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

Hao Xu
2022-05-07T16:01:33Z

在 2022/5/7 上午11:08, Jens Axboe 写道:
> On 5/6/22 8:33 PM, Jens Axboe wrote:
>> On 5/6/22 5:26 PM, Jens Axboe wrote:
>>> 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.
>>
>> One more thought on this - how is it supposed to work with
>> accept-direct? One idea would be to make it incrementally increasing.
>> But we need a good story for that, if it's exclusive to non-direct
>> files, then it's a lot less interesting as the latter is really nice win
>> for lots of files. If we can combine the two, even better.
>
> Running some quick testing, on an actual test box (previous numbers were
> from a vm on my laptop):
>
> Testing singleshot, normal files
> Did 10000 accepts
>
> ________________________________________________________
> Executed in 216.10 millis fish external
> usr time 9.32 millis 150.00 micros 9.17 millis
> sys time 110.06 millis 67.00 micros 109.99 millis
>
> Testing multishot, fixed files
> Did 10000 accepts
>
> ________________________________________________________
> Executed in 189.04 millis fish external
> usr time 11.86 millis 159.00 micros 11.71 millis
> sys time 93.71 millis 70.00 micros 93.64 millis
>
> That's about ~19 usec to accept a connection, pretty decent. Using
> singleshot and with fixed files, it shaves about ~8% off, ends at around
> 200msec.
>
> I think we can get away with using fixed files and multishot, attaching
I'm not following, do you mean we shouldn't do the multishot+fixed file
or we should use multishot+fixed to make the result better?
> the quick patch I did below to test it. We need something better than
Sorry Jens, I didn't see the quick patch, is there anything I misunderstand?
> this, otherwise once the space fills up, we'll likely end up with a
> sparse space and the naive approach of just incrementing the next slot
> won't work at all.

>