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

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Date:   Fri, 6 May 2022 17:26:44 -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 4:23 PM, Jens Axboe wrote:
> 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

Quick benchmark here, accepting 10k connections:

Stock kernel
real	0m0.728s
user	0m0.009s
sys	0m0.192s

Patched
real	0m0.684s
user	0m0.018s
sys	0m0.102s

Looks like a nice win for a highly synthetic benchmark. Nothing
scientific, was just curious.

-- 
Jens Axboe