicated to work with."

Related Articles

  • It Must be Crap on Relational Dabases Week
  • No to SQL? Anti-database movement gains steam
  • Anti-RDBMS: A list of distributed key-value stores
  • Thursday

    Learn How to Think at Scale

    Aaron Kimball of Cloudera gives a wonderful 23 minute presentation titled Cloudera Hadoop Training: Thinking at Scale Cloudera which talks about "common challenges and general best practices for scaling with your data." As a company Cloudera offers "enterprise-level support to users of Apache Hadoop." Part of that offering is a really useful series of tutorial videos on the Hadoop ecosystem.

    Like TV lawyer Perry Mason (or is it Harmon Rabb?), Aaron gradually builds his case. He opens with the problem of storing lots of data. Then a blistering cross examination of the problem of building distributed systems to analyze that data sets up a powerful closing argument. With so much testimony behind him, on closing Aaron really brings it home with why shared nothing systems like map-reduce are the right solution on how to query lots of data. They jury loved it.

    Here's the video Thinking at Scale. And here's a summary of some of the lessons learned from the talk:

    Lessons Learned

  • We can process data much faster than we can read it and much faster than we can write results back to disk.
    * Say 32 GB of RAM available to a machine. You can get 1-2 TB of data on disk. The amount of data a machine can store is greater than the amount it can manipulate in memory so you have to swap out RAM.
    * With an average job size of 180 GB it would take 45 minutes to read that data off of disk sequentially. Random access would be much slower.
    * An individual SATA drive can read at 75 MB/sec. To process 180 GB you would have to read at 75 MB/sec which leaves the CPU doing very little with the data.
  • The solution is to parallelize the reads. Have a 1000 hard drives working together and you can read 75 GB/sec.
  • With a parallel system in place the next step is to move computation to where the data is already stored.
    * Grids moved data to computation. Data was typically stored on a large filer/SAN.
    * The new large scale computing approach is to move computation to where the data is already stored. A file has limited processing power relative to storage size so not useful.
    * So move processing to individual nodes that store only a small amount of the data at a time.
    * This gets around implementation complexity and bandwidth limitations of a centralized filer. Distributed systems can drown themselves if they start sharing data.
  • Large distributed systems must be able to support partial failure and adapt to new additional capacity.
    * Failure with large systems is inevitable so partial progress must be kept for long jobs and jobs must be restarted when a failure is detected. Complex distributed systems make job restarting difficult because of the state that must be maintained.
    * Processing should be close to linear with the number of nodes. Losing 5% of nodes should not end up with a 50% loss in throughput. Doubling the size of the cluster should double the number of jobs that can be processed. No job should be able to nuke the system.
    * Workload should be transferred as new nodes are added and failures occur.
    * Node changes (failures, additions, new hard drives, more memory, etc) should be transparent to jobs. Users shouldn't have to deal with changes, the system should handle them transparently.
  • Solution to large scale data processing problems is to build a shared nothing architecture.
    * To get around the limits faced byMPI (message passing interface) based systems nothing is shared.
    * In map-reduce (MR) systems data is read locally and processed locally. Results are written back locally.
    * Nodes do not talk to each other.
    * Data is paritioned onto machines in advance and computations happen where data is stored.
    * In MPI communication is explicit. Programs know who they are talking to and what they are talking about. In MR communication is implicit. It is taken care of by the system. Data is routed where it needs to go. This simplifies programs by removing the complexity of explicit coordination. Allows developers to concentrate on solving their problem without know level network stack and programming details.
    * On multi-core computers each core would be treated as a separate node.
    * Goal is to have locality of reference. Tasks are processed on the same node as where the data is stored or at least on the same rack. This removes a load step. The data isn't loaded onto a filer. It's not then loaded onto a processing machines. It's already where spreat around the cluster where it needs to be used upfront.
    * In standard MR it's processing large files of data, typically 1 GB or more. This allows streaming reads from this disk. Typical file system block sizes are 4K, for MR they are 64MB to 256MB, which allows writing large linear chunks which reduces seeks on reading.
    * Tasks are restarted transparently on failure because tasks are independent of each other.
    * Data is replicated across nodes for fault tolerance purposes.
    * Task independence allows speculative task execution. The same task can be started on difference nodes and the fastest result can be used. This allows problems like broken disk controllers to be worked around.
    * If necessary inputs can be processed on another machine. There's a big penalty for going off node and off rack.
    * Nodes have no identity to the programmer. Nodes can run multiple jobs.

    Related Articles

  • Researchers: Databases still beat Google's MapReduce by Eric Lai
  • Relational Database Experts Jump The MapReduce Shark by Greg Jorgensen
  • Hadoop and HBAse vs RDBMS by Jonathan Gray
  • Database Technology for the Web: Part 1 – The MapReduce Debate by Colin White
  • Wednesday

    Strategy: Let Google and Yahoo Host Your Ajax Library - For Free

    Update: Offloading ALL JS Files To Google. Now you can let Google serve all your javascript files. This article tells you how to do it (upload to Google Code Project) and why it's a big win (cheap, fast, caching, parallel downloads, save bandwidth).

    Don't have a CDN? Why not let Google and Yahoo be your CDN? At least for Ajax libraries. No charge. Google runs a content distribution network and loading architecture for the most popular open source JavaScript libraries, which include: jQuery, prototype,, MooTools, and dojo. The idea is web pages directly include your library of choice from Google's global, fast, and highly available network. Some have found much better performance and others experienced slower performance. My guess is the performance may be slower if your data center is close to you, but far away users will be much happier. Some negatives: not all libraries are included, you'll load more than you need because all functionality is included. Yahoo has had a similar service for YUI for a while. Remember to have a backup plan for serving your libraries, just in case.


