blk-mq: implement hybrid poll mode for sync O_DIRECT
This patch enables a hybrid polling mode. Instead of polling after IO submission, we can induce an artificial delay, and then poll after that. For example, if the IO is presumed to complete in 8 usecs from now, we can sleep for 4 usecs, wake up, and then do our polling. This still puts a sleep/wakeup cycle in the IO path, but instead of the wakeup happening after the IO has completed, it'll happen before. With this hybrid scheme, we can achieve big latency reductions while still using the same (or less) amount of CPU. Signed-off-by:Jens Axboe <axboe@fb.com> Tested-By:
Stephen Bates <sbates@raithlin.com> Reviewed-By:
Stephen Bates <sbates@raithlin.com>
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189ce2b9
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