Re: Network slowdown due to CFS

From: Jarek Poplawski
Date: Thu Sep 27 2007 - 05:27:52 EST

On 26-09-2007 15:31, Ingo Molnar wrote:
> * David Schwartz <davids@xxxxxxxxxxxxx> wrote:
>>>> I think the real fix would be for iperf to use blocking network IO
>>>> though, or maybe to use a POSIX mutex or POSIX semaphores.
>>> So it's definitely not a bug in the kernel, only in iperf?
>> Martin:
>> Actually, in this case I think iperf is doing the right thing (though not
>> the best thing) and the kernel is doing the wrong thing. [...]
> it's not doing the right thing at all. I had a quick look at the source
> code, and the reason for that weird yield usage was that there's a
> locking bug in iperf's "Reporter thread" abstraction and apparently
> instead of fixing the bug it was worked around via a horrible yield()
> based user-space lock.
> the (small) patch below fixes the iperf locking bug and removes the
> yield() use. There are numerous immediate benefits of this patch:
> sched_yield() is almost always the symptom of broken locking or other
> bug. In that sense CFS does the right thing by exposing such bugs =B-)

...Only if it were under some DEBUG option. Even if iperf is doing
the wrong thing there is no explanation for such big difference in
the behavior between sched_compat_yield 1 vs. 0. It seems common
interfaces should work similarly and predictably on various
systems, and here, if I didn't miss something, linux looks like a
different kind?

Jarek P.
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