high_scalability high_scalability-2012 high_scalability-2012-1229 knowledge-graph by maker-knowledge-mining

1229 high scalability-2012-04-17-YouTube Strategy: Adding Jitter isn't a Bug


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Introduction: The adding jitter strategy was one of the most commented on techniques from 7 Years Of YouTube Scalability Lessons In 30 Minutes on HackerNews . Probably because it’s one of the emergent phenomena that you really can’t predict and is shocking when you see it in real life. Here’s the technique: Add Entropy Back into Your System If your system doesn’t jitter then you get thundering herds . Distributed applications are really weather systems. Debugging them is as deterministic as predicting the weather. Jitter introduces more randomness because surprisingly, things tend to stack up. For example, cache expirations. For a popular video they cache things as best they can. The most popular video they might cache for 24 hours. If everything expires at one time then every machine will calculate the expiration at the same time. This creates a thundering herd. By jittering you are saying  randomly expire between 18-30 hours. That prevents things from stackin


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1 The adding jitter strategy was one of the most commented on techniques from 7 Years Of YouTube Scalability Lessons In 30 Minutes on HackerNews . [sent-1, score-0.556]

2 Probably because it’s one of the emergent phenomena that you really can’t predict and is shocking when you see it in real life. [sent-2, score-0.255]

3 Here’s the technique: Add Entropy Back into Your System If your system doesn’t jitter then you get thundering herds . [sent-3, score-0.642]

4 Jitter introduces more randomness because surprisingly, things tend to stack up. [sent-6, score-0.161]

5 For a popular video they cache things as best they can. [sent-8, score-0.09]

6 The most popular video they might cache for 24 hours. [sent-9, score-0.09]

7 If everything expires at one time then every machine will calculate the expiration at the same time. [sent-10, score-0.147]

8 By jittering you are saying  randomly expire between 18-30 hours. [sent-12, score-0.171]

9 Systems have a tendency to self synchronize as operations line up and try to destroy themselves. [sent-15, score-0.219]

10 Each one actually removes entropy from the system so you have to add some back in. [sent-19, score-0.161]

11 Comments from HackerNews really help to fill out the topic with more detail: There were some other examples of the thundering herd problem : Adblock Plus had a thundering herd problem : Ad blocking lists check for updates every 5 days. [sent-20, score-0.744]

12 Updates that had been scheduled for Saturday or Sunday would spill over to Monday. [sent-22, score-0.093]

13 Facebook uses jitter a lot in their cache infrastructure. [sent-24, score-0.561]

14 The Multicast DNS extension makes extensive use of randomness to reduce network collisions. [sent-26, score-0.161]

15 Jitter is used in all sorts of applications, from cron jobs to configuration management to memcache key expiration. [sent-28, score-0.266]

16 Any time you have a lot of nodes that all need to do an operation at a specific time you rely on jitter to keep resources from bottlenecking. [sent-29, score-0.471]

17 I frequently find myself adding a sleep for PID mod some appropriate constant to the beginning of big distributed batch jobs in order to keep shared resources (NFS, database, etc) from getting hammered all at once. [sent-32, score-0.516]

18 Not everyone adds jitter : Jeff Dean at Google says they prefer to have a known hit every once in awhile, so they will have all the cron jobs go off at the same time versus introducing jitter. [sent-38, score-0.737]

19 The Linux kernel tries to schedule timer events for the same deadline time. [sent-40, score-0.455]

20 That allows the processor to sleep longer because the kernel doesn't need to wake up as often just to handle 1 or 2 timer events. [sent-41, score-0.451]


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Introduction: One of the benefits of web applications is that they are generally transported via TCP, which is a connection-oriented protocol designed to assure delivery. TCP has a variety of native mechanisms through which delivery issues can be addressed – from window sizes to selective acks to idle time specification to ramp up parameters. All these technical knobs and buttons serve as a way for operators and administrators to tweak the protocol, often at run time, to ensure the exchange of requests and responses upon which web applications rely. This is unlike UDP, which is more of a “fire and forget” protocol in which the server doesn’t really care if you receive the data or not. Now, voice and streaming video and audio over the web has always leveraged UDP and thus it has always been highly sensitive to jitter. Jitter is, without getting into layer one (physical) jargon, an undesirable delay in the otherwise consistent delivery of packets. It causes the delay of and sometimes

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