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    SubjectRe: [V4][PATCH 0/4]page fault retry with NOPAGE_RETRY

    On Mon, 13 Apr 2009, Ying Han wrote:
    > Benchmarks:
    > case 1. one application has a high count of threads each faulting in
    > different pages of a hugefile. Benchmark indicate that this double data
    > structure walking in case of major fault results in << 1% performance hit.
    > case 2. add another thread in the above application which in a tight loop
    > of mmap()/munmap(). Here we measure loop count in the new thread while other
    > threads doing the same amount of work as case one. we got << 3% performance
    > hit on the Complete Time(benchmark value for case one) and 10% performance
    > improvement on the mmap()/munmap() counter.
    > This patch helps a lot in cases we have writer which is waitting behind all
    > readers, so it could execute much faster.

    Hmm. I normally think of "<<" as "much smaller than", but the way you use
    it makes me wonder. In particular, "<< 3%" sounds very odd. If it's much
    smaller than 3%, I'd have expected "<< 1%" again. So it probably isn't.

    > benchmarks from Wufengguang:
    > Just tested the sparse-random-read-on-sparse-file case, and found the
    > performance impact to be 0.4% (8.706s vs 8.744s) in the worst case.
    > Kind of acceptable.

    Well, have you tried the obvious optimization of _not_ doing the RETRY
    path when atomic_read(&mm->counter) == 1?

    After all, if it's not a threaded app, and it doesn't have a possibility
    of concurrent mmap/fault, then why release the lock?


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