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    SubjectRe: madvise failure
    On Thu, 7 May 2009 14:10:17 +0200
    "Andries E. Brouwer" <> wrote:

    > In an application something like
    > p = mmap(0, sz, PROT_READ, MAP_PRIVATE, fd, 0);
    > madvise(p, sz, MADV_SEQUENTIAL);
    > is done for a small number of files, each with a size of a few GB.
    > A single sequential pass is done over these files - essentially a merge.
    > On an old machine the madvise is useful, and decreases total time.
    With the same kernel (Ubuntu/2.6.27) ? old one ?

    > But on a more recent machine, with more memory, the madvise makes
    > things worse. There, it seems better not to reveal to the kernel
    > that the data will be read sequentially.
    > Timing (six files of 4GB each, quadcore Intel Q9550, 16GB memory,
    > kernel 2.6.27 [Ubuntu], two other processes active):
    > with madvise, 7 runs: real time varying 9m10s - 37m29s,
    > without madvise, 6 runs: real time fairly constant 5m45s - 5m54s.
    Accessing each page one by one sequentially ? or
    Sequential but (may) skip some pages in each access ?


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