Messages in this thread | | | Date | Wed, 30 Apr 2008 12:33:53 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: Page Faults slower in 2.6.25-rc9 than 2.6.23 |
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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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