Re: [PATCH V3 5/9] io_uring: support SQE group

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

On 6/11/24 14:32, Ming Lei wrote:
> On Mon, Jun 10, 2024 at 02:55:22AM +0100, Pavel Begunkov wrote:
>> On 5/21/24 03:58, Ming Lei wrote:
>>> On Sat, May 11, 2024 at 08:12:08AM +0800, Ming Lei wrote:
>>>> SQE group is defined as one chain of SQEs starting with the first SQE that
>>>> has IOSQE_SQE_GROUP set, and ending with the first subsequent SQE that
>>>> doesn't have it set, and it is similar with chain of linked SQEs.
>>>>
>>>> Not like linked SQEs, each sqe is issued after the previous one is completed.
>>>> All SQEs in one group are submitted in parallel, so there isn't any dependency
>>>> among SQEs in one group.
>>>>
>>>> The 1st SQE is group leader, and the other SQEs are group member. The whole
>>>> group share single IOSQE_IO_LINK and IOSQE_IO_DRAIN from group leader, and
>>>> the two flags are ignored for group members.
>>>>
>>>> When the group is in one link chain, this group isn't submitted until the
>>>> previous SQE or group is completed. And the following SQE or group can't
>>>> be started if this group isn't completed. Failure from any group member will
>>>> fail the group leader, then the link chain can be terminated.
>>>>
>>>> When IOSQE_IO_DRAIN is set for group leader, all requests in this group and
>>>> previous requests submitted are drained. Given IOSQE_IO_DRAIN can be set for
>>>> group leader only, we respect IO_DRAIN by always completing group leader as
>>>> the last one in the group.
>>>>
>>>> Working together with IOSQE_IO_LINK, SQE group provides flexible way to
>>>> support N:M dependency, such as:
>>>>
>>>> - group A is chained with group B together
>>>> - group A has N SQEs
>>>> - group B has M SQEs
>>>>
>>>> then M SQEs in group B depend on N SQEs in group A.
>>>>
>>>> N:M dependency can support some interesting use cases in efficient way:
>>>>
>>>> 1) read from multiple files, then write the read data into single file
>>>>
>>>> 2) read from single file, and write the read data into multiple files
>>>>
>>>> 3) write same data into multiple files, and read data from multiple files and
>>>> compare if correct data is written
>>>>
>>>> Also IOSQE_SQE_GROUP takes the last bit in sqe->flags, but we still can
>>>> extend sqe->flags with one uring context flag, such as use __pad3 for
>>>> non-uring_cmd OPs and part of uring_cmd_flags for uring_cmd OP.
>>>>
>>>> Suggested-by: Kevin Wolf <kwolf@redhat.com>
>>>> Signed-off-by: Ming Lei <ming.lei@redhat.com>
>>>
>>> BTW, I wrote one link-grp-cp.c liburing/example which is based on sqe group,
>>> and keep QD not changed, just re-organize IOs in the following ways:
>>>
>>> - each group have 4 READ IOs, linked by one single write IO for writing
>>>     the read data in sqe group to destination file
>>
>> IIUC it's comparing 1 large write request with 4 small, and
> 
> It is actually reasonable from storage device viewpoint, concurrent
> small READs are often fast than single big READ, but concurrent small
> writes are usually slower.

It is, but that doesn't make the comparison apple to apple.
Even what I described, even though it's better (same number
of syscalls but better parallelism as you don't block next
batch of reads by writes), you can argues it's not a
completely fair comparison either since needs different number
of buffers, etc.

>> it's not exactly anything close to fair. And you can do same
>> in userspace (without links). And having control in userspace
> 
> No, you can't do it with single syscall.

That's called you _can_ do it. And syscalls is not everything,
context switching turned to be a bigger problem, and to execute
links it does exactly that.

-- 
Pavel Begunkov