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

[Ming Lei <ming.lei@redhat.com>]

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

- the 1st 12 groups have (4 + 1) IOs, and the last group have (3 + 1)
  IOs


Run the example for copying two block device(from virtio-blk to
virtio-scsi in my test VM):

1) buffered copy:
- perf is improved by 5%

2) direct IO mode
- perf is improved by 27%


[1] link-grp-cp.c example

https://github.com/ming1/liburing/commits/sqe_group_v2/


[2] one bug fixes(top commit) against V3

https://github.com/ming1/linux/commits/io_uring_sqe_group_v3/



Thanks,
Ming