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RE: Network slowdown due to CFS

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To: Linux-Kernel@Vger. Kernel. Org <linux-kernel@...>
Date: Wednesday, September 26, 2007 - 8:00 am

> > I think the real fix would be for iperf to use blocking network IO

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 calling
'sched_yield' to ensure that every other thread gets a chance to run before
the current thread runs again. CFS is not doing that, allowing the yielding
thread to hog the CPU to the exclusion of the other threads. (It can allow
the yielding thread to hog the CPU, of course, just not to the exclusion of
other threads.)

It's still better to use some kind of rational synchronization primitive
(like mutex/sempahore) so that the other threads can tell you when there's
something for you to do. It's still better to use blocking network IO, so
the kernel will let you know exactly when to try I/O and your dynamic
priority can rise.

Ingo:

Can you clarify what CFS' current default sched_yield implementation is and
what setting sched_compat_yield to 1 does? Which way do we get the right
semantics (all threads of equal static priority are scheduled, with some
possible SMP fuzziness, before this thread is scheduled again)?

DS


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Messages in current thread:
Network slowdown due to CFS, Martin Michlmayr, (Wed Sep 26, 4:52 am)
Re: Network slowdown due to CFS, Mike Galbraith, (Wed Sep 26, 6:23 am)
Re: Network slowdown due to CFS, Martin Michlmayr, (Wed Sep 26, 6:48 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Wed Sep 26, 7:21 am)
Re: Network slowdown due to CFS, Martin Michlmayr, (Wed Sep 26, 7:29 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Thu Sep 27, 5:49 am)
Re: Network slowdown due to CFS, Martin Michlmayr, (Thu Sep 27, 6:54 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Thu Sep 27, 6:56 am)
Re: Network slowdown due to CFS, Martin Michlmayr, (Thu Sep 27, 7:12 am)
RE: Network slowdown due to CFS, David Schwartz, (Wed Sep 26, 8:00 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Wed Sep 26, 9:31 am)
Re: Network slowdown due to CFS, Jarek Poplawski, (Thu Sep 27, 5:30 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Thu Sep 27, 5:46 am)
Re: Network slowdown due to CFS, Jarek Poplawski, (Thu Sep 27, 8:27 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Thu Sep 27, 9:31 am)
Re: Network slowdown due to CFS, Jarek Poplawski, (Thu Sep 27, 10:42 am)
Re: Network slowdown due to CFS, Nick Piggin, (Fri Sep 28, 2:10 am)
Re: Network slowdown due to CFS, Jarek Poplawski, (Mon Oct 1, 4:43 am)
Re: Network slowdown due to CFS, Jarek Poplawski, (Tue Oct 2, 5:26 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Mon Oct 1, 12:25 pm)
Re: Network slowdown due to CFS, Jarek Poplawski, (Tue Oct 2, 5:03 am)
Re: Network slowdown due to CFS, Jarek Poplawski, (Tue Oct 2, 9:39 am)
Re: Network slowdown due to CFS, Chris Friesen, (Mon Oct 1, 12:55 pm)
Re: Network slowdown due to CFS, Ingo Molnar, (Mon Oct 1, 1:09 pm)
Re: Network slowdown due to CFS, Chris Friesen, (Mon Oct 1, 1:45 pm)
Re: iperf yield usage, Ingo Molnar, (Mon Oct 1, 3:09 pm)
RE: Network slowdown due to CFS, David Schwartz, (Mon Oct 1, 12:49 pm)
RE: Network slowdown due to CFS, Rusty Russell, (Wed Oct 3, 8:31 pm)
Re: Network slowdown due to CFS, Helge Hafting, (Wed Oct 3, 7:31 am)
Re: Network slowdown due to CFS, Arjan van de Ven, (Mon Oct 1, 3:53 pm)
RE: Network slowdown due to CFS, David Schwartz, (Mon Oct 1, 6:17 pm)
Re: Network slowdown due to CFS, Arjan van de Ven, (Mon Oct 1, 6:35 pm)
RE: Network slowdown due to CFS, David Schwartz, (Mon Oct 1, 6:44 pm)
Re: Network slowdown due to CFS, Arjan van de Ven, (Mon Oct 1, 6:55 pm)
RE: Network slowdown due to CFS, David Schwartz, (Tue Oct 2, 11:37 am)
Re: Network slowdown due to CFS, Jarek Poplawski, (Wed Oct 3, 3:15 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Mon Oct 1, 1:31 pm)
RE: Network slowdown due to CFS, David Schwartz, (Mon Oct 1, 2:23 pm)
Re: yield API, Ingo Molnar, (Tue Oct 2, 2:46 am)
Re: yield API, Jesper Juhl, (Wed Dec 12, 6:39 pm)
Re: yield API, Kyle Moffett, (Thu Dec 13, 12:43 am)
RE: yield API, David Schwartz, (Thu Dec 13, 4:10 pm)
Re: yield API, Eric St-Laurent, (Tue Oct 2, 5:57 pm)
Re: yield API, linux-os (Dick Johnson), (Tue Oct 2, 7:50 am)
Re: yield API, Douglas McNaught, (Tue Oct 2, 11:24 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Tue Oct 2, 2:26 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Tue Oct 2, 2:08 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Tue Oct 2, 2:06 am)
Re: Network slowdown due to CFS, Jarek Poplawski, (Wed Oct 3, 4:02 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Wed Oct 3, 4:16 am)
Re: Network slowdown due to CFS, Jarek Poplawski, (Wed Oct 3, 4:56 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Wed Oct 3, 5:10 am)
Re: Network slowdown due to CFS, Casey Dahlin, (Thu Oct 4, 1:33 am)
Re: Network slowdown due to CFS, Jarek Poplawski, (Wed Oct 3, 5:50 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Sun Oct 7, 3:18 am)
Re: Network slowdown due to CFS, Dmitry Adamushko, (Wed Oct 3, 6:55 am)
Re: Network slowdown due to CFS, Jarek Poplawski, (Wed Oct 3, 7:40 am)
Re: yield, Ingo Molnar, (Wed Oct 3, 7:56 am)
Re: yield, Jarek Poplawski, (Wed Oct 3, 8:16 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Wed Oct 3, 7:22 am)
Re: Network slowdown due to CFS, Dmitry Adamushko, (Wed Oct 3, 6:58 am)
Re: Network slowdown due to CFS, Jarek Poplawski, (Wed Oct 3, 7:20 am)
Re: Network slowdown due to CFS, Andi Kleen, (Tue Oct 2, 2:47 am)
Re: Network slowdown due to CFS, Stephen Hemminger, (Wed Sep 26, 11:46 am)
Re: Network slowdown due to CFS, Stephen Hemminger, (Wed Sep 26, 11:40 am)
Re: Network slowdown due to CFS, Mike Galbraith, (Wed Sep 26, 6:20 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Wed Sep 26, 5:34 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Wed Sep 26, 5:47 am)
Re: Network slowdown due to CFS, Martin Michlmayr, (Wed Sep 26, 6:08 am)
Re: Network slowdown due to CFS, Ingo Molnar, (Wed Sep 26, 6:18 am)
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