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    SubjectRe: [PATCH, RFC 0/6] Avoid cache trashing on clearing huge/gigantic page
    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@linux.intel.com>
    > >
    > > 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@linux.intel.com -- Speaking for myself only


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