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Date:      Tue, 7 Feb 2006 17:30:51 -0500
From:      John Baldwin <jhb@freebsd.org>
To:        Andrew Gallatin <gallatin@cs.duke.edu>
Cc:        freebsd-current@freebsd.org, Andre Oppermann <andre@freebsd.org>
Subject:   Re: machdep.cpu_idle_hlt and SMP perf?
Message-ID:  <200602071730.53881.jhb@freebsd.org>
In-Reply-To: <17385.7187.845964.182297@grasshopper.cs.duke.edu>
References:  <17379.56708.421007.613310@grasshopper.cs.duke.edu> <200602071037.05314.jhb@freebsd.org> <17385.7187.845964.182297@grasshopper.cs.duke.edu>

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On Tuesday 07 February 2006 17:15, Andrew Gallatin wrote:
> John Baldwin writes:
>  > On Monday 06 February 2006 17:37, Andrew Gallatin wrote:
>  > > John Baldwin writes:
>  > >  > On Monday 06 February 2006 14:46, Andrew Gallatin wrote:
>  > >  > > Andre Oppermann writes:
>  > >  > >  > Andrew Gallatin wrote:
>  > >  > >  > > Why dooes machdep.cpu_idle_hlt=1 drop my 10GbE network rx
>  > >  > >  > > performance by a considerable amount (7.5Gbs -> 5.5Gbs)?
>  > >  >
>  > >  > You may be seeing problems because it might simply take a while for
>  > >  > the CPU to wake up from HLT when an interrupt comes in.  The 4BSD
>  > >  > scheduler tries to do IPIs to wakeup any sleeping CPUs when it
>  > >  > schedules a new thread, but that would add higher latency for
>  > >  > ithreads than just preempting directly to the ithread.  Oh, you
>  > >  > have to turn that on, it's off by default
>  > >  > (kern.sched.ipiwakeup.enabled=1).
>  > >
>  > > Hmm..  It seems to be on by default.  Unfortunately, it does not seem
>  > > to help.
>  >
>  > I'm not sure.
>
> One thing which really helps is disabling preemption.  If I do that,
> I get 7.7Gb/sec with machdep.cpu_idle_hlt=1.  This is slightly better
> than machdep.cpu_idle_hlt=0 and no PREEMPTION.
>
> BTW, net.isr.direct=1 in all testing.

Do you have very little userland activity in this test?

-- 
John Baldwin <jhb@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org



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