linux-kernel - Re: [PATCH] SLUB: revert direct page allocator pass through

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Message-Id: <200902031233.40987.nickpiggin@yahoo.com.au>
Date:	Tue, 3 Feb 2009 12:33:39 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	yanmin_zhang@...ux.intel.com, Andi Kleen <andi@...stfloor.org>,
	Matthew Wilcox <matthew@....cx>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] SLUB: revert direct page allocator pass through

On Tuesday 27 January 2009 04:17:28 Christoph Lameter wrote:
> On Sat, 24 Jan 2009, Nick Piggin wrote:

> > Other ones which could be changed but could introduce regressions are
> > watermarks, buddy merging, and struct page error checking and setup.
>
> Isnt it possible to defer that (queuing them (sigh)).

If you defer it then you lose merge opportunities and effectively increase
the rate of fragmentation.

> A bitmap could be
> used to avoid queuing and may even allow fully concurrent allocations
> without locks. Use a counter to check watermarks once in a while.

Bitmap? Involving locks or atomic operations, right?

> > (I did make that patch to enable refcounting to be avoided FWIW, which
> > avoids a couple of atomic operations, but I don't think it brought
> > performance up too much, but I still intend to dust it off at some
> > point).
>
> Well if the page stays with a refcount of one then we do not need to check
> the refcount at all but just push it in an out of some queue / bitmap or
> something.

I sent you the patch but I don't think you ever came back with numbers.
It's trivial, you just can avoid the put_page_testzero if the caller does
not need refcounting and the refcount remains at 0 (has to remain 0, not
1, because of speculative page references).

The page refcounting layer lives on top of the buddy/queueing/etc layers,
so the same technique works no matter what the underlying allocator looks
like.

The only reason I didn't merge it is that it added another branch. I wanted
to rework the APIs a little bit to avoid it.

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