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

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Date:   Fri, 6 May 2022 16:23:30 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Hao Xu <haoxu.linux@...il.com>, io-uring@...r.kernel.org
Cc:     Pavel Begunkov <asml.silence@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/5] fast poll multishot mode

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