The value of the I/O plugging (idling) timeout is used also as the
think-time threshold to decide whether a process has a short think
time.  In this respect, a good value of this timeout for rotational
drives is un the order of several ms. Yet, this is often too long a
time interval to be effective as a think-time threshold. This commit
mitigates this problem (by a lot, according to tests), by halving the
threshold.
Tested-by: NJan Kara <jack@suse.cz>
Signed-off-by: NPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: NJens Axboe <axboe@kernel.dk>