Messages in this thread | | | From | Petr Tesarik <> | Subject | Re: regression introduced by - timers: fix itimer/many thread hang | Date | Fri, 21 Nov 2008 19:42:43 +0100 |
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Dne Friday 07 of November 2008 11:29:04 Peter Zijlstra napsal(a): > (fwiw your email doesn't come across properly, evo refuses to display > them, there's some mangling of headers which makes it think there's an > attachment) > > On Thu, 2008-11-06 at 15:52 -0800, Frank Mayhar wrote: > > On Thu, 2008-11-06 at 16:08 +0100, Peter Zijlstra wrote: > > > On Thu, 2008-11-06 at 09:03 -0600, Christoph Lameter wrote: > > > > On Thu, 6 Nov 2008, Peter Zijlstra wrote: > > > > > Also, you just introduced per-cpu allocations for each > > > > > thread-group, while Christoph is reworking the per-cpu allocator, > > > > > with one unfortunate side-effect - its going to have a limited size > > > > > pool. Therefore this will limit the number of thread-groups we can > > > > > have. > > > > > > > > Patches exist that implement a dynamically growable percpu pool > > > > (using virtual mappings though). If the cost of the additional > > > > complexity / overhead is justifiable then we can make the percpu pool > > > > dynamically extendable. > > > > > > Right, but I don't think the patch under consideration will fly anyway, > > > doing a for_each_possible_cpu() loop on every tick on all cpus isn't > > > really healthy, even for moderate sized machines. > > > > I personally think that you're overstating this. First, the current > > implementation walks all threads for each tick, which is simply not > > scalable and results in soft lockups with large numbers of threads. > > This patch fixes a real bug. Second, this only happens "on every tick" > > for processes that have more than one thread _and_ that use posix > > interval timers. Roland and I went to some effort to keep loops like > > the on you're referring to out of the common paths. > > > > In any event, while this particular implementation may not be optimal, > > at least it's _right_. Whatever happened to "make it right, then make > > it fast?" > > Well, I'm not thinking you did it right ;-) > > While I agree that the linear loop is sub-optimal, but it only really > becomes a problem when you have hundreds or thousands of threads in your > application, which I'll argue to be insane anyway.
This is just not true. I've seen a very real example of a lockup with a very sane number of threads (one per CPU), but on a very large machine (1024 CPUs IIRC). The application set per-process CPU profiling with an interval of 1 tick, which translates to 1024 timers firing off with each tick...
Well, yes, that was broken, too, but that's the way one quite popular FORTRAN compiler works...
Petr
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