lkml.org 
[lkml]   [2007]   [Sep]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 1/6] cpuset write dirty map
On Sat, 15 Sep 2007 05:17:48 +0530
"Satyam Sharma" <satyam.sharma@gmail.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.

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-09-15 02:11    [W:0.103 / U:12.880 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site