Re: [PATCH v2 0/5] fast poll multishot mode

Jens Axboe
2022-05-06T22:23:38Z

On 5/6/22 1:00 AM, Hao Xu wrote:
> 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

Wish I had seen that, I wrote my own! But maybe that's good, you tend to
find other issues through that.

Anyway, works for me in testing, and I can see this being a nice win for
accept intensive workloads. I pushed a bunch of cleanup patches that
should just get folded in. Can you fold them into your patches and
address the other feedback, and post a v3? I pushed the test branch
here:

https://git.kernel.dk/cgit/linux-block/log/?h=fastpoll-mshot

--
Jens Axboe