Re: [PATCH, RFC 0/6] Avoid cache trashing on clearing huge/giganticpage

[Andi Kleen]

On Wed, Jul 25, 2012 at 01:51:01PM -0500, Christoph Lameter wrote:
> On Fri, 20 Jul 2012, Kirill A. Shutemov wrote:
> 
> > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> >
> > Clearing a 2MB huge page will typically blow away several levels of CPU
> > caches.  To avoid this only cache clear the 4K area around the fault
> > address and use a cache avoiding clears for the rest of the 2MB area.
> 
> why exempt the 4K around the fault address? Is there a regression if that
> is not exempted?

You would get an immediate cache miss when the faulting instruction
is reexecuted.

> 
> I guess for anonymous huge pages one may assume that there will be at
> least one write to one cache line in the 4k page. Is it useful to get all
> the cachelines in the page in the cache.

We did some measurements -- comparing 4K and 2MB with some tracing 
of fault patterns -- and a lot of apps don't use the full 2MB area. 
The apps with THP regressions usually used less than others.
The patchkit significantly reduced some of the regressions.

> 
> Also note that if we get later into hugepage use for the page cache we
> would want the cache to be cold because the contents have to come in from
> a storage medium.

Page cache is not cleared, so never runs this code.


-Andi

-- 
ak@xxxxxxxxxxxxxxx -- Speaking for myself only
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