• Paolo Valente's avatar
    block, bfq: limit sectors served with interactive weight raising · 8a8747dc
    Paolo Valente authored
    To maximise responsiveness, BFQ raises the weight, and performs device
    idling, for bfq_queues associated with processes deemed as
    interactive. In particular, weight raising has a maximum duration,
    equal to the time needed to start a large application. If a
    weight-raised process goes on doing I/O beyond this maximum duration,
    it loses weight-raising.
    
    This mechanism is evidently vulnerable to the following false
    positives: I/O-bound applications that will go on doing I/O for much
    longer than the duration of weight-raising. These applications have
    basically no benefit from being weight-raised at the beginning of
    their I/O. On the opposite end, while being weight-raised, these
    applications
    a) unjustly steal throughput to applications that may truly need
    low latency;
    b) make BFQ uselessly perform device idling; device idling results
    in loss of device throughput with most flash-based storage, and may
    increase latencies when used purposelessly.
    
    This commit adds a countermeasure to reduce both the above
    problems. To introduce this countermeasure, we provide the following
    extra piece of information (full details in the comments added by this
    commit). During the start-up of the large application used as a
    reference to set the duration of weight-raising, involved processes
    transfer at most ~110K sectors each. Accordingly, a process initially
    deemed as interactive has no right to be weight-raised any longer,
    once transferred 110K sectors or more.
    
    Basing on this consideration, this commit early-ends weight-raising
    for a bfq_queue if the latter happens to have received an amount of
    service at least equal to 110K sectors (actually, a little bit more,
    to keep a safety margin). I/O-bound applications that reach a high
    throughput, such as file copy, get to this threshold much before the
    allowed weight-raising period finishes. Thus this early ending of
    weight-raising reduces the amount of time during which these
    applications cause the problems described above.
    Tested-by: default avatarOleksandr Natalenko <oleksandr@natalenko.name>
    Tested-by: default avatarHolger Hoffstätte <holger@applied-asynchrony.com>
    Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    8a8747dc