linux-kernel - Re: [rfc] lru_add_drain

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Date:	Wed,  9 Sep 2009 13:27:17 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	kosaki.motohiro@...fujitsu.com,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...e.hu>,
	linux-mm <linux-mm@...ck.org>,
	Oleg Nesterov <onestero@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] lru_add_drain_all() vs isolation

> The usefulness of a scheme like this requires:
> 
> 1. There are cpus that continually execute user space code
>    without system interaction.
> 
> 2. There are repeated VM activities that require page isolation /
>    migration.
> 
> The first page isolation activity will then clear the lru caches of the
> processes doing number crunching in user space (and therefore the first
> isolation will still interrupt). The second and following isolation will
> then no longer interrupt the processes.
> 
> 2. is rare. So the question is if the additional code in the LRU handling
> can be justified. If lru handling is not time sensitive then yes.

Christoph, I'd like to discuss a bit related (and almost unrelated) thing.
I think page migration don't need lru_add_drain_all() as synchronous, because
page migration have 10 times retry.

Then asynchronous lru_add_drain_all() cause

  - if system isn't under heavy pressure, retry succussfull.
  - if system is under heavy pressure or RT-thread work busy busy loop, retry failure.

I don't think this is problematic bahavior. Also, mlock can use asynchrounous lru drain.

What do you think?


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