Messages in this thread | | | Subject | Re: regression introduced by - timers: fix itimer/many thread hang | From | Frank Mayhar <> | Date | Mon, 10 Nov 2008 10:00:09 -0800 |
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On Mon, 2008-11-10 at 15:42 +0100, Peter Zijlstra wrote: > On Mon, 2008-11-10 at 08:38 -0600, Christoph Lameter wrote: > > Can we at least somehow make sure that nothing significantly happens in a > > timer interrupt on a processor if the thread has not scheduled any events > > or not odone any system calls? > Do threads actually scale that far? I thought mmap_sem contention and > other shared state would render threads basically useless on these very > large machines. > > But afaiu this stuff, the per-cpu loop is only done when an itimer is > actually active.
Correct.
> The detail I've not looked at is, if when this itimer is indeed active > and we are running 256 threads of the same application on all cpus do we > then do the per-cpu loop for each tick on each cpu?
The answer to this question is, "that depends." You can have an itimer for a single thread or for the whole thread group. In the former case, it never happens; it only does the loops for the thread group case. If there is a thread group itimer then of course we have to sum the tick count across all CPUs to determine whether the timer has expired.
Personally, I would argue that it's silly to have an itimer running when you have many threads, and if you care about performance it's even _more_ silly. But it's sillier yet to be able to wedge the kernel by running a program in user space.
As far as Christoph's concern regarding latency for 8- and 16-processor systems, my belief (supported by data I can't discuss, sigh) is that the loop adds negligible latency. In fact, it can't really be discussed in this way since the existing implementation adds *lots* of latency when an itimer is running, since it sums the values across all threads. I never collected latency versus number of threads data but it's bad enough that at about 4500 threads (on a dual amd64) it took longer than a tick to do a tick's worth of processing. -- Frank Mayhar <fmayhar@google.com> Google, Inc.
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