Re: pgd_free, pmd_free, and pte_free trapping memory.

From: Christoph Hellwig <hch_at_infradead.org>
Date: 2004-03-17 01:48:20
On Tue, Mar 16, 2004 at 05:24:24AM -0600, Robin Holt wrote:
> Looking through the code, we have identified the source of the problem.
> The fork is occuring on one cpu where the pgd, pmd, and pte allocations
> get pages of memory local to that cpu.  The worker thread is then
> migrated to a different cpu where it exits.  The pages are then placed
> on the cpu which is very distant from where the memory is located.
> 
> I looked at the i386 code which appears to have been very similar to the
> ia64 at one point in time, but no longer.  They appear to have completely
> eliminated the quicklists.  Is this the right direction for ia64?
> 
> Since, when the pgd, pmd, and pte are ready to be freed, they are
> zeroed out again, I understand the benefit to keeping the entry around
> to save the time for zeroing out the page again.  Why not have a single
> quicklist where all three are placed.  How would node locality best play
> into placing items on the lists?  Should we have one quicklist on
> each cpu that a cpu returns node local pages and then a node quicklist
> where we place pages that are not node local using cmpxchg?

Tjis quicklist thing is a workaround for not having per-cpu pages in
Linux <= 2.4.  Your patch is a workaround for a workaround and gets a little
ugly.  I'd say just rip the quicklists out like x86 and benchmark it.

That's less code and thus less complexity which is always good.  Now if
the pre-zeroing actually makes a difference we might have to keep small
pre-zeroed list around, but I doubt this is really good idea (or even
nessecary)

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Received on Tue Mar 16 10:30:34 2004

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