Hi!
I'm playing with io-uring, and I found the io-wrk thread situation confusing.
(A) In one case, I have a SOCK_DGRAM socket (blocking), over which I
do IORING_OP_RECVMSG. This works well, and unless I mark the sqe as
IOSQE_ASYNC, it doesn't seem to start an io-wrk kernel thread.
(B) However, the same can't be said of another situation. In the
second case I have a tap file descriptor (blocking), which doesn't
support "Socket operations on non-socket", so I must do
IORING_OP_READV. This however seems to start a new io-wrk for each
readv request:
$ pstree -pt `pidof tapuring`
tapuring(44932)─┬─{iou-wrk-44932}(44937)
├─{iou-wrk-44932}(44938)
├─{iou-wrk-44932}(44939)
├─{iou-wrk-44932}(44940)
├─{iou-wrk-44932}(44941)
├─{iou-wrk-44932}(44942)
I would expect both situations to behave the same way.
The manpage for IOSQE_ASYNC:
IOSQE_ASYNC
Normal operation for io_uring is to try and issue an sqe
as non-blocking first, and if that fails, execute it in an
async manner. To support more efficient overlapped
operation of requests that the application knows/assumes
will always (or most of the time) block, the application
can ask for an sqe to be issued async from the start. Note
that this flag immediately causes the SQE to be offloaded
to an async helper thread with no initial non-blocking
attempt. This may be less efficient and should not be
used liberally or without understanding the performance
and efficiency tradeoffs.
This seems to cover the tap file descriptor case. It tries to readv
and when that fails a new io-wrk is spawned. Fine. However, as I
described it seems this is not true for sockets, as without
IOSQE_ASYNC the io-wrk thread is _not_ spawned there?
Is the behaviour different due to socket vs non-socket or readv vs recvmsg?
Please advise.
Marek
