[PATCHSET RFC 0/3] Add plug based request allocation batching

Hi,

Even if the caller knows that N requests will be submitted, we still
have to go all the way to tag allocation for each one. That's somewhat
inefficient, when we could just grab as many tags as we need initially
instead.

This small series allows request caching in the blk_plug. We add a new
helper for passing in how many requests we expect to submit,
blk_start_plug_nr_ios(), and then we can use that information to
populate the cache on the first request allocation. Subsequent queue
attempts can then just grab a pre-allocated request out of the cache
rather than go into the tag allocation again.

This brings single core performance on my setup from:

sudo taskset -c 0,16  t/io_uring -b512 -d128 -s32 -c32 -p1 -F1 -B1 -n2 /dev/nvme1n1 /dev/nvme2n1
Added file /dev/nvme1n1 (submitter 0)
Added file /dev/nvme2n1 (submitter 1)
polled=1, fixedbufs=1, register_files=1, buffered=0, QD=128
Engine=io_uring, sq_ring=128, cq_ring=256
submitter=0, tid=1206
submitter=1, tid=1207
IOPS=5806400, BW=2835MiB/s, IOS/call=32/32, inflight=(77 32)
IOPS=5860352, BW=2861MiB/s, IOS/call=32/31, inflight=(102 128)
IOPS=5844800, BW=2853MiB/s, IOS/call=32/32, inflight=(102 32)

to:

taskset -c 0,16  t/io_uring -b512 -d128 -s32 -c32 -p1 -F1 -B1 -n2 /dev/nvme1n1 /dev/nvme2n1
Added file /dev/nvme1n1 (submitter 0)
Added file /dev/nvme2n1 (submitter 1)
polled=1, fixedbufs=1, register_files=1, buffered=0, QD=128
Engine=io_uring, sq_ring=128, cq_ring=256
submitter=0, tid=1220
submitter=1, tid=1221
IOPS=6061248, BW=2959MiB/s, IOS/call=32/31, inflight=(114 82)
IOPS=6100672, BW=2978MiB/s, IOS/call=32/31, inflight=(128 128)
IOPS=6082496, BW=2969MiB/s, IOS/call=32/32, inflight=(77 38)

which is about a 4-5% improvement in single core performance. Looking
at profiles, we're spending _less_ time in sbitmap even though we're
doing more work.

-- 
Jens Axboe