Re: [PATCH] x86: cache pollution aware __copy_from_user_ll()

[Dave Jones]

On Sat, Jun 24, 2006 at 04:42:32PM +0200, Arjan van de Ven wrote:
 > On Fri, 2006-06-23 at 15:01 +0000, Linux Kernel Mailing List wrote:
 > > commit c22ce143d15eb288543fe9873e1c5ac1c01b69a1
 > > tree dc7d457b8952fc50dfc90df659b35de4117c61fc
 > > parent 7dbdf43cfa635ddc3701cc8d1eab07597cd731c0
 > > author Hiro Yoshioka <hyoshiok@xxxxxxxxxxxxxxxx> Fri, 23 Jun 2006 16:04:16 -0700
 > > committer Linus Torvalds <torvalds@xxxxxxxxxxx> Fri, 23 Jun 2006 21:42:56 -0700
 > > 
 > > [PATCH] x86: cache pollution aware __copy_from_user_ll()
 > > 
 > > Use the x86 cache-bypassing copy instructions for copy_from_user().
 > > 
 > > Some performance data are
 > > 
 > > Total of GLOBAL_POWER_EVENTS (CPU cycle samples)
 > > 
 > > 2.6.12.4.orig    1921587
 > > 2.6.12.4.nt      1599424
 > > 1599424/1921587=83.23% (16.77% reduction)
 > 
 > Hi,
 > 
 > while this patch will reduce the number of cycles spent in the kernel,
 > it's just pushing the cache miss to userspace (by virtue of doing a
 > cache flush effectively)... is this really the right thing? The total
 > memory bandwidth will actually increase with this patch if you're
 > unlucky (eg if userspace decides to write to this memory eventually)....

But this is a copy _from_ userspace. The userspace app has likely already
brought it into cache before we do the __copy_from_user_ll

		Dave

-- 
http://www.codemonkey.org.uk
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