• Paolo Valente's avatar
    block, bfq: do not merge queues on flash storage with queueing · 8cacc5ab
    Paolo Valente authored
    To boost throughput with a set of processes doing interleaved I/O
    (i.e., a set of processes whose individual I/O is random, but whose
    merged cumulative I/O is sequential), BFQ merges the queues associated
    with these processes, i.e., redirects the I/O of these processes into a
    common, shared queue. In the shared queue, I/O requests are ordered by
    their position on the medium, thus sequential I/O gets dispatched to
    the device when the shared queue is served.
    
    Queue merging costs execution time, because, to detect which queues to
    merge, BFQ must maintain a list of the head I/O requests of active
    queues, ordered by request positions. Measurements showed that this
    costs about 10% of BFQ's total per-request processing time.
    
    Request processing time becomes more and more critical as the speed of
    the underlying storage device grows. Yet, fortunately, queue merging
    is basically useless on the very devices that are so fast to make
    request processing time critical. To reach a high throughput, these
    devices must have many requests queued at the same time. But, in this
    configuration, the internal scheduling algorithms of these devices do
    also the job of queue merging: they reorder requests so as to obtain
    as much as possible a sequential I/O pattern. As a consequence, with
    processes doing interleaved I/O, the throughput reached by one such
    device is likely to be the same, with and without queue merging.
    
    In view of this fact, this commit disables queue merging, and all
    related housekeeping, for non-rotational devices with internal
    queueing. The total, single-lock-protected, per-request processing
    time of BFQ drops to, e.g., 1.9 us on an Intel Core i7-2760QM@2.40GHz
    (time measured with simple code instrumentation, and using the
    throughput-sync.sh script of the S suite [1], in performance-profiling
    mode). To put this result into context, the total,
    single-lock-protected, per-request execution time of the lightest I/O
    scheduler available in blk-mq, mq-deadline, is 0.7 us (mq-deadline is
    ~800 LOC, against ~10500 LOC for BFQ).
    
    Disabling merging provides a further, remarkable benefit in terms of
    throughput. Merging tends to make many workloads artificially more
    uneven, mainly because of shared queues remaining non empty for
    incomparably more time than normal queues. So, if, e.g., one of the
    queues in a set of merged queues has a higher weight than a normal
    queue, then the shared queue may inherit such a high weight and, by
    staying almost always active, may force BFQ to perform I/O plugging
    most of the time. This evidently makes it harder for BFQ to let the
    device reach a high throughput.
    
    As a practical example of this problem, and of the benefits of this
    commit, we measured again the throughput in the nasty scenario
    considered in previous commit messages: dbench test (in the Phoronix
    suite), with 6 clients, on a filesystem with journaling, and with the
    journaling daemon enjoying a higher weight than normal processes. With
    this commit, the throughput grows from ~150 MB/s to ~200 MB/s on a
    PLEXTOR PX-256M5 SSD. This is the same peak throughput reached by any
    of the other I/O schedulers. As such, this is also likely to be the
    maximum possible throughput reachable with this workload on this
    device, because I/O is mostly random, and the other schedulers
    basically just pass I/O requests to the drive as fast as possible.
    
    [1] https://github.com/Algodev-github/STested-by: default avatarHolger Hoffstätte <holger@applied-asynchrony.com>
    Tested-by: default avatarOleksandr Natalenko <oleksandr@natalenko.name>
    Tested-by: default avatarFrancesco Pollicino <fra.fra.800@gmail.com>
    Signed-off-by: default avatarAlessio Masola <alessio.masola@gmail.com>
    Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    8cacc5ab
bfq-iosched.c 214 KB