• Jan Kara's avatar
    blk-flush: Queue through IO scheduler when flush not required · 1b2ff19e
    Jan Kara authored
    Currently blk_insert_flush() just adds flush request to q->queue_head
    when flush is not required. That completely bypasses IO scheduler so
    e.g. CFQ can be idling waiting for new request to arrive and will idle
    through the whole window unnecessarily. Luckily this only happens in
    rare cases as usually checks in generic_make_request_checks() clear
    FLUSH and FUA flags early if they are not needed.
    When no flushing is actually required, we can easily fix the problem by
    properly queueing the request through the IO scheduler. Ideally IO
    scheduler should be also made aware of requests queued via
    blk_flush_queue_rq(). However inserting flush request through IO
    scheduler can have unwanted side-effects since due to flush batching
    delaying the flush request in IO scheduler will delay all flush requests
    possibly coming from other processes. So we keep adding the request
    directly to q->queue_head.
    Signed-off-by: default avatarJan Kara <jack@suse.com>
    Reviewed-by: default avatarJeff Moyer <jmoyer@redhat.com>
    Signed-off-by: default avatarJens Axboe <axboe@fb.com>
blk-flush.c 15.2 KB