linux-kernel - Re: [patch 07/21] Xen-paravirt: remove ctor for pgd cache

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Date:	Fri, 16 Feb 2007 09:19:35 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Pekka Enberg <penberg@...helsinki.fi>, Andi Kleen <ak@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, virtualization@...ts.osdl.org,
	xen-devel@...ts.xensource.com, Chris Wright <chrisw@...s-sol.org>,
	Zachary Amsden <zach@...are.com>
Subject: Re: [patch 07/21] Xen-paravirt: remove ctor for pgd cache

Nick Piggin wrote:
> Pekka Enberg wrote:
>> On 2/16/07, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>>
>>> Remove the ctor for the pgd cache.  There's no point in having the
>>> cache machinery do this via an indirect call when all pgd are freed in
>>> the one place anyway.
>>
>>
>> The reason we have slab constructors and destructors is to _avoid_
>> reinitializing every time we allocate an object. AFAICT your changing
>> the code now to do _more_ work than before, so is there some other
>> reason why you want to do this than avoiding an indirect call?
>
> Sometimes  it is better for the caches to initialise an object
> immediately, but in this case I think it is better to use the
> slab ctor because it is very unlikely to use many cachelines
> immediately anyway.
>
> It would be nice to put the "why" in the changelogs, rather than
> "what". Not everyone wants to go through the whole patchset to
> decipher why Xen possibly needs something.

Hm, I think I was mislead by looking at kmem_cache_alloc in slob.c
rather than the one that's actually used in slab.c.  There's no
particular Xen reason for this patch, so I can drop it.

    J

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