Messages in this thread | | | Subject | Re: regression introduced by - timers: fix itimer/many thread hang | From | Peter Zijlstra <> | Date | Fri, 07 Nov 2008 11:29:04 +0100 |
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(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.
But with your new scheme it'll be a problem regardless of how many threads you have, as long as each running application will have at least 2 (not uncommon these days).
Furthermore, the memory requirements for your solution now also scale with cpus instead of threads, again something not really appreciated.
Therefore I say your solution is worse than the one we had.
You should optimize for the common case, and ensure the worst case doesn't suck. You did it backwards.
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