[PATCHSET,RFC,0/2] Cache issue side time querying

Message

Jens Axboe Jan. 15, 2024, 9:53 p.m. UTC

Hi,

When I run my peak testing to see if we've regressed, my test script
always does:

	echo 1 > /sys/block/$DEV/queue/iostats
	echo 2 > /sys/block/$DEV/queue/nomerges

for each device being used. It's unfortunate that we need to disable
iostats, but without doing that, I lose about 12% performance. The main
reason for that is the time querying we need to do, when iostats are
enabled. As it turns out, lots of other block code is quite trigger
happy with querying time as well. We do have some nice batching in place
which helps ammortize that, but it's not perfect.

This trivial patchset simply caches the current time in struct blk_plug,
on the premise that any issue side time querying can get adequate
granularity through that. Nobody really needs nsec granularity on the
timestamp.

Results in patch 2, but tldr is a more than 6% improvement (108M -> 115M
IOPS) for my test case, which doesn't even enable most of the costly
block layer items that you'd typically find in a distro and which would
further increase the number of issue side time calls.