Pavel Begunkov wrote: > On 4/9/24 02:33, Oliver Crumrine wrote: > > Pavel Begunkov wrote: > >> On 4/7/24 20:14, Oliver Crumrine wrote: > >>> Oliver Crumrine wrote: > >>>> Pavel Begunkov wrote: > >>>>> On 4/5/24 21:04, Oliver Crumrine wrote: > >>>>>> Pavel Begunkov wrote: > >>>>>>> On 4/4/24 23:17, Oliver Crumrine wrote: > >>>>>>>> In his patch to enable zerocopy networking for io_uring, Pavel Begunkov > >>>>>>>> specifically disabled REQ_F_CQE_SKIP, as (at least from my > >>>>>>>> understanding) the userspace program wouldn't receive the > >>>>>>>> IORING_CQE_F_MORE flag in the result value. > >>>>>>> > >>>>>>> No. IORING_CQE_F_MORE means there will be another CQE from this > >>>>>>> request, so a single CQE without IORING_CQE_F_MORE is trivially > >>>>>>> fine. > >>>>>>> > >>>>>>> The problem is the semantics, because by suppressing the first > >>>>>>> CQE you're loosing the result value. You might rely on WAITALL > >>>>>> That's already happening with io_send. > >>>>> > >>>>> Right, and it's still annoying and hard to use > >>>> Another solution might be something where there is a counter that stores > >>>> how many CQEs with REQ_F_CQE_SKIP have been processed. Before exiting, > >>>> userspace could call a function like: io_wait_completions(int completions) > >>>> which would wait until everything is done, and then userspace could peek > >>>> the completion ring. > >>>>> > >>>>>>> as other sends and "fail" (in terms of io_uring) the request > >>>>>>> in case of a partial send posting 2 CQEs, but that's not a great > >>>>>>> way and it's getting userspace complicated pretty easily. > >>>>>>> > >>>>>>> In short, it was left out for later because there is a > >>>>>>> better way to implement it, but it should be done carefully > >>>>>> Maybe we could put the return values in the notifs? That would be a > >>>>>> discrepancy between io_send and io_send_zc, though. > >>>>> > >>>>> Yes. And yes, having a custom flavour is not good. It'd only > >>>>> be well usable if apart from returning the actual result > >>>>> it also guarantees there will be one and only one CQE, then > >>>>> the userspace doesn't have to do the dancing with counting > >>>>> and checking F_MORE. In fact, I outlined before how a generic > >>>>> solution may looks like: > >>>>> > >>>>> https://github.com/axboe/liburing/issues/824 > >>>>> > >>>>> The only interesting part, IMHO, is to be able to merge the > >>>>> main completion with its notification. Below is an old stash > >>>>> rebased onto for-6.10. The only thing missing is relinking, > >>>>> but maybe we don't even care about it. I need to cover it > >>>>> well with tests. > >>>> The patch looks pretty good. The only potential issue is that you store > >>>> the res of the normal CQE into the notif CQE. This overwrites the > >>>> IORING_CQE_F_NOTIF with IORING_CQE_F_MORE. This means that the notif would > >>>> indicate to userspace that there will be another CQE, of which there > >>>> won't. > >>> I was wrong here; Mixed up flags and result value. > >> > >> Right, it's fine. And it's synchronised by the ubuf refcounting, > >> though it might get more complicated if I'd try out some counting > >> optimisations. > >> > >> FWIW, it shouldn't give any performance wins. The heavy stuff is > >> notifications waking the task, which is still there. I can even > >> imagine that having separate CQEs might be more flexible and would > >> allow more efficient CQ batching. > > I've actaully been working on this issue for a little while now. My current > > idea is that an id is put into the optval section of the SQE, and then it > > can be used to tag that req with a certain group. When a req has a flag > > set on it, it can request for all of group's notifs to be "flushed" in one > > notif that encompasses that entire group. If the id is zero, it won't be > > associated with a group and will generate a notif. LMK if you see anything > > in here that could overcomplicate userspace. I think it's pretty simple, > > but you've had a crack at this before so I'd like to hear your opinion. > > You can take a look at early versions of the IORING_OP_SEND_ZC, e.g. > patchset v1, probably even later ones. It was basically doing what > you've described with minor uapi changes, like you had to declare groups > (slots) in advance, i.e. register them. My idea is that insead of allocating slots before making requests, "slots" will be allocated as the group ids show up. Instead of an array of slots, a linked list can be used so things can be kmalloc'ed on the fly to make the uapi simpler. > > More flexible and so performant in some circumstances, but the overall > feedback from people trying it is that it's complicated. The user should > allocate group ids, track bound requests / buffers, do other management. > The next question is how the user should decide what bind to what. There > is some nastiness in using the same group for multiple sockets, and then Then maybe we find a way to prevent multiple sockets on one group. > what's the cut line to flush the previous notif? I probably forgot a I'd make it the max for a u32 -- I'm (probably) going to use an atomic_t to store the counter of how many reqs have been completed, so a u32 max would make sense. > couple more complaints. > > TL;DR; > > The performance is a bit of a longer story, problems are mostly coming > from the async nature of io_uring, and it'd be nice to solve at least a > part of it generically, not only for sendzc. The expensive stuff is > waking up the task, it's not unique to notifications, recv will trigger > it with polling as well as other opcodes. Then the key is completion > batching. Maybe the interface is made for sendzc first, and people could test it there. Then if it is considered beneficial to other places, it could be implemented there. > > What's interesting, take for example some tx only toy benchmark with > DEFER_TASKRUN (recommended to use in any case). If you always wait for > sends without notifications and add eventual *_get_events(), that would > completely avoid the wake up overhead if there are enough buffers, > and if it's not it can 1:1 replace tx polling. Seems like an interesting way to eliminate waiting overhead. > > Try groups, see if numbers are good. And a heads up, I'm looking at I will. Working hard to have the code done by Sunday. > improving it a little bit for TCP because of a report, not changing > uapi but might change performance math. > > -- > Pavel Begunkov
