linux-kernel - Re: Page Faults slower in 2.6.25-rc9 than 2.6.23

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Pine.LNX.4.64.0804301215080.4651@blonde.site>
Date:	Wed, 30 Apr 2008 12:33:53 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	Ross Biro <rossb@...gle.com>, linux-mm@...ck.org,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: Page Faults slower in 2.6.25-rc9 than 2.6.23

On Wed, 30 Apr 2008, KAMEZAWA Hiroyuki wrote:
> On Tue, 29 Apr 2008 09:10:36 -0400
> "Ross Biro" <rossb@...gle.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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/