Re: [POC RFC 0/3] support graph like dependent sqes

[Pavel Begunkov <asml.silence@gmail.com>]

On 12/16/21 16:55, Hao Xu wrote:
> 在 2021/12/15 上午2:16, Pavel Begunkov 写道:
>> On 12/14/21 16:53, Hao Xu wrote:
>>> 在 2021/12/14 下午11:21, Pavel Begunkov 写道:
>>>> On 12/14/21 05:57, Hao Xu wrote:
>>>>> This is just a proof of concept which is incompleted, send it early for
>>>>> thoughts and suggestions.
>>>>>
>>>>> We already have IOSQE_IO_LINK to describe linear dependency
>>>>> relationship sqes. While this patchset provides a new feature to
>>>>> support DAG dependency. For instance, 4 sqes have a relationship
>>>>> as below:
>>>>>        --> 2 --
>>>>>       /        \
>>>>> 1 ---          ---> 4
>>>>>       \        /
>>>>>        --> 3 --
>>>>> IOSQE_IO_LINK serializes them to 1-->2-->3-->4, which unneccessarily
>>>>> serializes 2 and 3. But a DAG can fully describe it.
>>>>>
>>>>> For the detail usage, see the following patches' messages.
>>>>>
>>>>> Tested it with 100 direct read sqes, each one reads a BS=4k block data
>>>>> in a same file, blocks are not overlapped. These sqes form a graph:
>>>>>        2
>>>>>        3
>>>>> 1 --> 4 --> 100
>>>>>       ...
>>>>>        99
>>>>>
>>>>> This is an extreme case, just to show the idea.
>>>>>
>>>>> results below:
>>>>> io_link:
>>>>> IOPS: 15898251
>>>>> graph_link:
>>>>> IOPS: 29325513
>>>>> io_link:
>>>>> IOPS: 16420361
>>>>> graph_link:
>>>>> IOPS: 29585798
>>>>> io_link:
>>>>> IOPS: 18148820
>>>>> graph_link:
>>>>> IOPS: 27932960
>>>>
>>>> Hmm, what do we compare here? IIUC,
>>>> "io_link" is a huge link of 100 requests. Around 15898251 IOPS
>>>> "graph_link" is a graph of diameter 3. Around 29585798 IOPS
>>
>> Diam 2 graph, my bad
>>
>>
>>>> Is that right? If so it'd more more fair to compare with a
>>>> similar graph-like scheduling on the userspace side.
>>>
>>> The above test is more like to show the disadvantage of LINK
>>
>> Oh yeah, links can be slow, especially when it kills potential
>> parallelism or need extra allocations for keeping state, like
>> READV and WRITEV.
>>
>>
>>> But yes, it's better to test the similar userspace  scheduling since
>>>
>>> LINK is definitely not a good choice so have to prove the graph stuff
>>>
>>> beat the userspace scheduling. Will test that soon. Thanks.
>>
>> Would be also great if you can also post the benchmark once
>> it's done
> 
> Wrote a new test to test nop sqes forming a full binary tree with (2^10)-1 nodes,
> which I think it a more general case.  Turns out the result is still not stable and
> the kernel side graph link is much slow. I'll try to optimize it.

That's expected unfortunately. And without reacting on results
of previous requests, it's hard to imagine to be useful. BPF may
have helped, e.g. not keeping an explicit graph but just generating
new requests from the kernel... But apparently even with this it's
hard to compete with just leaving it in userspace.

> Btw, is there any comparison data between the current io link feature and the
> userspace scheduling.

Don't remember. I'd try to look up the cover-letter for the patches
implementing it, I believe there should've been some numbers and
hopefully test description.

fwiw, before io_uring mailing list got established patches/etc.
were mostly going through linux-block mailing list. Links are old, so
patches might be there.

-- 
Pavel Begunkov