Delay and Server reliability
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Delay and Server reliability
 
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Hi -
We recently had some "fun" chasing server lockups (with truly awful uptimes of about a day or less before things went downhill) and were finally able to track a huge portion of it down to problems with Delay. The effect we were seeing on our servers was that the system would randomly lock up and either go down to 0% CPU or 100% CPU. After poking it with a USR1 signal (which, in our VMs is hooked up such that it prints all the call stacks in the image; it's a life- safer if you need to debug these issues) we usually found that all processes were waiting on Delay's AccessProtect (0%) or alternatively found that a particular process (the event tickler) would sit in a tight loop swallowing repeated errors complaining that "this delay is already scheduled". After hours and hours of testing, debugging, and a little stroke of luck we finally found out that all of these issues were caused by the fact that Delay's internal structures are updated by the calling process (insertion into and removal from SuspendedDelays) which renders the process susceptible to being terminated in the midst of updating these structures. If you look at the code, this is obviously an issue because if (for example) the calling process gets terminated while it's resorting SuspendedDelays the result is unpredictable. This is in particular an issue because the calling process is often running at a relatively low priority so interruption by other, high-priority processes is a common case. And if any of these higher priority processes kills the one that just happens to execute SortedCollection>>remove: anything can happen - from leaving a later delay in front of an earlier one (one of the cases we had indicated that this was just what had happened) to errors when doing the next insert/remove ("trying to evaluate a block that is already evaluated") to many more weirdnesses. Unfortunately, it is basically impossible to recreate this problem under any kind of controlled circumstances, mostly because you need a source of events that is truly independent from your time source. As a consequence of our findings we rewrote Delay to deal with these issues properly and, having deployed the changes about ten days ago on our servers, all of these sources of problems simply vanished. We haven't had a single server problem which we couldn't attribute to our own stupidity (such as running out of disk space ;-) The changes will in particular be helpful to you if you: * run network servers * fork processes to handle network requests * terminate these processes explicitly (on error conditions for example) * use Semaphore>>waitTimeoutMsecs: (all socket functions use this) If you have seen random, unexplained lockups of your server (0% CPU load while being locked up is a dead giveaway[*]) I'd recommend using the attached changes (which work best on top of a VM with David Lewis' 64bit fixes applied) and see if that helps. For us, they made the difference between running the server in Squeak and rewriting it in Java. I've also filed this as http://bugs.squeak.org/view.php?id=6576 [*] The 0% CPU lockups have sometimes been attributed to issues with Linux wait functions. After having seen the havoc that Delay wrecks on the system I don't buy these explanations any longer. A much simpler (and more likely) explanation is that Delay went wild. Cheers, - Andreas _______________________________________________ Seaside mailing list [hidden email] http://lists.squeakfoundation.org/cgi-bin/mailman/listinfo/seaside |
SafeDelay.cs (8K)
Re: Delay and Server reliability
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Re: Delay and Server reliability
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