Re: [PATCH] vmscan: synchronous lumpy reclaim don't call congestion_wait()

[Minchan Kim <minchan.kim@gmail.com>]

Hi KOSAKI, 

On Sun, Aug 01, 2010 at 06:12:47PM +0900, KOSAKI Motohiro wrote:
> rebased onto Wu's patch
> 
> ----------------------------------------------
> From 35772ad03e202c1c9a2252de3a9d3715e30d180f Mon Sep 17 00:00:00 2001
> From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Date: Sun, 1 Aug 2010 17:23:41 +0900
> Subject: [PATCH] vmscan: synchronous lumpy reclaim don't call congestion_wait()
> 
> congestion_wait() mean "waiting for number of requests in IO queue is
> under congestion threshold".
> That said, if the system have plenty dirty pages, flusher thread push
> new request to IO queue conteniously. So, IO queue are not cleared
> congestion status for a long time. thus, congestion_wait(HZ/10) is
> almostly equivalent schedule_timeout(HZ/10).
Just a nitpick. 
Why is it a problem?
HZ/10 is upper bound we intended.  If is is rahter high, we can low it. 
But totally I agree on this patch. It would be better to remove it 
than lowing. 

> 
> If the system 512MB memory, DEF_PRIORITY mean 128kB scan and It takes 4096
> shrink_page_list() calls to scan 128kB (i.e. 128kB/32=4096) memory.
> 4096 times 0.1sec stall makes crazy insane long stall. That shouldn't.

128K / (4K * SWAP_CLUSTER_MAX) = 1

> 
> In the other hand, this synchronous lumpy reclaim donesn't need this
> congestion_wait() at all. shrink_page_list(PAGEOUT_IO_SYNC) cause to
> call wait_on_page_writeback() and it provide sufficient waiting.

Absolutely I agree on you. 

> 
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

-- 
Kind regards,
Minchan Kim