linux-kernel - Re: [PATCH/RFC] MM slub: add a sysfs entry to show the calculated number of fallback slabs

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Message-ID: <1289578964.1972.43.camel@castor.rsk>
Date:	Fri, 12 Nov 2010 16:22:44 +0000
From:	Richard Kennedy <richard@....demon.co.uk>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Pekka Enberg <penberg@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH/RFC] MM slub: add a sysfs entry to show the calculated
 number of fallback slabs

On Fri, 2010-11-12 at 09:13 -0600, Christoph Lameter wrote:
> On Fri, 12 Nov 2010, Richard Kennedy wrote:
> 
> > On my desktop workloads (kernel compile etc) I'm seeing surprisingly
> > little slab fragmentation. Do you have any suggestions for test cases
> > that will fragment the memory?
> 
> Do a massive scan through huge amounts of files that triggers inode and
> dentry reclaim?

thanks, I'll give it a try.

> > + * Note that this can give the wrong answer if the user has changed the
> > + * order of this slab via sysfs.
> 
> Not good. Maybe have an additional counter in kmem_cache_node instead?

I know it's not ideal. Of course there already is a counter in
CONFIG_SLUB_STATS but it only counts the total number of fallback slabs
issued since boot time.
I'm not sure if I can reliably decrement a fallback counter when a slab
get freed. If the size was changed then we could have slabs with several
different sizes, and off the top of my head I'm not sure if I can
identify which ones were created as fallback slabs. I don't suppose
there's a spare flag anywhere. 

I'll give this some more thought.

regards
Richard   

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