linux-kernel - Re: [-RT] multiple streams have degraded performance

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Date:	Tue, 19 Jun 2007 07:25:29 -0700
From:	Vernon Mauery <vernux@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>, Jeff Garzik <jeff@...zik.org>
Subject: Re: [-RT] multiple streams have degraded performance

On Monday 18 June 2007 11:51:38 pm Peter Zijlstra wrote:
> On Mon, 2007-06-18 at 22:12 -0700, Vernon Mauery wrote:
> > In looking at the performance characteristics of my network I found that
> > 2.6.21.5-rt15 suffers from degraded thoughput with multiple threads.  The
> > test that I did this with is simply invoking 1, 2, 4, and 8 instances of
> > netperf at a time and measuring the total throughput.  I have two 4-way
> > machines connected with 10GbE cards.  I tested several kernels (some
> > older and some newer) and found that the only thing in common was that
> > with -RT kernels the performance went down with concurrent streams.
> >
> > While the test was showing the numbers for receiving as well as sending,
> > the receiving numbers are not reliable because that machine was running a
> > -RT kernel for these tests.
> >
> > I was just wondering if anyone had seen this problem before or would have
> > any idea on where to start hunting for the solution.
>
> could you enable CONFIG_LOCK_STAT
>
> echo 0 > /proc/lock_stat
>
> <run your test>
>
> and report the output of (preferably not 80 column wrapped):
>
> grep : /proc/lock_stat | head

/proc/lock_stat stayed empty for the duration of the test.  I am guessing this 
means there was no lock contention.

I do see this on the console:
BUG: scheduling with irqs disabled: IRQ-8414/0x00000000/9494
caller is wait_for_completion+0x85/0xc4

Call Trace:
 [<ffffffff8106f3b2>] dump_trace+0xaa/0x32a
 [<ffffffff8106f673>] show_trace+0x41/0x64
 [<ffffffff8106f6ab>] dump_stack+0x15/0x17
 [<ffffffff8106566f>] schedule+0x82/0x102
 [<ffffffff81065774>] wait_for_completion+0x85/0xc4
 [<ffffffff81092043>] set_cpus_allowed+0xa1/0xc8
 [<ffffffff810986e2>] do_softirq_from_hardirq+0x105/0x12d
 [<ffffffff810ca6cc>] do_irqd+0x2a8/0x32f
 [<ffffffff8103469d>] kthread+0xf5/0x128
 [<ffffffff81060f68>] child_rip+0xa/0x12

INFO: lockdep is turned off.

I haven't seen this until I enabled lock_stat and ran the test.

> or otherwise if there are any highly contended network locks listed?

Any other ideas for debugging this?

--Vernon
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