    Strategy: Devirtualize for More Vroom

    Virtualization offers a lot of benefits, but it also comes with a cost (memory, CPU, network, IO, licensing). If you are in or running a cloud then some form of virtualization may not even be an option. But if you are running your own string of servers you can choose to go without. Free will and all that. Should you or shouldn't you?

    In a detailed comparison the folks at 37signals found that running their Rails application servers without virtualization resulted in A 66% reduction in the response time while handling multiples of the traffic is beyond what I expected.

    As is common 37signals runs their big database servers without virtualization. They use a scale-up approach at the database tier so extracting every bit of performance out of those servers is key. Application servers typically use a scale-out approach for scalability, which is virtualization friendly, but that says nothing about performance.

    Finding performance increases, especially when you are running on a dynamic language is a challenge. The improvements found by 37signals were significant. Something to consider when you are casting around wondering how you can get more bang for your buck.


    37signals Architecture

    Update 7: Basecamp, now with more vroom. Basecamp application servers running Ruby code were upgraded and virtualization was removed. The result: A 66 % reduction in the response time while handling multiples of the traffic is beyond what I expected. They still use virtualization (Linux KVM), just less of it now.
    Update 6: Things We’ve Learned at 37Signals. Themes: less is more; don't worry be happy.
    Update 5: Nuts & Bolts: HAproxy . Nice explanation (post, screencast) by Mark Imbriaco of why HAProxy (load balancing proxy server) is their favorite (fast, efficient, graceful configuration, queues requests when Mongrels are busy) for spreading dynamic content between Apache web servers and Mongrel application servers.
    Update 4: O'Rielly's Tim O'Brien interviews David Hansson, Rails creator and 37signals partner. Says BaseCamp scales horizontally on the application and web tier. Scales up for the database, using one "big ass" 128GB machine. Says: As technology moves on, hardware gets cheaper and cheaper. In my mind, you don't want to shard unless you positively have to, sort of a last resort approach.
    Update 3: The need for speed: Making Basecamp faster. Pages now load twice as fast, cut CPU usage by a third and database time by about half. Results achieved by: Analysis, Caching, MySQL optimizations, Hardware upgrades.
    Update 2: customer support is handled in real-time using Campfire.
    Update: highly useful information on creating a customer billing system.

    In the giving spirit of Christmas the folks at 37signals have shared a bit about how their system works. 37signals is most famous for loosing Ruby on Rails into the world and they've use RoR to make their very popular Basecamp, Highrise, Backpack, and Campfire products. RoR takes a lot of heat for being a performance dog, but 37signals seems to handle a lot of traffic with relatively normal sounding resources. This is just an initial data dump, they promise to add more details later. As they add more I'll update it here.


    Information Sources

  • Ask 37signals: Numbers?
  • Ask 37signals: How do you process credit cards?
  • Behind the scenes at 37signals: Support
  • Ask 37signals: Why did you restart Highrise?


  • Ruby on Rails
  • Memcached
  • Xen
  • MySQL
  • S3 for image storage

    The Stats

  • 30 servers ranging from single processor file servers to 8 CPU application servers for about 100 CPUs and 200GB of RAM.
  • Plan to diagonally scale by reducing the number of servers to 16 for about 92 CPU cores (each significantly faster than what are used today) and 230 GB of combined RAM.
  • Xen virtualization will be used to improve system management.
  • Basecamp (web based project management)
    * 2,000,000 people with accounts
    * 1,340,000 projects
    * 13,200,000 to-do items
    * 9,200,000 messages
    * 5,500,000 time tracking entries
    * 4,000,000 milestones

  • Backpack (personal and small business information management)
    * Just under 1,000,000 pages
    * 6,800,000 to-do items
    * 1,500,000 notes
    * 829,000 photos
    * 370,000 files

  • Overall storage stats (Nov 2007)
    * 5.9 terabytes of customer-uploaded files
    * 888 GB files uploaded (900,000 requests)
    * 2 TB files downloaded (8,500,000 requests)

    The Architecture

  • Memcached caching is used and they are looking to add more. Yields impressive performance results.
  • URL helper methods are used rather than building the URLs by hand.
  • Standard ActiveRecord built queries are used, but for performance reasons they will also "dig in and use" find_by_sql when necessary.
  • They fix Rails when they run into performance problems. It pays to be king :-)
  • Amazon’s S3 is used for storage of files upload by users. Extremely happy with results.

    Credit Card Processing Process

  • Bill monthly. It makes credit card companies more comfortable because they won't be on the hook for a large chunk of change if your company goes out of business. Customers also like it better because it costs less up front and you don't need a contract. Just pay as long as you want the service.

  • Get a Merchant Account. One is needed to process credit cards. They use Chase Bank. Use someone you trust and later negotiate rates when you get enough volume that it matters.
  • is the gateway they use to process the credit card charge.
  • A custom built system handles the monthly billing. It runs each night and bills the appropriate people and records the result.
  • On success an invoice is sent via email.
  • On failure an explanation is sent to the customer.
  • If the card is declined three times the account is frozen until a valid card number is provided.
  • Error handling is critical because problems with charges are common. Freeze to fast is bad, freezing too slow is also bad.
  • All products are being converted to using a centralized billing service.
  • You need to be PCI DSS (Payment Card Industry Data Security Standard) compliant.
  • Use a gateway service that makes it so you don't have to store credit card numbers on your site. That makes your life easier because of the greater security. Some gateway services do have reoccurring billing so you don't have to do it yourself.

