linux-kernel - Re: [RFC 0/8] Cpuset aware writeback

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Date:	Tue, 16 Jan 2007 18:34:06 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Christoph Lameter <clameter@....com>
Cc:	menage@...gle.com, linux-kernel@...r.kernel.org,
	nickpiggin@...oo.com.au, linux-mm@...ck.org, ak@...e.de,
	pj@....com, dgc@....com
Subject: Re: [RFC 0/8] Cpuset aware writeback

> On Tue, 16 Jan 2007 17:30:26 -0800 (PST) Christoph Lameter <clameter@....com> wrote:
> > Nope.  You've completely omitted the little fact that we'll do writeback in
> > the offending zone off the LRU.  Slower, maybe.  But it should work and the
> > system should recover.  If it's not doing that (it isn't) then we should
> > fix it rather than avoiding it (by punting writeback over to pdflush).
> 
> pdflush is not running *at* all nor is dirty throttling working. That is 
> correct behavior? We could do background writeback but we choose not to do 
> so? Instead we wait until we hit reclaim and then block (well it seems 
> that we do not block the blocking there also fails since we again check 
> global ratios)?

I agree that it is a worthy objective to be able to constrain a cpuset's
dirty memory levels.  But as a performance optimisation and NOT as a
correctness fix.

Consider: non-exclusive cpuset A consists of mems 0-15, non-exclusive
cpuset B consists of mems 0-3.  A task running in cpuset A can freely dirty
all of cpuset B's memory.  A task running in cpuset B gets oomkilled.

Consider: a 32-node machine has nodes 0-3 full of dirty memory.  I create a
cpuset containing nodes 0-2 and start using it.  I get oomkilled.

There may be other scenarios.

IOW, we have a correctness problem, and we have a probable,
not-yet-demonstrated-and-quantified performance problem.  Fixing the latter
(in the proposed fashion) will *not* fix the former.

So what I suggest we do is to fix the NFS bug, then move on to considering
the performance problems.

On reflection, I agree that your proposed changes are sensible-looking for
addressing the probable, not-yet-demonstrated-and-quantified performance
problem.  The per-inode (should be per-address_space, maybe it is?) node
map is unfortunate.  Need to think about that a bit more.  For a start, it
should be dynamically allocated (from a new, purpose-created slab cache):
most in-core inodes don't have any dirty pages and don't need this
additional storage.

Also, I worry about the worst-case performance of that linear search across
the inodes.

But this is unrelated to the NFS bug ;)

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