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    SubjectRe: [PATCH 0/2][RFC] New version of shared page tables
    Brian Twichell wrote:

    > Hugh Dickins wrote:
    >> Let me say (while perhaps others are still reading) that I'm seriously
    >> wondering whether you should actually restrict your shared pagetable
    >> work
    >> to the hugetlb case. I realize that would be a disappointing limitation
    >> to you, and would remove the 25%/50% improvement cases, leaving only the
    >> 3%/4% last-ounce-of-performance cases.
    >> But it's worrying me a lot that these complications to core mm code will
    >> _almost_ never apply to the majority of users, will get little testing
    >> outside of specialist setups. I'd feel safer to remove that "almost",
    >> and consign shared pagetables to the hugetlb ghetto, if that would
    >> indeed remove their handling from the common code paths. (Whereas,
    >> if we didn't have hugetlb, I would be arguing strongly for shared pts.)
    > Hi,
    > In the case of x86-64, if pagetable sharing for small pages was
    > eliminated, we'd lose more than the 27-33% throughput improvement
    > observed when the bufferpools are in small pages. We'd also lose a
    > significant chunk of the 3% improvement observed when the bufferpools
    > are in hugepages. This occurs because there is still small page
    > pagetable sharing being achieved, minimally for database text, when
    > the bufferpools are in hugepages. The performance counters indicated
    > that ITLB and DTLB page walks were reduced by 28% and 10%,
    > respectively, in the x86-64/hugepage case.

    Aside, can you just enlighten me as to how TLB misses are improved on
    x86-64? As far as
    I knew, it doesn't have ASIDs so I wouldn't have thought it could share
    TLBs anyway...
    But I'm not up to scratch with modern implementations.

    > To be clear, all measurements discussed in my post were performed with
    > kernels config'ed to share pagetables for both small pages and hugepages.
    > If we had to choose between pagetable sharing for small pages and
    > hugepages, we would be in favor of retaining pagetable sharing for
    > small pages. That is where the discernable benefit is for customers
    > that run with "out-of-the-box" settings. Also, there is still some
    > benefit there on x86-64 for customers that use hugepages for the
    > bufferpools.

    Of course if it was free performance then we'd want it. The downsides
    are that it
    is a significant complexity for a pretty small (3%) performance gain for
    your apparent
    target workload, which is pretty uncommon among all Linux users.

    Ignoring the complexity, it is still not free. Sharing data across
    processes adds to
    synchronisation overhead and hurts scalability. Some of these page fault
    scenarios have shown to be important enough that we have introduced
    complexity _there_.

    And it seems customers running "out-of-the-box" settings really want to
    start using
    hugepages if they're interested in getting the most performance
    possible, no?


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