    Customer Support

  • Campfire is used for customer service. Campfire is a web-based group chat tool, password-protectable, with chatting, file sharing, image previewing, and decision making.
  • Issues discussed are used to drive code changes and the subversion commit is shown in the conversation. Seems to skip a bug tracking system, which would make it hard to manage bugs and features in any traditional sense, ie, you can't track subversion changes back to a bug and you can't report what features and bugs are in a release.
  • Support can solve problems by customers uploading images, sharing screens, sharing files, and chatting in real-time.
  • Developers are always on within Campfire addressing problems in real-time with the customers.

    Lessons Learned

  • Take a lesson from Amazon and build internal functions as services from the start. This make it easier to share them across all product lines and transparently upgrade features.
  • Don't store credit card numbers on your site. This greatly reduces your security risk.
  • Developers and customers should interact in real-time on a public forum. Customers get better service as developers handle issues as they come up in the normal flow of their development cycle. Several layers of the usual BS are removed. Developers learn what customers like and dislike which makes product development more agile. Customers can see the responsiveness of the company to customers by reading the interactions. This goes a long ways to give potential customers the confidence and the motivation to sign up.
  • Evolve your software by actual features needed by users instead of making up features someone might need someday. Otherwise you end up building something that nobody wants and won't work anyway.
  • Monday

    Handle 700 Percent More Requests Using Squid and APC Cache

    This post on documents some impressive system performance improvements by the addition of Squid Cache (a caching proxy) and APC Cache (opcode cache for PHP).

    • Apache is able to deliver roughly 700% more requests per second with Squid when serving 1KB and 100KB images.
    • Server load is reduced using Squid because the server does not have to create a bunch of Apache processes to handle the requests.
    • APC Cache took a system that could barely handle 10-20 requests per second to handling 50-60 requests per second. A 400% increase.
    • APC allowed the load times to remain under 5 seconds even with 200 concurrent threads slamming on the server.
    • These two caches are easy to setup and install and allow you to get a lot more performance out of them.
    The post has an in-depth discussion and a number of supporting charts. The primary point is how simple it can be to improve performance and scalability by adding caching.


    Latency is Everywhere and it Costs You Sales - How to Crush it

    Update 8: The Cost of Latency by James Hamilton. James summarizing some latency info from  Steve Souder, Greg Linden, and Marissa Mayer.  Speed [is] an undervalued and under-discussed asset on the web.

    Update 7: How do you know when you need more memcache servers?. Dathan Pattishall talks about using memcache not to scale, but to reduce latency and reduce I/O spikes, and how to use stats to know when more servers are needed.
    Update 6: Stock Traders Find Speed Pays, in Milliseconds. Goldman Sachs is making record profits off a 500 millisecond trading advantage. Yes, latency matters. As an interesting aside, Libet found 500 msecs is about the time it takes the brain to weave together an experience of consciousness from all our sensor inputs.
    Update 5: Shopzilla's Site Redo - You Get What You Measure. At the Velocity conference Phil Dixon, from Shopzilla, presented data showing a 5 second speed up resulted in a 25% increase in page views, a 10% increase in revenue, a 50% reduction in hardware, and a 120% increase traffic from Google. Built a new service oriented Java based stack. Keep it simple. Quality is a design decision. Obsessively easure everything. Used agile and built the site one page at a time to get feedback. Use proxies to incrementally expose users to new pages for A/B testing. Oracle Coherence Grid for caching. 1.5 second page load SLA. 650ms server side SLA. Make 30 parallel calls on server. 100 million requests a day. SLAs measure 95th percentile, averages not useful. Little things make a big difference.
    Update 4: Slow Pages Lose Users. At the Velocity Conference Jake Brutlag (Google Search) and Eric Schurman (Microsoft Bing) presented study data showing delays under half a second impact business metrics and delay costs increase over time and persist. Page weight not key. Progressive rendering helps a lot.
    Update 3: Nati Shalom's Take on this article. Lots of good stuff on designing architectures for latency minimization.
    Update 2: Why Latency Lags Bandwidth, and What it Means to Computing by David Patterson. Reasons: Moore's Law helps BW more than latency; Distance limits latency; Bandwidth easier to sell; Latency help BW, but not vice versa; Bandwidth hurts latency; OS overhead hurts latency more than BW. Three ways to cope: Caching, Replication, Prediction. We haven't talked about prediction. Games use prediction, i.e, project where a character will go, but it's not a strategy much used in websites.
    Update: Efficient data transfer through zero copy. Copying data kills. This excellent article explains the path data takes through the OS and how to reduce the number of copies to the big zero.

    Latency matters. Amazon found every 100ms of latency cost them 1% in sales. Google found an extra .5 seconds in search page generation time dropped traffic by 20%. A broker could lose $4 million in revenues per millisecond if their electronic trading platform is 5 milliseconds behind the competition.

    The Amazon results were reported by Greg Linden in his presentation Make Data Useful. In one of Greg's slides Google VP Marissa Mayer, in reference to the Google results, is quoted as saying "Users really respond to speed." And everyone wants responsive users. Ka-ching! People hate waiting and they're repulsed by seemingly small delays.

    The less interactive a site becomes the more likely users are to click away and do something else. Latency is the mother of interactivity. Though it's possible through various UI techniques to make pages subjectively feel faster, slow sites generally lead to higher customer defection rates, which lead to lower conversation rates, which results in lower sales. Yet for some reason latency isn't a topic talked a lot about for web apps. We talk a lot about about building high-capacity sites, but very little about how to build low-latency sites. We apparently do so at the expense of our immortal bottom line.

