linux-kernel - Re: [PATCH 1/6] cpuset write dirty map

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Message-Id: <20070914170733.dbe89493.akpm@linux-foundation.org>
Date:	Fri, 14 Sep 2007 17:07:33 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Satyam Sharma" <satyam.sharma@...il.com>
Cc:	"Ethan Solomita" <solo@...gle.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	"Christoph Lameter" <clameter@....com>
Subject: Re: [PATCH 1/6] cpuset write dirty map

On Sat, 15 Sep 2007 05:17:48 +0530
"Satyam Sharma" <satyam.sharma@...il.com> wrote:

> > It's unobvious why the break point is at MAX_NUMNODES = BITS_PER_LONG and
> > we might want to tweak that in the future.  Yet another argument for
> > centralising this comparison.
> 
> Looks like just an optimization to me ... Ethan wants to economize and not bloat
> struct address_space too much.
> 
> So, if sizeof(nodemask_t) == sizeof(long), i.e. when:
> MAX_NUMNODES <= BITS_PER_LONG, then we'll be adding only sizeof(long)
> extra bytes to the struct (by plonking the object itself into it).
> 
> But even when MAX_NUMNODES > BITS_PER_LONG, because we're storing
> a pointer, and because sizeof(void *) == sizeof(long), so again the maximum
> bloat addition to struct address_space would only be sizeof(long) bytes.

yup.

Note that "It's unobvious" != "It's unobvious to me".  I review code for
understandability-by-others, not for understandability-by-me.

> I didn't see the original mail, but if the #ifdeffery for this
> conditional is too much
> as a result of this optimization, Ethan should probably just do away
> with all of it
> entirely, and simply put a full nodemask_t object (irrespective of MAX_NUMNODES)
> into the struct. After all, struct task_struct does the same unconditionally ...
> but admittedly, there are several times more address_space struct's resident in
> memory at any given time than there are task_struct's, so this optimization does
> make sense too ...

I think the optimisation is (probably) desirable, but it would be best to
describe the tradeoff in the changelog and to add some suitable
code-commentary for those who read the code in a year's time and to avoid
sprinkling the logic all over the tree.

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