linux-kernel - Re: quicklists confuse meminfo

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Message-ID: <8763vv1wdp.fsf@basil.nowhere.org>
Date:	09 Mar 2008 21:21:38 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Lameter <clameter@....com>,
	Bart Van Assche <bart.vanassche@...il.com>
Subject: Re: quicklists confuse meminfo

Andrew Morton <akpm@...ux-foundation.org> writes:

> On Sun, 9 Mar 2008 13:34:32 +0100 Ingo Molnar <mingo@...e.hu> wrote:
> 
> > the right solution is to get rid of quicklists altogether
> 
> Yes, I think so.
> 
> - They are pretty marginal from a performance POV (iirc)

One general issue -- as noted again by Christoph Lameter recently --
is that the order 0 fast path in page_alloc.c isn't actually very 
fast. That is why people keep inventing their own...

 > - As I said when we merged them (under protest): Private object caches
>   like this are just a bad idea - caches should be *shared*, because some
>   other code path which wants a zeroed page wants a cache-warm one, not a
>   cache-cold one from the allocator (iirc there was doubt over how
>   cache-warm these pages are, however).
> 
>   Making __GFP_ZERO smarter/more efficient would be a preferable way of
>   addressing any performance problems we have in there.

To do the same as quicklists you would need a __free_pages_zeroed()
and separate buddy lists I think.  Later is probably somewhat ugly.
Or perhaps do it only for order 0?

Or perhaps idle time zeroing should be reinvestigated on modern CPUs,
but I'm always a little sceptical of that.

-Andi
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