    I wondered if latency went to zero if sales would be infinite? But alas, as Dan Pritchett says, Latency Exists, Cope!. So we can't hide the "latency problem" by appointing a Latency Czar to conduct a nice little war on latency. Instead, we need to learn how to minimize and manage latency. It turns out a lot of problems are better solved that way.

    How do we recover that which is most meaningful--sales--and build low-latency systems?

    I'm excited that the topic of latency came up. There are a few good presentations on this topic I've been dying for a chance to reference. And latency is one of those quantifiable qualities that takes real engineering to create. A lot of what we do is bolt together other people's toys. Building high-capacity low-latency system takes mad skills. Which is fun. And which may also account for why we see latency a core design skill in real-time and market trading type systems, but not web systems. We certainly want our nuclear power plant plutonium fuel rod lowering hardware to respond to interrupts with sufficient alacrity. While less serious, trading companies are always in a technological arms race to create lower latency systems. He with the fastest system creates a sort of private wire for receiving and acting on information faster than everyone else. Knowing who has the bestest price the firstest is a huge advantage. But if our little shopping cart takes an extra 500 milliseconds to display, the world won't end. Or will it?

    Latency Defined

    My unsophisticated definition of latency is that it is the elapsed time between A and B where A and B are something you care about. Low-latency and high-latency are relative terms. The latency requirements for a femptosecond laser are far different than for mail delivery via the pony express, yet both systems can be characterized by latency. A system has low-latency if it's low enough to meet requirements, otherwise it's a high-latency system.

    Latency Explained

    The best explanation of latency I've ever read is still It's the Latency, Stupid by admitted network wizard Stuart Cheshire. A wonderful and detailed rant explaining latency as it relates to network communication, but the ideas are applicable everywhere.

    Stuart's major point: If you have a network link with low bandwidth then it's an easy matter of putting several in parallel to make a combined link with higher bandwidth, but if you have a network link with bad latency then no amount of money can turn any number of them into a link with good latency.

    I like the parallel with sharding in this observation. We put shards in parallel to increase capacity, but request latency through the system remains the same. So if we want to increase interactivity we have to address every component in the system that introduces latency and minimize or remove it's contribution. There's no "easy" scale-out strategy for fixing latency problems.

    Sources of Latency

    My parents told me latency was brought by Santa Clause in the dead of night, but that turns out not to be true! So where does latency come from?

  • Low Level Infrastructure. Includes OS / Kernel, Processors / CPU's, Memory, Storage related I/O, and Network related I/O.
  • High Level Infrastructure. Analysis of sources of latency in downloading web pages by Marc Abrams. The study examines several sources of latency: DNS, TCP, Web server, network links, and routers. Conclusion: In most cases, roughly half of the time is spent from the moment the browser sends the acknowledgment completing the TCP connection establishment until the first packet containing page content arrives. The bulk of this time is the round trip delay, and only a tiny portion is delay at the server. This implies that the bottleneck in accessing pages over the Internet is due to the Internet itself, and not the server speed.
  • Software Processing. Software processing accounts for much of the difficult to squeeze out latency in a system. In very rough terms a 2.0 GHz microprocessor can execute a few hundred lines of code every microsecond. Before a packet is delivered to an endpoint many thousands of instructions have probably already been executed. Then the handling software will spend many thousands more processing the message and then sending a reply. It all can add up to a substantial part of the latency budget. Included in this category are support services like databases, search engines, etc.
  • Frontend. 80-90% of the end-user response time is spent on the frontend, so it makes sense to concentrate efforts there before heroically rewriting the backend.
  • Service Dependency Latency. Dependent components increase latency. If component A calls compont B then the latency is the sum of the latency for each component and overall availability is reduced.
  • Propagation Latency. The speed at which data travels through a link. For fibre optic cable, the rate of signal propagation is roughly two-thirds the speed of light in vacuum. Every 20km takes about 100 microseconds of propagation latency. To reduce latency your only choice is to reduce the distance between endpoints.
  • Transmission Latency. The speed at which a data is transmitted on a communication link. On a 1Gbps network a 1000 bit packet takes about one millionth of a second to transmit. It's not dependent on distance. To reduce latency you need a faster link.
  • Geographical Distribution. BCP (Business Continuity Planning) requires running in multiple datacenters which means added WAN latency constraints.
  • Messaging Latency. The folks at 29west provide a great list forces that increase message latency: Intermediaries, Garbage Collection, Retransmissions, Reordering, Batching, CPU Scheduling, Socket Buffers, Network Queuing, Network Access Control, Serialization, Speed of Light.

    Draw out the list of every hop a client request takes and the potential number of latency gremlins is quite impressive.

    The Downsides of Latency

    Lower sales may be the terminal condition of latency problems, but the differential diagnosis is made of many and varied ailments. As latency increases work stays queued at all levels of the system which puts stress everywhere. It's like dementia, the system forgets how to do anything. Some of the problems you may see are: Queues grow; Memory grows; Timeouts cascade; Memory grows; Paging increases; Retries cascade; State machines reset; Locks are held longer; Threads block; Deadlock occurs; Predictability declines; Throughput declines; Messages drop; Quality plummets.

    For a better list take a look at The Many Flavors of System Latency.. along the Critical Path of Peak Performance by Todd Jobson. A great analysis of the subject.

    Managing Latency

    The general algorithm for managing latency is:
  • Continually map, monitor, and characterize all sources of latency.
  • Remove and/or minimize all latency sources that are found.

