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

From: Minchan Kim
Date: Wed Jul 20 2011 - 01:53:25 EST

On Wed, Jul 20, 2011 at 11:53 AM, Shaohua Li <> 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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