Re: [PATCH] ext2/ext3: allocate ->s_blockgroup_lock separately toavoid wasting space

[Pekka Enberg]

Peter Zijlstra wrote:
> On Fri, 2008-11-14 at 11:17 +0200, Pekka J Enberg wrote:
> > From: Pekka Enberg <penberg@xxxxxxxxxxxxxx>
> >
> > As spotted by kmemtrace, struct ext2_sb_info is 17024 bytes and ext3_sb_info is
> > 17152 bytes on 64-bit which makes them a very bad fit for SLAB allocators. In
> > fact, both allocations are round up to the next available page size of 
> > order 3 which is 32 KB.
> >
> > The culprit if the wasted memory is the ->s_blockgroup_lock which can be as big
> > as 16 KB when CONFIG_NR_CPUS is set to 32. As struct blockgroup_lock is a
> > perfect fit for order 2 page in the worst case, allocate ->s_blockgroup_lock
> > separately to avoid wasting space.
>
> And here I was thinking that NR_CPUS=4096 is currently our worst
> case ;-)

Sure but look at <linux/blockgroup_lock.h>. NR_BG_LOCKS is capped to 128 
for >= 32 CPUs.
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