On Mon, Apr 22, 2024 at 12:27:28PM -0600, Jens Axboe wrote: > On 4/7/24 7:03 PM, Ming Lei wrote: > > SQE group is defined as one chain of SQEs starting with the first sqe that > > has IOSQE_EXT_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. > > > > The 1st SQE is group leader, and the other SQEs are group member. The group > > leader is always freed after all members are completed. Group members > > aren't submitted until the group leader is completed, and there isn't any > > dependency among group members, and IOSQE_IO_LINK can't be set for group > > members, same with IOSQE_IO_DRAIN. > > > > Typically the group leader provides or makes resource, and the other members > > consume the resource, such as scenario of multiple backup, the 1st SQE is to > > read data from source file into fixed buffer, the other SQEs write data from > > the same buffer into other destination files. SQE group provides very > > efficient way to complete this task: 1) fs write SQEs and fs read SQE can be > > submitted in single syscall, no need to submit fs read SQE first, and wait > > until read SQE is completed, 2) no need to link all write SQEs together, then > > write SQEs can be submitted to files concurrently. Meantime application is > > simplified a lot in this way. > > > > Another use case is to for supporting generic device zero copy: > > > > - the lead SQE is for providing device buffer, which is owned by device or > > kernel, can't be cross userspace, otherwise easy to cause leak for devil > > application or panic > > > > - member SQEs reads or writes concurrently against the buffer provided by lead > > SQE > > In concept, this looks very similar to "sqe bundles" that I played with > in the past: > > https://git.kernel.dk/cgit/linux/log/?h=io_uring-bundle Indeed, so looks it is something which io_uring needs. > > Didn't look too closely yet at the implementation, but in spirit it's > about the same in that the first entry is processed first, and there's > no ordering implied between the test of the members of the bundle / > group. Yeah. > > I do think that's a flexible thing to support, particularly if: > > 1) We can do it more efficiently than links, which are pretty horrible. Agree, link is hard to use in async/.await of modern language per my experience. Also sqe group won't break link, and the group is thought as a whole wrt. linking. > 2) It enables new worthwhile use cases > 3) It's done cleanly > 4) It's easily understandable and easy to document, so that users will > actually understand what this is and what use cases it enable. Part > of that is actually naming, it should be readily apparent what a > group is, what the lead is, and what the members are. Using your > terminology here, definitely worth spending some time on that to get > it just right and self evident. All are nice suggestions, and I will follow above and make them in V2. Thanks, Ming
