linux-kernel - Re: [patch 2/3] slub: scan partial list for free slabs when thrashing

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Date:	Tue, 31 Mar 2009 11:49:11 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Christoph Lameter <cl@...ux.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Martin Bligh <mbligh@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/3] slub: scan partial list for free slabs when
	thrashing

Hi David,

On Mon, 2009-03-30 at 10:37 -0400, Christoph Lameter wrote:
> > > That adds fastpath overhead and it shows for small objects in your tests.

On Tue, 31 Mar 2009, Pekka Enberg wrote:
> > Yup, and looking at this:
> > 
> > +       u16 fastpath_allocs;    /* Consecutive fast allocs before slowpath */
> > +       u16 slowpath_allocs;    /* Consecutive slow allocs before watermark */
> > 
> > How much do operations on u16 hurt on, say, x86-64?

On Tue, 2009-03-31 at 01:23 -0700, David Rientjes wrote:
> As opposed to unsigned int?  These simply use the word variations of the 
> mov, test, cmp, and inc instructions instead of long.  It's the same 
> tradeoff when using the u16 slub fields within struct page except it's not 
> strictly required in this instance because of size limitations, but rather 
> for cacheline optimization.

I was thinking of partial register stalls. But looking at it on x86-64,
the generated asm seems sane. I see tons of branch instructions, though,
so simplifying this somehow:

+       if (is_empty) {
+               if (c->fastpath_allocs < s->min_free_watermark)
+                       c->slowpath_allocs++;
+               else if (c->slowpath_allocs)
+                       c->slowpath_allocs--;
+       } else
+               c->slowpath_allocs = 0;
+       c->fastpath_allocs = 0;

would be most welcome.

			Pekka

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