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  • linux-rt
  • block
  • mq-deadline.c
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  • Jens Axboe's avatar
    Revert "block/mq-deadline: Prioritize high-priority requests" · 7b05bf77
    Jens Axboe authored Aug 26, 2021
    This reverts commit fb926032.
    
    Zhen reports that this commit slows down mq-deadline on a 128 thread
    box, going from 258K IOPS to 170-180K. My testing shows that Optane
    gen2 IOPS goes from 2.3M IOPS to 1.2M IOPS on a 64 thread box.
    
    Looking in detail at the code, the main culprit here is needing to sum
    percpu counters in the dispatch hot path, leading to very high CPU
    utilization there. To make matters worse, the code currently needs to
    sum 2 percpu counters, and it does so in the most naive way of iterating
    possible CPUs _twice_.
    
    Since we're close to release, revert this commit and we can re-do it
    with regular per-priority counters instead for the 5.15 kernel.
    
    Link: https://lore.kernel.org/linux-block/20210826144039.2143-1-thunder.leizhen@huawei.com/
    
    
    Reported-by: default avatarZhen Lei <thunder.leizhen@huawei.com>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    7b05bf77