    Hardly a revelation, but it's actually rare for applications to view their work flow in terms of latency. This is part of the Log Everything All the Time mantra. Time stamp every part of your system. Look at mean latency, standard deviation, and outliers. See if you can't make the mean a little nicer, pinch in that standard deviation, and chop off some of those spikes. With latency variability is the name of the game, but that doesn't mean that variability can't be better controlled and managed. Target your latency slimming efforts where it matters the most and you get the most bang for your buck.

    Next we will talk about various ideas for what you can do about latency once you've found it.

    Dan Pritchett's Lessons for Managing Latency

    Dan Pritchett is one of the few who has openly written on architecting for latency. Here are some of Dan's suggestions for structuring systems to manage latency:
  • Loosely Couple Components. Loose coupling has a number of benifits: Tightly coupled systems are impossible distribute across data centers, tightly couples systems fail together, and loosely coupled systems can be independently scaled and engineered for latency.
  • Use Asynchronous Interfaces. Set an expectation of asynchronous behavior between components. This allows you to add latency when you need to make changes. Getting users on hooked on synchronous low-latency interactions doesn't allow for architecture flexibility. So start from the beginning with asynch semantics.
  • Horizontally Scale from the Start. It's very difficult to change a monolithic schema once you meet a scaling wall. Start with a horizontal architecture so you don't build in too many problems that will be hard to remove later.
  • Create an Active/Active Architecture. Most approaches to BCP take an active/passive approach, only one data center is active at a time. Creating an active/active system, where all data centers operate simultaneously allows users to be served from the closest data center which decreases latency.
  • Use a BASE (basically available, soft state, eventually consistent) Instead of ACID (atomicity, consistency, isolation, durability) Shared Storage Model. BASE is derived from the CAP Theorem which is the highly counter intuitive notion that database services cannot ensure all three of the following properties at once: Consistency, Availability, Partition tolerance. A BASE based system is more tolerant to latency because it is an inherently partitioned and loosely coupled architecture and it uses eventual consistency. With eventual consistency you can make an update to one partition and return. You don't have to coordinate a transaction across multiple database servers, which makes a system have a higher and more variable latency.

    Clearly each of these principles is a major topic all on their own. For more details please read: Dan Pritchett has written a few excellent papers on managing latency: The Challenges of Latency, Architecting for Latency, Latency Exists, Cope!.

    GigaSpaces Lessons for Lowering Latency

    GigsSpaces is an in-memory grid vendor and as such is on the vanguard of the RAM is the New Disk style of application building. In this approach disk is pushed aside for keeping all data in RAM. Following this line of logic GigaSpaces came up with these low latency architecture principles:

  • Co-location of the tiers (logic, data, messaging, presentation) on the same physical machine (but with a shared-nothing architecture so that there is minimal communication between machines)
  • Co-location of services on the same machine
  • Maintaining data in memory (caching)
  • Asynch communication to a persistent store and across geographical locations

    The thinking is the primary source of latency in a system centers around accessing disk. So skip the disk and keep everything in memory. Very logical. As memory is an order of magnitude faster than disk it's hard to argue that latency in such a system wouldn't plummet.

    Latency is minimized because objects are in kept memory and work requests are directed directly to the machine containing the already in-memory object. The object implements the request behavior on the same machine. There's no pulling data from a disk. There isn't even the hit of accessing a cache server. And since all other object requests are also served from in-memory objects we've minimized the Service Dependency Latency problem as well.

    GigaSpaces isn't the only player in this market. You might want to also take a look at: Scaleout Software, Grid Gain, Teracotta, GemStone, and Coherence. We'll have more on some of these products later.

    Miscellaneous Latency Reduction Ideas

  • Cache. No, really? Well it had to be said. See A Bunch of Great Strategies for Using Memcached and MySQL Better Together.
  • Use a CDN. No, really? See What CDN would you recommend?.
  • Use a Caching Proxy Server. At least this is a little less obvious. See Strategy: Front S3 with a Caching Proxy.
  • Enhance Your Web Operations Capability. There are plenty of available tools to help you pinpoint and correct operation related problems. See Velocity Conference for more information.
  • Use Yslow to Make Your Pages Go. Yslow is a tool to show sources of latency on the client side and suggest ways to fix any problems found. See Yslow to speed up your web pages.
  • Use an Edge DNS Accelerator. This type of service "will ensure that a name server most accessible to the end user will pick up the request and respond." See Edge Acceleration Strategies: Akamai.
  • Optimize Virtual Machines. People often forget VM's exact a performance tax. Virtualized I/O can suffer a substantial performance penalty. See if it can't be tuned.
  • Use Ajax to minimize perceived latency to the user. Clever UI design can make a site feel faster than it really is.
  • Use a faster network. A high speed InfiniBand link can have an end-to end latency of about 1 microsecond. Another option is a 10 GigE network.
  • Scale up. Faster processors means less software induced latency.
  • Optimize firewalls. An often hidden latency enhancer is your firewall system.
  • Use Small Memory Chunks When Using Java. GC in Java kills latency. One way to minimize the impact of garbage collection on latency is to use more VMs and less memory in each VM instead of VM with a lot of memory. This prevents a large GC run and makes latency more predictable.
  • Use a TCP Offload Engine (TOE). TOE tech offloads the TCP/IP stack from the main CPU and puts it on the network controller. This means network adapters can respond faster which means faster end-to-end communication. Network adapters respond faster because bus wait time is reduced as the number of transactions across the system I/O bus and memory bus are reduced.
  • Design low latency network topoligies. Phil Dykstra in Issues Impacting Gigabit Networks:
    Why don't most users experience high data rates?
    pinpoints poor network design as one major source of latency: On a single high performance network today, measured latencies are typically ~1.5x - 3x that expected from the speed of light in fiber. This is mostly due to taking longer than line-of-site paths. Between different networks (via NAPs) latency is usually much worse. Some extra distance is required, based on the availability of fiber routes and interconnects, but much more attention should be given to minimizing latency as we design our network topologies and routing.
  • Make TCP Faster. FastTCP, for example, tweaks TCP to provide smoother and faster data delivery.
  • Copy Data Zero Times. Efficient data transfer through zero copy. Copying data kills. This excellent article explains the path data takes through the OS and how to reduce the number of copies to the big zero.
  • Increase the speed of light. Warp capability could really help speed up communication. Get to work on that!

