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From: Jens Axboe <axboe@kernel.dk>
To: "Carter Li 李通洲" <carter.li@eoitek.com>,
	io-uring <io-uring@vger.kernel.org>
Subject: Re: [ISSUE] The time cost of IOSQE_IO_LINK
Date: Wed, 12 Feb 2020 10:29:41 -0700
Message-ID: <4f1cdfd5-4947-39d0-c21b-01e694b47e87@kernel.dk> (raw)
In-Reply-To: <c0bf9bf4-9932-0c74-aa74-72b9cfa488b0@kernel.dk>

On 2/12/20 10:22 AM, Jens Axboe wrote:
> On 2/12/20 10:11 AM, Jens Axboe wrote:
>> On 2/12/20 9:31 AM, Carter Li 李通洲 wrote:
>>> Hi everyone,
>>>
>>> IOSQE_IO_LINK seems to have very high cost, even greater then io_uring_enter syscall.
>>>
>>> Test code attached below. The program completes after getting 100000000 cqes.
>>>
>>> $ gcc test.c -luring -o test0 -g -O3 -DUSE_LINK=0
>>> $ time ./test0
>>> USE_LINK: 0, count: 100000000, submit_count: 1562500
>>> 0.99user 9.99system 0:11.02elapsed 99%CPU (0avgtext+0avgdata 1608maxresident)k
>>> 0inputs+0outputs (0major+72minor)pagefaults 0swaps
>>>
>>> $ gcc test.c -luring -o test1 -g -O3 -DUSE_LINK=1
>>> $ time ./test1
>>> USE_LINK: 1, count: 100000110, submit_count: 799584
>>> 0.83user 19.21system 0:20.90elapsed 95%CPU (0avgtext+0avgdata 1632maxresident)k
>>> 0inputs+0outputs (0major+72minor)pagefaults 0swaps
>>>
>>> As you can see, the `-DUSE_LINK=1` version emits only about half io_uring_submit calls
>>> of the other version, but takes twice as long. That makes IOSQE_IO_LINK almost useless,
>>> please have a check.
>>
>> The nop isn't really a good test case, as it doesn't contain any smarts
>> in terms of executing a link fast. So it doesn't say a whole lot outside
>> of "we could make nop links faster", which is also kind of pointless.
>>
>> "Normal" commands will work better. Where the link is really a win is if
>> the first request needs to go async to complete. For that case, the
>> next link can execute directly from that context. This saves an async
>> punt for the common case.
> 
> Case in point, if I just add the below patch, we're a lot closer:
> 
> [root@archlinux liburing]# time test/nop-link 0
> Using link: 0
> count: 100000000, submit_count: 1562500
> 
> 
> real	0m7.934s
> user	0m0.740s
> sys	0m7.157s
> [root@archlinux liburing]# time test/nop-link 1
> Using link: 1
> count: 100000000, submit_count: 781250
> 
> 
> real	0m9.009s
> user	0m0.710s
> sys	0m8.264s
> 
> The links are still a bit slower, which is to be expected as the
> nop basically just completes, it doesn't do anything at all and
> it never needs to go async.

Pinning the test for more reliable results and we're basically even.

[root@archlinux liburing]# time taskset -c 0 test/nop-link 1
Using link: 1
count: 100000000, submit_count: 781250


real	0m8.251s
user	0m0.680s
sys	0m7.536s

[root@archlinux liburing]# time taskset -c 0 test/nop-link 0
Using link: 0
count: 100000000, submit_count: 1562500


real	0m7.986s
user	0m0.610s
sys	0m7.340s

For the intended case (outlined above), it'll definitely be a
win.

-- 
Jens Axboe


  reply index

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-12 16:31 Carter Li 李通洲
2020-02-12 17:11 ` Jens Axboe
2020-02-12 17:22   ` Jens Axboe
2020-02-12 17:29     ` Jens Axboe [this message]
2020-02-13  0:33   ` Carter Li 李通洲
2020-02-13 15:08     ` Pavel Begunkov
2020-02-13 15:14       ` Jens Axboe
2020-02-13 15:51         ` Carter Li 李通洲
2020-02-14  1:25           ` Carter Li 李通洲
2020-02-14  2:45             ` Jens Axboe
2020-02-14  5:03               ` Jens Axboe
2020-02-14 15:32                 ` Peter Zijlstra
2020-02-14 15:47                   ` Jens Axboe
2020-02-14 16:18                     ` Jens Axboe
2020-02-14 17:52                       ` Jens Axboe
2020-02-14 20:44                         ` Jens Axboe
2020-02-15  0:16                           ` Carter Li 李通洲
2020-02-15  1:10                             ` Jens Axboe
2020-02-15  1:25                               ` Carter Li 李通洲
2020-02-15  1:27                                 ` Jens Axboe
2020-02-15  6:01                                   ` Jens Axboe
2020-02-15  6:32                                     ` Carter Li 李通洲
2020-02-15 15:11                                       ` Jens Axboe
2020-02-16 19:06                                     ` Pavel Begunkov
2020-02-16 22:23                                       ` Jens Axboe
2020-02-17 10:30                                         ` Pavel Begunkov
2020-02-17 19:30                                           ` Jens Axboe
2020-02-16 23:06                                       ` Jens Axboe
2020-02-16 23:07                                         ` Jens Axboe
2020-02-17 12:09                           ` Peter Zijlstra
2020-02-17 16:12                             ` Jens Axboe
2020-02-17 17:16                               ` Jens Axboe
2020-02-17 17:46                                 ` Peter Zijlstra
2020-02-17 18:16                                   ` Jens Axboe
2020-02-18 13:13                                     ` Peter Zijlstra
2020-02-18 14:27                                       ` [PATCH] asm-generic/atomic: Add try_cmpxchg() fallbacks Peter Zijlstra
2020-02-18 14:40                                         ` Peter Zijlstra
2020-02-20 10:30                                         ` Will Deacon
2020-02-20 10:37                                           ` Peter Zijlstra
2020-02-20 10:39                                             ` Will Deacon
2020-02-18 14:56                                       ` [ISSUE] The time cost of IOSQE_IO_LINK Oleg Nesterov
2020-02-18 15:07                                         ` Oleg Nesterov
2020-02-18 15:38                                           ` Peter Zijlstra
2020-02-18 16:33                                             ` Jens Axboe
2020-02-18 15:07                                         ` Peter Zijlstra
2020-02-18 15:50                                           ` [PATCH] task_work_run: don't take ->pi_lock unconditionally Oleg Nesterov
2020-02-20 16:39                                             ` Peter Zijlstra
2020-02-20 17:22                                               ` Oleg Nesterov
2020-02-20 17:49                                                 ` Peter Zijlstra
2020-02-21 14:52                                                   ` Oleg Nesterov
2020-02-24 18:47                                                     ` Jens Axboe
2020-02-28 19:17                                                       ` Jens Axboe
2020-02-28 19:25                                                         ` Peter Zijlstra
2020-02-28 19:28                                                           ` Jens Axboe
2020-02-28 20:06                                                             ` Peter Zijlstra
2020-02-28 20:15                                                               ` Jens Axboe
2020-02-18 16:46                                       ` [ISSUE] The time cost of IOSQE_IO_LINK Jens Axboe
2020-02-18 16:52                                         ` Jens Axboe
2020-02-18 13:13                               ` Peter Zijlstra

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