linux-kernel - Re: [PATCH]vmscan: add block plug for page reclaim

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Date:	Thu, 28 Jul 2011 09:04:20 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jens Axboe <jaxboe@...ionio.com>,
	Minchan Kim <minchan.kim@...il.com>,
	"mgorman@...e.de" <mgorman@...e.de>, linux-mm <linux-mm@...ck.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH]vmscan: add block plug for page reclaim

On Thu, 2011-07-28 at 07:45 +0800, Andrew Morton wrote:
> On Sat, 23 Jul 2011 20:49:10 +0200
> Jens Axboe <jaxboe@...ionio.com> wrote:
> 
> > > I can observe the average request size changes. Before the patch, the
> > > average request size is about 90k from iostat (but the variation is
> > > big). With the patch, the request size is about 100k and variation is
> > > small.
> > 
> > That's a good win right there, imho.
> 
> yup.  Reduced CPU consumption on that path isn't terribly exciting IMO,
> but improved request size is significant.
> 
> Using an additional 44 bytes of stack on that path is also
> significant(ly bad).  But we need to fix that problem anyway.  One way
> we could improve things in mm/vmscan.c is to move the blk_plug into
> scan_control then get the scan_control off the stack in some manner. 
> That's easy for kswapd: allocate one scan_control per kswapd at
> startup.  Doing it for direct-reclaim would be a bit trickier...
unfortunately, the direct-reclaim case is what cares about stack.

BTW, the scan_control can be dieted. may_unmap/may_swap/may_writepage
can be a bit. swappiness < 100, so can be a char. order <= 11, can be a
char. should I do it to cut the size?

> And I have the usual maintainability whine.  If someone comes up to
> vmscan.c and sees it calling blk_start_plug(), how are they supposed to
> work out why that call is there?  They go look at the blk_start_plug()
> definition and it is undocumented.  I think we can do better than this?
the block plug is a little tricky, we definitely should document it.
Jens, if you don't mind, I'll add comments there.

Thanks,
Shaohua

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