    Application Server Architecture Matters Again

    With the general move over the past few years to a standard shared nothing two-tierish architecture, discussion of application server architectures has become a neglected topic, mainly because there weren't application servers anymore. Web requests came in, data was retrieved from the database, and results were calculated and returned to the user. No application server. The web server became the application server. This was quite a change from previous architectures which were more application server oriented. Though they weren't called application servers, they were call daemons or even just servers (as in client-server).

    Let's say we buy into RAM is the New Disk. This means we'll have many persistent processes filled with many objects spread over many boxes. A stream of requests are directed at each process and those requests must be executed in each process. How should those processes be designed?

    Sure, having objects in memory reduces latency, but it's very easy through poor programming practice to lose all of that advantage. And then some. Fortunately we have a ton of literature on how to structure servers. I have a more thorough discussion here in Architecture Discussion. Also take a look at SEDA, an architecture for highly concurrent servers and ACE, an OO network programming toolkit in C++.

    A few general suggestions:
  • Stop Serializing/Deserializing Messages. It boggles my mind why we still serialize and deserialize messages. Leave messages in a binary compressed format and decode only on access. Very few activities waste more CPU and cause more lock contention through the memory library than does serialization.
  • Load Balance Across Read Replicas. The more copies of objects you have the more work you can perform in parallel. Consider keeping objects replicas for both high availability and high scalability. This is the same strategy distributed file systems use handle more load. It works in-memory as well.
  • Don't Block. The goal for a program is to use its whole CPU time quanta when it's scheduled to run. Don't block. Don't give the processor back to the OS for someone else to get it. Block for any reason and your performance tanks because not only do you incur the latency of the operation but there's added rescheduling latency as well. Who knows when your thread will get scheduled again?
  • Minimize Paging. Thrashing is when a system experiences excessive page faults. More work is spent on moving memory around than is being given to tasks to perform real work. It's usually three orders of magnitude slower to access a page from disk instead of memory. Unfortunately, memory managers in most languages make reducing paging difficult as you have no control over where memory is placed or how it us used. With CPU speeds these days basically an operation is free when you are operating on paged-in memory.
  • Minimize/Remove locking. Locks add latency and variability to a processing pipeline. A lock is a blocking operation. So you are choosing not to run when you have the CPU which means you incur a number of different forms of latency. Select a server architecture that minimizes the need for locks.


    Locating applications together reduces latency by reducing data hops. The number and location of network hops a message has to travel through is a big part of the end-to-end latency of a system.

    For example, from New York to the London Stock Exchange a round trip message takes 84 milliseconds to send, from Frankfurt it take 18 milliseconds, and from Tokyo it takes 208 milliseconds. If you want to minimize latency then the clear strategy is to colocate your service in the London Stock Exchange. Distance is minimized and you can probably use a faster network too.

    Virtualization technology makes it easier than ever to compose separate systems together. Add a cloud infrastructure to that and it becomes almost easy to dramatically lower latencies by colocating applications.

    Minimize the Number of Hops

    Latency increases with each hop in a system. The fewer hops the less latency. So put those hops on a diet. Some hop reducing ideas are:
  • Colocation. Colocation is one hop reducing strategy. It reduces the number of WAN links, routers, etc that a message has to go through. If a router takes 400 microsecond for each packet, for example, getting rid of that router reduces latency. Colocation also works for code and data, as in the GigaSpaces architecture. They maintain a sharded in-memory object cache so an extra database hop is avoided when executing an operation.
  • Simplify Software Architecture. Remove intermediate daemons, brokers and other latency adding components. Dispatch work to where it will be processed as simply and fast as possible. Peer-to-peer architectures and sharding approaches are good at this. Avoid sending work into a hub for central dispatching. Dispatch as far out at the edge as possible.
  • Open a New Datacenter. Facebook opened a new datacenter on the east coast in order to save 70 milliseconds.

    Build Your own Field-programmable Gate Array (FPGA)

    This one may seem a little off the wall, but creating your own custom FPGA may be a killer option for some problems. A FPGA is a semiconductor device containing programmable logic. Typical computer programs are a series of instructions that are loaded and interpreted by a general purpose microprocessor, like the one in your desk top computer. Using a FPGA it's possible to bypass the overhead of a general purpose microprocessor and code your application directly into silicon. For some classes of problems the performance increases can be dramatic.

    FPGAs are programmed with your task specific algorithm. Usually something compute intensive like medical imaging, modeling bond yields, cryptography, and matching patterns for deep packet inspections. I/O heavy operations probably won't benefit from FPGAs. Sure, the same algorithm could be run on a standard platform, but the advantage FPGAs have is even though they may run at a relatively low clock rates, FPGAs can perform many calculations in parallel. So perhaps orders-of-magnitude more work is being performed each clock cycle. Also, FPGAs often use content addressable memory which provides a significant speedup for indexing, searching, and matching operations. We also may see a move to FPGAs because they use less power. Stay lean and green.

