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    SubjectRe: CFQ idling kills I/O performance on ext4 with blkio cgroup controller
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    On 5/22/19 2:09 AM, Paolo Valente wrote:
    >
    > First, thank you very much for testing my patches, and, above all, for
    > sharing those huge traces!
    >
    > According to the your traces, the residual 20% lower throughput that you
    > record is due to the fact that the BFQ injection mechanism takes a few
    > hundredths of seconds to stabilize, at the beginning of the workload.
    > During that setup time, the throughput is equal to the dreadful ~60-90 KB/s
    > that you see without this new patch. After that time, there
    > seems to be no loss according to the trace.
    >
    > The problem is that a loss lasting only a few hundredths of seconds is
    > however not negligible for a write workload that lasts only 3-4
    > seconds. Could you please try writing a larger file?
    >

    I tried running dd for longer (about 100 seconds), but still saw around
    1.4 MB/s throughput with BFQ, and between 1.5 MB/s - 1.6 MB/s with
    mq-deadline and noop. But I'm not too worried about that difference.

    > In addition, I wanted to ask you whether you measured BFQ throughput
    > with traces disabled. This may make a difference.
    >

    The above result (1.4 MB/s) was obtained with traces disabled.

    > After trying writing a larger file, you can try with low_latency on.
    > On my side, it causes results to become a little unstable across
    > repetitions (which is expected).
    >
    With low_latency on, I get between 60 KB/s - 100 KB/s.

    Regards,
    Srivatsa
    VMware Photon OS

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