Messages in this thread | | | Subject | Re: regression introduced by - timers: fix itimer/many thread hang | From | Petr Tesarik <> | Date | Mon, 24 Nov 2008 09:46:43 +0100 |
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Peter Zijlstra píše v Ne 23. 11. 2008 v 15:24 +0100: > On Fri, 2008-11-21 at 19:42 +0100, Petr Tesarik wrote: > > > > > 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... > > I'm not sure what side you're arguing...
In this particular case I'm arguing against both, it seems. The old behaviour is broken and the new one is not better. :(
> The current (per-cpu) code is utterly broken on large machines too, I've > asked SGI to run some tests on real numa machines (something multi-brick > altix) and even moderately small machines with 256 cpus in them grind to > a halt (or make progress at a snails pace) when the itimer stuff is > enabled. > > Furthermore, I really dislike the per-process-per-cpu memory cost, it > bloats applications and makes the new per-cpu alloc work rather more > difficult than it already is. > > I basically think the whole process wide itimer stuff is broken by > design, there is no way to make it work on reasonably large machines, > the whole problem space just doesn't scale. You simply cannot maintain a > global count without bouncing cachelines like mad, so you might as well > accept it and do the process wide counter and bounce only a single line, > instead of bouncing a line per-cpu.
Very true. Unfortunately per-process itimers are prescribed by the Single Unix Specification, so we have to cope with them in some way, while not permitting a non-privileged process a DoS attack. This is going to be hard, and we'll probably have to twist the specification a bit to still conform to its wording. :((
I really don't think it's a good idea to set a per-process ITIMER_PROF to one timer tick on a large machine, but the kernel does allow any process to do it, and then it can even cause hard freeze on some hardware. This is _not_ acceptable.
What is worse, we can't just limit the granularity of itimers, because threads can come into being _after_ the itimer was set.
> Furthermore, I stand by my claims that anything that runs more than a > hand-full of threads per physical core is utterly braindead and deserves > all the pain it can get. (Yes, I'm a firm believer in state machines and > don't think just throwing threads at a problem is a sane solution).
Yes, anything with many threads per-core is badly designed. My point is that it's not the only broken case.
Petr Tesarik
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