Re: Network slowdown due to CFS

[Jarek Poplawski <jarkao2@o2.pl>]

On 26-09-2007 15:31, Ingo Molnar wrote:
> * David Schwartz <davids@webmaster.com> 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?

Regards,
Jarek P.