[RFC PATCH 0/2] Batch free work in io-wq

[Bui Quang Minh <minhquangbui99@gmail.com>]

When benchmarking a echo server using io_uring with force async to
spawn io workers and without IORING_SETUP_DEFER_TASKRUN, I saw a small
contention in tctx->task_list in the io_worker_handle_work loop. After
handling work, we call free_work, which will add a task work to the
shared tctx->task_list. The idea of this patchset is that each io worker
will queue the freed works in its local list and batches multiple free
work in one call when the number of freed works reaches
IO_REQ_ALLOC_BATCH.

=========
Benchmark
=========

Setup:
- Host: Intel(R) Core(TM) i7-10750H CPU with 12 CPUs
- Guest: Qemu KVM with 4 vCPUs, multiqueues virtio-net (each vCPUs has
  its own tx/rx pair)
- Test source code: https://github.com/minhbq-99/toy-echo-server

In guest machine, run the `./io_uring_server -a` (number of unbound io
worker is limited to number of CPUs, 4 in the benchmark environment).

In host machine, run `for i in $(seq 1 10); do ./client --client 8
--packet.size 2000 --duration 30s -ip 192.168.31.3; done;`. This will
create 8 TCP client sockets.

Result:
- Before: 55,885.56 +- 1,782.51 req/s
- After:  59,926.25 +-   312.60 req/s (+7.23%)

Though the result shows the difference is statistically significant, the
improvement is quite small. I really appreciate any further suggestions.

Thanks,
Quang Minh.

Bui Quang Minh (2):
  io_uring: make io_req_normal_work_add accept a list of requests
  io_uring/io-wq: try to batch multiple free work

 io_uring/io-wq.c    | 62 +++++++++++++++++++++++++++++++++++++++++++--
 io_uring/io-wq.h    |  4 ++-
 io_uring/io_uring.c | 36 +++++++++++++++++++-------
 io_uring/io_uring.h |  8 +++++-
 4 files changed, 97 insertions(+), 13 deletions(-)

-- 
2.43.0