[PATCH v2 0/5] fast poll multishot mode

Hao Xu
2022-05-06T07:01:04Z

Let multishot support multishot mode, currently only add accept as its
first comsumer.
theoretical analysis:
1) when connections come in fast
- singleshot:
add accept sqe(userpsace) --> accept inline
^ |
|-----------------|
- multishot:
add accept sqe(userspace) --> accept inline
^ |
|--*--|

we do accept repeatedly in * place until get EAGAIN

2) when connections come in at a low pressure
similar thing like 1), we reduce a lot of userspace-kernel context
switch and useless vfs_poll()

tests:
Did some tests, which goes in this way:

server client(multiple)
accept connect
read write
write read
close close

Basically, raise up a number of clients(on same machine with server) to
connect to the server, and then write some data to it, the server will
write those data back to the client after it receives them, and then
close the connection after write return. Then the client will read the
data and then close the connection. Here I test 10000 clients connect
one server, data size 128 bytes. And each client has a go routine for
it, so they come to the server in short time.
test 20 times before/after this patchset, time spent:(unit cycle, which
is the return value of clock())
before:
1930136+1940725+1907981+1947601+1923812+1928226+1911087+1905897+1941075
+1934374+1906614+1912504+1949110+1908790+1909951+1941672+1969525+1934984
+1934226+1914385)/20.0 = 1927633.75
after:
1858905+1917104+1895455+1963963+1892706+1889208+1874175+1904753+1874112
+1874985+1882706+1884642+1864694+1906508+1916150+1924250+1869060+1889506
+1871324+1940803)/20.0 = 1894750.45

(1927633.75 - 1894750.45) / 1927633.75 = 1.65%

A liburing test is here:
https://github.com/HowHsu/liburing/blob/multishot_accept/test/accept.c

v1->v2:
- re-implement it against the reworked poll code

Hao Xu (5):
io_uring: add IORING_ACCEPT_MULTISHOT for accept
io_uring: add REQ_F_APOLL_MULTISHOT for requests
io_uring: let fast poll support multishot
io_uring: add a helper for poll clean
io_uring: implement multishot mode for accept

fs/io_uring.c | 121 ++++++++++++++++++++++++++--------
include/uapi/linux/io_uring.h | 5 ++
2 files changed, 100 insertions(+), 26 deletions(-)

base-commit: f2e030dd7aaea5a937a2547dc980fab418fbc5e7
--
2.36.0