Re: regression introduced by - timers: fix itimer/many thread hang

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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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