Pavel Begunkov wrote: > On 4/11/24 01:52, Oliver Crumrine wrote: > > 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. > > You don't have to explicitly prevent it unless there are other reasons, > it's just not given a real app would be able to use it this way. > > >> 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. > > To be clear, the question raised is entirely for userspace to decide > if we're talking about the design when the user has to flush a group > notificaiton via flag or so. Atomics or not is a performance side, > that's separate. > > >> 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. > > Good, and here is the patchset I mentioned: > > https://lore.kernel.org/io-uring/cover.1712923998.git.asml.silence@xxxxxxxxx/T/ Wow! 6x improvment is crazy. I just finished the code for notif grouping, and will be benchmarking it in the upcoming hours/days. It's still in a pre-alpha state, so I'll have to put a little more work into it. (pre-alpha means leaking memory. I have 32 gigs in my system. Assuming I don't go too crazy on the benchmarking I should be fine) Either way, my patch will need a little bit of work to be compatible with yours, as it modifies the ubuf callback, and yours does too. > > >> improving it a little bit for TCP because of a report, not changing > >> uapi but might change performance math. > > -- > Pavel Begunkov
