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    SubjectRe: Minutes from 10/1 LSE Call
    Hanna Linder <hannal@us.ibm.com> wrote:
    >
    > In mainline, once block size is over 32k our throuput actually drops off.
    > It levels off around 128k but at a greater cpu utilization.
    > Dont really understand why that is.

    Probably thrashing the CPU's L1 cache.

    > In mm tree, maintains throughput for all block size but the cpu utilzation
    > keeps going up to do the same throughput. Readprofile shows the extra time
    > is being spent in copy_to_user (in mm tree). Backing out readahead patch
    > reduces cpu by 10% for all block sizes but still shows the spike. So that
    > isnt the main problem.

    If you have a loop like:

    char *buf;

    for (lots) {
    read(fd, buf, size);
    }


    the optimum value of `size' is small: as little as 8k. Once `size' gets
    close to half the size of the L1 cache you end up pushing the memory at
    `buf' out of CPU cache all the time.

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