linux-kernel - Re: Mainline kernel OLTP performance update

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Date:	Thu, 15 Jan 2009 11:00:53 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	"Wilcox, Matthew R" <matthew.r.wilcox@...el.com>,
	chinang.ma@...el.com, linux-kernel@...r.kernel.org,
	sharad.c.tripathi@...el.com, arjan@...ux.intel.com,
	andi.kleen@...el.com, suresh.b.siddha@...el.com,
	harita.chilukuri@...el.com, douglas.w.styner@...el.com,
	peter.xihong.wang@...el.com, hubert.nueckel@...el.com,
	chris.mason@...cle.com, srostedt@...hat.com,
	linux-scsi@...r.kernel.org,
	Andrew Vasquez <andrew.vasquez@...gic.com>,
	Anirban Chakraborty <anirban.chakraborty@...gic.com>
Subject: Re: Mainline kernel OLTP performance update

On Thu, Jan 15, 2009 at 09:44:42AM -0800, Andrew Morton wrote:
> > Me too.  Anecdotally, I haven't noticed this in my lab machines, but
> > what I have noticed is on someone else's laptop (a hyperthreaded atom)
> > that I was trying to demo powertop on was that IPI reschedule interrupts
> > seem to be out of control ... they were ticking over at a really high
> > rate and preventing the CPU from spending much time in the low C and P
> > states.  To me this implicates some scheduler problem since that's the
> > primary producer of IPI reschedules ... I think it wouldn't be a
> > significant extrapolation to predict that the scheduler might be the
> > cause of the above problem as well.
> > 
> 
> Good point.
> 
> The context switch rate actually went down a bit.
> 
> I wonder if the Intel test people have records of /proc/interrupts for
> the various kernel versions.

I think Chinang does, but he's out of office today.  He did say in an
earlier reply:

> I took a quick look at the interrupts figure between 2.6.24 and 2.6.27.
> i/o interuputs is slightly down in 2.6.27 (due to reduce throughput).
> But both NMI and reschedule interrupt increased.  Reschedule interrupts
> is 2x of 2.6.24.

So if the reschedule interrupt is happening twice as often, and the
context switch rate is basically unchanged, I guess that means the
scheduler is doing a lot more work to get approximately the same
results.  And that seems like a bad thing.

Again, it's worth bearing in mind that these are all RT tasks, so the
underlying problem may be very different from the one that both James and
I have observed with an Atom laptop running predominantly non-RT tasks.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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