Re: [-RT] multiple streams have degraded performance

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2007-06-19 at 07:25 -0700, Vernon Mauery wrote:
> 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.

Most likely caused by the issue below.

> 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.

I think this is what causes the lack of output. It disables all lock
tracking features...

> > or otherwise if there are any highly contended network locks listed?
> 
> Any other ideas for debugging this?

fixing the above bug would help :-)

Ingo says that should be fixed in -rt17, so if you could give that a
spin...