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    Subject[PATCH -mm -v3 0/6] mm, swap: VMA based swap readahead
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    The swap readahead is an important mechanism to reduce the swap in
    latency. Although pure sequential memory access pattern isn't very
    popular for anonymous memory, the space locality is still considered
    valid.

    In the original swap readahead implementation, the consecutive blocks
    in swap device are readahead based on the global space locality
    estimation. But the consecutive blocks in swap device just reflect
    the order of page reclaiming, don't necessarily reflect the access
    pattern in virtual memory space. And the different tasks in the
    system may have different access patterns, which makes the global
    space locality estimation incorrect.

    In this patchset, when page fault occurs, the virtual pages near the
    fault address will be readahead instead of the swap slots near the
    fault swap slot in swap device. This avoid to readahead the unrelated
    swap slots. At the same time, the swap readahead is changed to work
    on per-VMA from globally. So that the different access patterns of
    the different VMAs could be distinguished, and the different readahead
    policy could be applied accordingly. The original core readahead
    detection and scaling algorithm is reused, because it is an effect
    algorithm to detect the space locality.

    In addition to the swap readahead changes, some new sysfs interface is
    added to show the efficiency of the readahead algorithm and some other
    swap statistics.

    This new implementation will incur more small random read, on SSD, the
    improved correctness of estimation and readahead target should beat
    the potential increased overhead, this is also illustrated in the test
    results below. But on HDD, the overhead may beat the benefit, so the
    original implementation will be used by default.

    The test and result is as follow,

    Common test condition
    =====================

    Test Machine: Xeon E5 v3 (2 sockets, 72 threads, 32G RAM)
    Swap device: NVMe disk

    Micro-benchmark with combined access pattern
    ============================================

    vm-scalability, sequential swap test case, 4 processes to eat 50G
    virtual memory space, repeat the sequential memory writing until 300
    seconds. The first round writing will trigger swap out, the following
    rounds will trigger sequential swap in and out.

    At the same time, run vm-scalability random swap test case in
    background, 8 processes to eat 30G virtual memory space, repeat the
    random memory write until 300 seconds. This will trigger random
    swap-in in the background.

    This is a combined workload with sequential and random memory
    accessing at the same time. The result (for sequential workload) is
    as follow,

    Base Optimized
    ---- ---------
    throughput 345413 KB/s 414029 KB/s (+19.9%)
    latency.average 97.14 us 61.06 us (-37.1%)
    latency.50th 2 us 1 us
    latency.60th 2 us 1 us
    latency.70th 98 us 2 us
    latency.80th 160 us 2 us
    latency.90th 260 us 217 us
    latency.95th 346 us 369 us
    latency.99th 1.34 ms 1.09 ms
    ra_hit% 52.69% 99.98%

    The original swap readahead algorithm is confused by the background
    random access workload, so readahead hit rate is lower. The VMA-base
    readahead algorithm works much better.

    Linpack
    =======

    The test memory size is bigger than RAM to trigger swapping.

    Base Optimized
    ---- ---------
    elapsed_time 393.49 s 329.88 s (-16.2%)
    ra_hit% 86.21% 98.82%

    The score of base and optimized kernel hasn't visible changes. But
    the elapsed time reduced and readahead hit rate improved, so the
    optimized kernel runs better for startup and tear down stages. And
    the absolute value of readahead hit rate is high, shows that the space
    locality is still valid in some practical workloads.

    Changelogs:

    v3:

    - Rebased on latest -mm tree
    - Use percpu_counter for swap readahead statistics per Dave Hansen's comment.

    Best Regards,
    Huang, Ying

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