    In embedded projects FPGAs and ASICS (application-specific integrated circuit) are avoided like the plague. If you can get by with an off-the-shelf microprocessors (Intel, AMD, ARM, PPC, etc) you do it. It's a time-to-market issue. Standard microprocessors are, well, standard, so that makes them easy to work with. Operating systems will already have board support packages for standard processors, which makes building a system faster and cheaper. Once custom hardware is involved it becomes a lot of work to support the new chip in hardware and software. Creating a software only solution is much more flexible in a world where constant change rules. Hardware resists change. So does software, but since people think it doesn't we have to act like software is infinitely malleable.

    Sometimes hardware is the way to go. If you are building a NIC that has to process packets at line speed the chances are an off-the-shelf processor won't be cost effective and may not be fast enough. Your typical high end graphics card, for example, is a marvel of engineering. Graphics cards are so powerful these days distributed computation projects like Folding@home get a substantial amount of their processing power from graphics cards. Traditional CPUs are creamed by NVIDIA GeForce GPUs which perform protein-folding simulations up to 140 times faster. The downside is GPUs require very specialized programming, so it's easier to write for a standard CPU and be done with it.

    That same protein folding power can be available to your own applications. ACTIV Financial, for example, uses a custom FGPA for low latency processing of high speed financial data flows. ACTIV's competitors use a traditional commodity box approach where financial data is processed by a large number of commodity servers. Let's say an application takes 12 servers. Using a FPGA the number of servers can be collapsed down to one because more instructions are performed simultaneously which means fewer machines ar needed. Using the FPGA architecture they process 20 times more messages than they did before and have reduced latency from one millisecond down to less than 100 microseconds.

    Part of the performance improvement comes from the high speed main memory and network IO access FPGAs enjoy with the processor. Both Intel and AMD make it relatively easy to connect FPGAs to their chips. Using these mechanisms data moves back and forth between your processing engine and the main processor with minimal latency. In a standard architecture all this communication and manipulation would happen over a network.

    FPGAs are programmed using hardware description languages like Verilog and VHDL. You can't get away from the hardware when programming FPGAs, which is a major bridge to cross for us software types. Many moons ago I took a Verilog FPGA programming class. It's not easy, nothing is ever easy, but it is possible. And for the right problem it might even be worth it.

    Related Articles

  • The Challenges of Latency by Dan Pritchett
  • Latency Exists, Cope! by Dan Pritchett
  • Architecting for Latency by Dan Pritchett
  • BASE: An ACID Alternative by Dan Pritchett, eBay
  • Comet: Sub-Second Latency with 10K+ Concurrent Users by Alexander Olaru
  • It's the Latency, Stupid by Stuart Cheshire
  • Latency and the Quest for Interactivity by Stuart Cheshire
  • The importance of bandwidth versus latency by Dion Almaer
  • Fallacies of Distributed Computing - The second fallacy is "Latency is Zero"
  • List of device bandwidths
  • Computing over a high-latency network means you have to bulk up by Raymond Chen
  • AJAX Latency problems: myth or reality? by Jep Castelein
  • RAM Guide: Part I DRAM and SRAM Basics and Part 2 by Jon "Hannibal" Stokes
  • The Many Flavors of System Latency.. along the Critical Path of Peak Performance and Processors and Performance : Chips, MIPS, and Sizing blips.. by Todd Jobson. A very detailed and helpful analysis of latency sources.
  • Ethernet Latency: The Hidden Performance Killer by Kevin Burton
  • Network latency vs. end-to-end latency by Nati Shalom
  • Low-Latency Delivery Enters Mainstream; But Standard Measurement Remains Elusive by Andrew Delaney
  • The three faces of latency by By Scott Parsons, Chief Scientist at Exegy, Inc.
  • Architecture Discussion
  • The JVM needs Value Types - Solving the next bottleneck. Value types use less space, less paging, less memory allocation, better cache usage, better garbage collection profile.
  • Latency, Bandwidth, and Response Times by Chris Loosley.
  • Anatomy of real-time Linux architectures by M. Time Jones.
  • True Cost of Latency by GemStone
  • Tuesday

    Paper: Parallelizing the Web Browser

    There have been reports that software engineering is dead. Maybe, like the future, software engineering is simply not evenly distributed? When you read this paper I think you'll agree there is some real engineering going on, it's just that most of the things we need to build do not require real engineering. Much like my old childhood tree fort could be patched together and was "good enough." This brings to mind the old joke: If a software tree falls in the woods would anyone hear it fall? Only if it tweeted on the way down...

    What this paper really showed me is we need not only to change programming practices and constructs, but we also need to design solutions that allow for deep parallelism to begin with. Grafting parallelism on later is difficult. Parallel execution requires knowing precisely how components are dependent on each other and that level of precision tends to go far beyond the human attention span.

    In particular this paper deals with how to parallelize the browser on cell phones. We are entering a multi-core smartphone dominated world. As network connections become faster, applications, like the browser, become CPU bound:

    On an equivalent network connection, the iPhone browser is 5 to 10 times slower than Firefox on a fast laptop. The browser is CPU-bound because it is a compiler (for HTML), a page layout engine (for CSS), and an interpreter (for JavaScript); all three tasks are on a user’s critical path.

    To speed up the browser they worked on: offloading computation, removing the abstraction tax, and parallelizing the browser using energy efficient data and task approaches. The problem is technologies like HTML, CSS, DOM, Javascript, events, and page layout were not designed to be parallel. They were designed to be run on a single CPU. And the paper goes to brilliant and heroic lengths to parallelize this part of the stack. They designed new work-efficient FSM algorithms, speculative parallelization for flow layouts, eliminating as much shared state as possible, callback dependency analysis, using actors to implement behaviours, and many more.

