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    SubjectRe: [PATCH 0/3] Virtual huge zero page
    On Mon, Oct 01, 2012 at 08:34:28AM -0700, H. Peter Anvin wrote:
    > On 09/29/2012 06:48 AM, Andrea Arcangeli wrote:
    > >
    > > There would be a small cache benefit here... but even then some first
    > > level caches are virtually indexed IIRC (always physically tagged to
    > > avoid the software to notice) and virtually indexed ones won't get any
    > > benefit.
    > >
    >
    > Not quite. The virtual indexing is limited to a few bits (e.g. three
    > bits on K8); the right way to deal with that is to color the zeropage,
    > both the regular one and the virtual one (the virtual one would circle
    > through all the colors repeatedly.)
    >
    > The cache difference, therefore, is *huge*.

    Kirill measured the cache benefit and it provided a 6% gain, not very
    huge but certainly significant.

    > It's a performance tradeoff, and it can, and should, be measured.

    I now measured the other side of the trade, by touching only one
    character every 4k page in the range to simulate a very seeking load,
    and doing so the physical huge zero page wins with a 600% margin, so
    if the cache benefit is huge for the virtual zero page, the TLB
    benefit is massive for the physical zero page.

    Overall I think picking the solution that risks to regress the least
    (also compared to current status of no zero page) is the safest.

    Thanks!
    Andrea


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