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

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

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


>> submit(req={1});
>> wait(nr=1);
>> submit({2-99});
>> wait(nr=98);
>> submit(req={100});
>> wait(nr=1);
>>

-- 
Pavel Begunkov