Re: [PATCH V6 03/17] io_uring: add generic IORING_OP_FUSED_CMD

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

On Thu, Mar 30, 2023 at 07:36:16PM +0800, Ming Lei wrote:
> Multiple requests submitted as one whole request logically, and the 1st one
> is primary command(IORING_OP_FUSED_CMD), and the others are secondary
> requests, which number can be retrieved from primary SQE.
> 
> Primary command is responsible for providing resources and submitting
> secondary requests, which depends on primary command's resources, and
> primary command won't be completed until all secondary requests are done.
> 
> The provided resource has same lifetime with primary command, and it
> lifetime won't cross multiple OPs, and this way provides safe way for
> secondary OPs to use the resource.
> 
> Add generic IORING_OP_FUSED_CMD for modeling this primary/secondary
> relationship among requests.

BTW, this model also solves 1:N dependency problem of io_uring.

Current io_uring can't handle this kind of dependency(1:N) efficiently,
and simply convert it into one linked list:

- N requests(1~n) depends on one request(0), and there isn't dependency among
these N requests

- current io_uring converts the dependency to linked list of (N + 1) requests
(0, 1, ...., n), in which all requests are just issued one by one from 0 to n,
inefficiently

The primary/secondary model solves it by issuing request 0 first, then issues
all other N requests concurrently.


Thanks,
Ming