[PATCHSET,0/4] blk-mq: per-ctx tag caching

Message

Jens Axboe Dec. 30, 2019, 6:14 p.m. UTC

This is something I've been toying with yesterday, and got it cleaned
up a bit just now. It's still very much a testing thing, and there's a
few debug checks in there and items that need improving.

Anyway, the idea here is that we can reduce the cost of getting a tag
for a new request, if we don't get them piecemeal. Add a per-ctx tag
cache, and grab batches of tags if it's empty. If it's not empty, we
can just find a free bit there.

/sys/kernel/debug/block/<dev>/<hctx>/<cpu>/tag_hit holds some stats
associated with this, so you can check how it's doing.

I've seen nice improvements with this in testing.