linux-kernel - Re: [GIT PULL] SLAB changes for v3.10

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Message-ID: <0000013e85852607-fd84223c-112e-46b9-a60f-73d9597d90ac-000000@email.amazonses.com>
Date:	Wed, 8 May 2013 19:01:35 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Chris Mason <clmason@...ionio.com>
cc:	Pekka Enberg <penberg@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Tony Lindgren <tony@...mide.com>
Subject: Re: [GIT PULL] SLAB changes for v3.10

On Wed, 8 May 2013, Chris Mason wrote:

> > You correctly moved the checks out of the if (!kmalloc_cacheS())
> > condition so that the caches are created properly.
>
> But if the ordering is required at all, why is it ok to create cache 2
> after cache 6 instead of after cache 7?

The power of two caches are 2^x beginning with KMALLOC_MIN_SHIFT. The non
power of two caches were folded into number 1 + 2 since they do not fit
into the scheme and they are special cased throughout. This works since
the minimal slab cache size is 8 bytes.

> IOW if we can safely do cache 2 after cache 6, why can't we just do both
> cache 1 and cache 2 after the loop?

Because the cache creation in SLAB can cause the use of a fractional slab
size if kmem_cache_create() thinks its better to put the metadata on a
different slab cache (OFF_SLAB type) because data will align better that
way. Its weird I know but its due to the way that SLAB aligns data in the
page frame.

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