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fe682e16e9
A buffered read that hits the page cache completes inline inside
`io_uring_enter`, so the thread driving a ring performs that read's
memcpy. One ring per disk therefore capped cache-hit reads at a single
core's memory bandwidth: measured on a 16-core host, one driver thread sat
pinned at 100% CPU while throughput stayed flat at ~5 GB/s regardless of
read size, against 50 GB/s for the blocking-pool baseline.
rustfs/uring#6 taught the driver to hold N independent rings, each with its
own thread, pending table, backpressure semaphore, and eventfd. Wire it up:
`UringBackend::try_new` now calls `probe_and_start_sharded`, and
`RUSTFS_IO_URING_SHARDS` selects the count per disk.
The default is a quarter of the available parallelism clamped to `1..=4`,
because the cost is `disks × shards` driver threads (each normally blocked
in `poll(2)`). Any override is clamped to `1..=16`, so a mistyped value can
neither disable the driver (0) nor spawn threads without bound; an
unparseable value falls back to the default.
Effect (warm page cache, 16-core, rustfs/uring's concurrent_pread_bench):
1 MiB, conc 8: 1 shard 4911 MB/s -> 8 shards 47361 MB/s (9.6x);
the blocking-pool baseline is 50662 MB/s
64 KiB, conc 32: StdBackend 153678 IOPS, p999 3030 us
8 shards 345402 IOPS, p999 897 us
64 KiB, conc 128: StdBackend 135155 IOPS, p999 10716 us
8 shards 389047 IOPS, p999 4092 us
Sharding removes the throughput deficit *and* keeps io_uring's tail-latency
advantage, rather than trading one for the other.
Unchanged: io_uring read stays gray-off by default
(`RUSTFS_IO_URING_READ_ENABLE`), reads are byte-for-byte identical to
StdBackend, the per-disk degradation latches and probe cache (backlog#1101)
and the O_DIRECT tiered fallback (backlog#1102) all still apply. Rings stay
per-disk, so a stalled disk cannot starve another disk's rings
(backlog#1055). Bumps the rustfs-uring pin to the merged #6 commit.
Verified on a real Linux host (16-core, real io_uring): cargo clippy
--tests -D warnings clean; disk::local tests 132 passed, 0 failed —
including the existing io_uring and O_DIRECT cases now running on the
sharded driver, plus a new test covering the shard-count default, override,
and clamping.
Co-authored-by: heihutu <heihutu@gmail.com>