linux-kernel - Re: [PATCH 02/10] mm: system wide ALLOC_NO

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Message-ID: <Pine.LNX.4.64.0708071513290.3683@schroedinger.engr.sgi.com>
Date:	Tue, 7 Aug 2007 15:18:36 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Daniel Phillips <phillips@...nq.net>
cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Matt Mackall <mpm@...enic.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Phillips <phillips@...gle.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Steve Dickson <SteveD@...hat.com>
Subject: Re: [PATCH 02/10] mm: system wide ALLOC_NO_WATERMARK

On Mon, 6 Aug 2007, Daniel Phillips wrote:

> > AFAICT: This patchset is not throttling processes but failing
> > allocations.
> 
> Failing allocations?  Where do you see that?  As far as I can see, 
> Peter's patch set allows allocations to fail exactly where the user has 
> always specified they may fail, and in no new places.  If there is a 
> flaw in that logic, please let us know.

See the code added to slub: Allocations are satisfied from the reserve 
patch or they are failing.

> > The patchset does not reconfigure the memory reserves as 
> > expected.
> 
> What do you mean by that?  Expected by who?

What would be expected it some recalculation of min_freekbytes?

> > And I suspect that we  
> > have the same issues as in earlier releases with various corner cases
> > not being covered.
> 
> Do you have an example?

Try NUMA constraints and zone limitations.
 
> > Code is added that is supposedly not used.
> 
> What makes you think that?

Because the argument is that performance does not matter since the code 
patchs are not used.

> > If it  ever is on a large config then we are in very deep trouble by
> > the new code paths themselves that serialize things in order to give
> > some allocations precendence over the other allocations that are made
> > to fail ....
> 
> You mean by allocating the reserve memory on the wrong node in NUMA?  

No I mean all 1024 processors of our system running into this fail/succeed 
thingy that was added.

> That is on a code path that avoids destroying your machine performance 
> or killing the machine entirely as with current kernels, for which a 

As far as I know from our systems: The current kernels do not kill the 
machine if the reserves are configured the right way.

> few cachelines pulled to another node is a small price to pay.  And you 
> are free to use your special expertise in NUMA to make those fallback 
> paths even more efficient, but first you need to understand what they 
> are doing and why.

There is your problem. The justification is not clear at all and the 
solution likely causes unrelated problems.


 
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