linux-kernel - Re: [PATCH] mm: make VM_MAX

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Date:	Mon, 12 Oct 2009 14:23:17 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
Cc:	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: make VM_MAX_READAHEAD configurable

On Mon, Oct 12, 2009 at 01:53:01PM +0800, Christian Ehrhardt wrote:
> Wu Fengguang wrote:
> > Hi Martin,
> >
> > On Fri, Oct 09, 2009 at 09:49:50PM +0800, Martin Schwidefsky wrote:
> >   
> >> On Fri, 9 Oct 2009 14:29:52 +0200
> >> Jens Axboe <jens.axboe@...cle.com> wrote:
> >>
> >>     
> >>> On Fri, Oct 09 2009, Peter Zijlstra wrote:
> >>>       
> >>>> On Fri, 2009-10-09 at 13:19 +0200, Ehrhardt Christian wrote:
> >>>>         
> >>>>> From: Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
> >>>>>
> >>>>> On one hand the define VM_MAX_READAHEAD in include/linux/mm.h is just a default
> >>>>> and can be configured per block device queue.
> >>>>> On the other hand a lot of admins do not use it, therefore it is reasonable to
> >>>>> set a wise default.
> >>>>>
> >>>>> This path allows to configure the value via Kconfig mechanisms and therefore
> >>>>> allow the assignment of different defaults dependent on other Kconfig symbols.
> >>>>>
> >>>>> Using this, the patch increases the default max readahead for s390 improving
> >>>>> sequential throughput in a lot of scenarios with almost no drawbacks (only
> >>>>> theoretical workloads with a lot concurrent sequential read patterns on a very
> >>>>> low memory system suffer due to page cache trashing as expected).
> >>>>>           
> > [snip]
> >   
> >> The patch from Christian fixes a performance regression in the latest
> >> distributions for s390. So we would opt for a larger value, 512KB seems
> >> to be a good one. I have no idea what that will do to the embedded
> >> space which is why Christian choose to make it configurable. Clearly
> >> the better solution would be some sort of system control that can be
> >> modified at runtime. 
> >>     
> >
> > May I ask for more details about your performance regression and why
> > it is related to readahead size? (we didn't change VM_MAX_READAHEAD..)
> >   
> Sure, the performance regression appeared when comparing Novell SLES10 
> vs. SLES11.
> While you are right Wu that the upstream default never changed so far, 
> SLES10 had a
> patch applied that set 512.

I see. I'm curious why SLES11 removed that patch. Did it experienced
some regressions with the larger readahead size?

> As mentioned before I didn't expect to get a generic 128->512 patch 
> accepted,therefore
> the configurable solution. But after Peter and Jens replied so quickly 
> stating that
> changing the default in kernel would be the wrong way to go I already 
> looked out for
> userspace alternatives. At least for my issues I could fix it with 
> device specific udev rules
> too.

OK.

> And as Andrew mentioned the diversity of devices cause any default to be 
> wrong for one
> or another installation. To solve that the udev approach can also differ 
> between different
> device types (might be easier on s390 than on other architectures 
> because I need to take
> care of two disk types atm - and both shold get 512).

I guess it's not a general solution for all. There are so many
devices in the world, and we have not yet considered the
memory/workload combinations.

> The testcase for anyone who wants to experiment with it is almost too 
> easy, the biggest
> impact can be seen with single thread iozone - I get ~40% better 
> throughput when
> increasing the readahead size to 512 (even bigger RA sizes don't help 
> much in my
> environment, probably due to fast devices).

That's impressive number - I guess we need a larger default RA size.
But before that let's learn something from SLES10's experiences :)

Thanks,
Fengguang
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