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

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Date:	Wed, 20 Jul 2011 14:53:16 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	Jens Axboe <jaxboe@...ionio.com>,
	Andrew Morton <akpm@...ux-foundation.org>, 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 Wed, Jul 20, 2011 at 11:53 AM, Shaohua Li <shaohua.li@...el.com> wrote:
> per-task block plug can reduce block queue lock contention and increase request
> merge. Currently page reclaim doesn't support it. I originally thought page
> reclaim doesn't need it, because kswapd thread count is limited and file cache
> write is done at flusher mostly.
> When I test a workload with heavy swap in a 4-node machine, each CPU is doing
> direct page reclaim and swap. This causes block queue lock contention. In my
> test, without below patch, the CPU utilization is about 2% ~ 7%. With the
> patch, the CPU utilization is about 1% ~ 3%. Disk throughput isn't changed.

Why doesn't it enhance through?
It means merge is rare?

> This should improve normal kswapd write and file cache write too (increase
> request merge for example), but might not be so obvious as I explain above.

CPU utilization enhance on  4-node machine with heavy swap?
I think it isn't common situation.

And I don't want to add new stack usage if it doesn't have a benefit.
As you know, direct reclaim path has a stack overflow.
These days, Mel, Dave and Christoph try to remove write path in
reclaim for solving stack usage and enhance write performance.

-- 
Kind regards,
Minchan Kim
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