Re: [-RT] multiple streams have degraded performance

[Vernon Mauery]

On Tuesday 19 June 2007 8:38:50 am Peter Zijlstra wrote:
> 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...

I just tested with -rt17 and the BUG message is gone, but I still don't see 
any entries in.  I don't see any other suspicious message so I think it is 
really strange that is not listing any locks at all.

--Vernon
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