From: Jens Axboe <[email protected]>
To: "Carter Li 李通洲" <[email protected]>,
io-uring <[email protected]>
Subject: Re: [ISSUE] The time cost of IOSQE_IO_LINK
Date: Wed, 12 Feb 2020 10:29:41 -0700 [thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
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
next prev parent reply other threads:[~2020-02-12 17:29 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-12 16:31 [ISSUE] The time cost of IOSQE_IO_LINK 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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