linux-kernel - Re: The performance and behaviour of the anti-fragmentation related patches

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Message-Id: <20070302145906.653d3b82.akpm@linux-foundation.org>
Date:	Fri, 2 Mar 2007 14:59:06 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Bill Irwin <bill.irwin@...cle.com>,
	Christoph Lameter <clameter@...r.sgi.com>,
	Mel Gorman <mel@...net.ie>, npiggin@...e.de, mingo@...e.hu,
	jschopp@...tin.ibm.com, arjan@...radead.org,
	torvalds@...ux-foundation.org, mbligh@...igh.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: The performance and behaviour of the anti-fragmentation related
 patches

On Fri, 02 Mar 2007 17:34:31 -0500
Rik van Riel <riel@...hat.com> wrote:

> >>>> The main reason they end up pounding the LRU locks is the
> >>>> swappiness heuristic.  They scan too much before deciding
> >>>> that it would be a good idea to actually swap something
> >>>> out, and with 32 CPUs doing such scanning simultaneously...
> >>> What kernel version?
> >> Customers are on the 2.6.9 based RHEL4 kernel, but I believe
> >> we have reproduced the problem on 2.6.18 too during stress
> >> tests.
> > 
> > The prev_priority fixes were post-2.6.18
> 
> We tested them.  They only alleviate the problem slightly in
> good situations, but things still fall apart badly with less
> friendly workloads.

What is it with vendors finding MM problems and either not fixing them or
kludging around them and not telling the upstream maintainers about *any*
of it?

> >> I have no reason to believe we should stick our heads in the
> >> sand and pretend it no longer exists on 2.6.21.
> > 
> > I have no reason to believe anything.  All I see is handwaviness,
> > speculation and grand plans to rewrite vast amounts of stuff without even a
> > testcase to demonstrate that said rewrite improved anything.
> 
> Your attitude is exactly why the VM keeps falling apart over
> and over again.
> 
> Fixing "a testcase" in the VM tends to introduce problems for
> other test cases, ad infinitum.

In that case it was a bad fix.  The aim is to fix known problems without
introducing regressions in other areas.  A perfectly legitimate approach.

You seem to be saying that we'd be worse off if we actually had a testcase.

> There's a reason we end up
> fixing the same bugs over and over again.

No we don't.

> I have been looking through a few hundred VM related bugzillas
> and have found the same bugs persist over many different
> versions of Linux, sometimes temporarily fixed, but they seem
> to always come back eventually...
> 
> > None of this is going anywhere, is is it?
> 
> I will test my changes before I send them to you, but I cannot
> promise you that you'll have the computers or software needed
> to reproduce the problems.  I doubt I'll have full time access
> to such systems myself, either.
> 
> 32GB is pretty much the minimum size to reproduce some of these
> problems. Some workloads may need larger systems to easily trigger

32GB isn't particularly large.

Somehow I don't believe that a person or organisation which is incapable of
preparing even a simple testcase will be capable of fixing problems such as
this without breaking things.
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