linux-kernel - Re: [RFC 0/7] Postphone reclaim laundry to write at high water marks

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Message-ID: <Pine.LNX.4.64.0708211400341.3082@schroedinger.engr.sgi.com>
Date:	Tue, 21 Aug 2007 14:03:28 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Dave McCracken <dave.mccracken@...cle.com>
cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 0/7] Postphone reclaim laundry to write at high water marks

On Tue, 21 Aug 2007, Dave McCracken wrote:

> On Monday 20 August 2007, Christoph Lameter wrote:
> > 1. First reclaiming non dirty pages. Dirty pages are deferred until reclaim
> >    has reestablished the high marks. Then all the dirty pages (the laundry)
> >    is written out.
> 
> I don't buy it.  What happens when there aren't enough clean pages in the 
> system to achieve the high water mark?  I'm guessing we'd get a quick OOM (as 
> observed by Peter).

We reclaim the clean pages that there are (removing the executable 
pages from memory) and then we do writeback.

The quick OOM is due to throttling not working right AFAIK.

> > 2. Reclaim is essentially complete during the writeout phase. So we remove
> >    PF_MEMALLOC and allow recursive reclaim if we still run into trouble
> >    during writeout.
> 
> You're assuming the system is static and won't allocate new pages behind your 
> back.  We could be back to critically low memory before the write happens.

Yes and that occurs now too.

> More broadly, we need to be proactive about getting dirty pages cleaned before 
> they consume the system.  Deferring the write just makes it harder to keep 
> up.

Cleaning dirty pages through writeout consumes memory. Writing dirty pages 
out early makes the memory situation even worse.