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

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Date:	Thu, 28 Jul 2011 09:34:52 +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 09:15 +0800, Andrew Morton wrote:
> On Thu, 28 Jul 2011 09:04:20 +0800 Shaohua Li <shaohua.li@...el.com> wrote:
> 
> > > 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?
> 
> All five will fit in a 32-bit word, at some expense in code size.
oh, I missed the code size will increase, so it's not good then.

> But I think first it would be better to work on a way of getting it all
> off the stack, along with the blk_plug.
> 
> Could be done with a per-cpu array and CPU pinning, but CPU pinning is
> a bit expensive nowadays.  Could put a scan_control* into the
> tack_struct, but that's dopey.
looks it should be per task, as reclaim could sleep. either putting it
to task_struct or allocating it, both are not good.

Thanks,
Shaohua

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