    What's clear though is their job would have been a heck of a lot easier if the stack would have been designed with parallelization in mind from the beginning.

    Leo Meyerovich, one of the authors of the paper, talks about the need for a more rigorous underpinning in blog postThe Point of Semantics:

    As part of the preparation for a paper submission, I'm finishing up my formalization of a subset of CSS 2.1 (blocks, inlines, inline-blocks, and floats) from last year. My first two, direct formalization approaches failed the smell test so Ras and I created a more orthogonal kernel language. It's small, and as the CSS spec is a scattered hodge-podge of prose and visual examples riddled with ambiguities, we phrase it as a total and deterministic attribute grammar that is easy to evaluate in parallel. 

    I asked Leo what rules we could follow to create more parallelizable constructs from the beginning and he said that's what he'll be working on for the next couple years :-) Some advice he had was:

  • Be clear on what you want to parallelize. Figuring out where the parallelism should be, at a conceptual level, is always the first step.
  • Understand how it should run in parallel.
  • Focus on making it easy to do just that (and worry about the rest later).
  • It's better to completely solve a problem for some folks than almost solve a problem for many: you can help more and more in the former, but with the latter, you might never end up helping anybody.

    Some things Leo will be working on are:
    I've been enjoying higher-order data flow models (Flapjax) and task parallelism (Cilk++) for awhile now and have been thinking about this, including support for controlled sharing (e.g., SharC for type qualifiers and I'm still trying to figure out implicitly transactional flows for FRP). For a browser, I think it will remain as specialized libraries written in privileged languages where good engineers can rock and put together and be exposed in higher-level languages. Hopefully gradually typing will extend into lower levels to support this. The above hints at a layered framework with the bulk in the high-level -- think parallel scripting. However, as a community, we don't know how to include performance guides in large software, so parallelism is a challenge. I prototyped one of my algorithms in a parallel python variant: the sequential C was magnitudes faster than then 20-core python. Of course, the parallel C++ was even faster :) 

    Related Articles

  • Parallelizing the Web Browser by Christopher Grant Jones, Rose Liu, Leo Meyerovich, Krste Asanovi´c, Rastislav Bodík.
  • Parallelizing the Web Browser
    Browsing Web 3.0 on 3 Watts
  • Leo Meyerovich's Project Website
  • Flapjax - a new programming language designed around the demands of modern, client-based Web applications: event-driven, reactive evaluation, event-stream abstraction for communicating with web services, interfaces to external web services.
  • Bell's Law of Computer Classes
  • Leo Meyerovich's Blog
  • Monday

    A Scalability Lament

    In Scalability issues for dummies Alex Barrera talks movingly about the challenges he faces trying to scale his startup inkzee as the lone developer. Inkzee is an online news reader that automatically groups similar topics. This is a cool problem and is one you know right away is going to have some killer scalability problems as the number of feeds and the number of users increase. And these problems lead to the point of the post, to explain here what are scalability problems and how deep the repercussions are for a small company, which Alex does admirably.

    Some takeaways:

  • Sites are composed of a frontend and backend. The backend isn't visible to users, but it does all the work and this is where the scalability problems show up.
  • As more and more users use a site it becomes slow because more users reveal bottlenecks in the system that weren't visible before.
  • There can be many many reasons for these bottlenecks. They are often very hard to find because the backend systems are very complex and have a lot of complex interactions.
  • It takes a while to find the bottlenecks and create fixes for them. You are never quite sure if the fixes will really work. Many of these problems are unique to the problem space so pre-canned solutions aren't always available. And because you don't want to destroy your production servers it takes a while to put fixes into the system. This means your release cycle is slow which means progress on your site is slow.
  • The process of redesign sucks up all your resources so progress on the site stalls almost completely, especially for a small development group. This stops growth as you can't handle new customers and your existing customers become disenchanted.
  • The process is unfortunately iterative. Solving one problem just puts you in the queue for the next problem. There's a reason it took Twitter a while to get on their feet.
  • Scalability problems aren’t something you can discard as being ONLY technical, it’s roots might be technical but its effects will shake the whole company.

    I found Alex's commentary quite touching and familiar. As I imagine many of you do too. It's the modern equivalent of an explorer following a dream. Going alone into uncharted territory where the Dragons live and trying to survive when everything seems against you. For every great returning hero there are 10 who do not make it back. And that's hard to deal with.

    Everyone will certainly have their ideas on how to "fix" the problem, as that's what engineers do. But it also doesn't hurt to use our Venus brain for a moment and simply recognize the toll this process can take. It can be dispiriting. The continual stream of problems and lack of positive feedback can wear you down after a while. To stick with it takes a bit of craziness in the heart.

    Switching back to being Mars brained I might suggest:
  • Find a partner. There's always a Jerry to go along with Ben. A Martin to go along with Lewis. And Rambo is just a movie. Going it alone is hard hard hard. Partners can pick each other up and give each other a breather when necessary.
  • Use the Cloud. Whatever your opinions of the economics of the cloud, it makes testing of the type Alex was needing a breeze. Architect for the cloud and you can spin up a system in parallel, run a load generator, and never touch your production system at all. And do it all for cheap. This is one of the biggest wins for the cloud and would seem to solve a lot of problems.

    Related Articles

  • Scalable Web Architectures and Application State by Marton Trencseni
  • Scalability for dummies by Royans Tharakan