Re: Page Faults slower in 2.6.25-rc9 than 2.6.23

[Hugh Dickins <hugh@veritas.com>]

On Wed, 30 Apr 2008, KAMEZAWA Hiroyuki wrote:
> On Tue, 29 Apr 2008 09:10:36 -0400
> "Ross Biro" <rossb@google.com> wrote:
> > I don't know if this has been noticed before.  I was benchmarking my
> > page table relocation code and I noticed that on 2.6.25-rc9 page
> > faults take 10% more time than on 2.6.22.  This is using lmbench
> > running on an intel x86_64 system.  The good news is that the page
> > table relocation code now only adds a 1.6% slow down to page faults.
> 
> It seems lmbench's pagefault program uses 'page fault by READ'.
> Then, this patch affects. (this patch was added at 2.6.24-rc?.)
> ==
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=557ed1fa2620dc119adb86b34c614e152a629a80
> ==
> By it, ZERO_PAGE is not used for page fault in anonymous mapping.

I'd wondered about that one too, but no: lmbench lat_pagefault uses
a shared mmap of an ordinary file (not /dev/zero), so the ZERO_PAGE
changes should have no effect on it whatsoever.

I notice that test is expecting msync(,,MS_INVALIDATE) to do something
it's never done on Linux (a kind of drop caches for the range).  We've
never done anything with MS_INVALIDATE, beyond permitting the flag:
I think you find problems however you try to go about implementing
it (and it might even originate from a UNIX which couldn't do shared
mmap coherently).  So I wonder if that test is erratic because of it.

Hugh

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