On 31/01/2021 09:10, dormando wrote: > Hey, > > I'm trying to puzzle out an architecture on top of io_uring for a tcp > proxy I'm working on. I have a high level question, then I'll explain what > I'm doing for context: > > - How (is?) order maintained for write()'s to the same FD from different > SQE's to a network socket? ie; I get request A and queue a write(), later Without IOSQE_LINK or anything -- no ordering guarantees. Even if CQEs came in some order actual I/O may have been executed in reverse. > request B comes in and gets queued, A finishes short. There was no chance > to IOSQE_LINK A to B. Does B cancel? This makes sense for disk IO but I > can't wrap my head around it for network sockets. > > The setup: > > - N per-core worker threads. Each thread handles X client sockets. > - Y backend sockets in a global shared pool. These point to storage > servers (or other proxyes/anything). > > - client sockets wake up with requests for an arbitrary number of keys (1 > to 100 or so). > - each key is mapped to a backend (like keyhash % Y). > - new requests are dispatched for each key to each backend socket. > - the results are put back into order and returned to the client. > > The workers are designed such that they should not have to wait for a > large request set before processing the next ready client socket. ie; > thread N1 gets a request for 100 keys; it queues that work off, and then > starts on a request for a single key. it picks up the results of the > original request later and returns it. Else we get poor long tail latency. > > I've been working out a test program to mock this new backend. I have mock > worker threads that submit batches of work from fake connections, and then > have libevent or io_uring handle things. > > In libevent/epoll mode: > - workers can directly call write() to backend sockets while holding a > lock around a descriptive structure. this ensures order. > - OR workers submit stacks to one or more threads which the backends > sockets are striped across. These threads lock and write(). this mode > helps with latency pileup. > - a dedicated thread sits in epoll_wait() on EPOLLIN for each backend > socket. This avoids repeated calls to epoll_add()/mod/etc. As responses > are parsed, completed sets of requests are shipped back to the worker > threads. > > In uring mode: > - workers should submit to a single (or few) threads which have a private > ring. sqe's are stacked and submit()'ed in a batch. Ideally saving all of > the overhead of write()'ing to a bunch of sockets. (not working yet) > - a dedicated thread with its own ring is sitting on recv() for each > backend socket. It handles the same as epoll mode, except after each read > I have to re-submit a new SQE for the next read. > > (I have everything sharing the same WQ, for what it's worth) > > I'm trying to figure out uring mode's single submission thread, but > figuring out the IO ordering issues is blanking my mind. Requests can come > in interleaved as the backends are shared, and waiting for a batch to > complete before submitting the next one defeats the purpose (I think). > > What would be super nice but I'm pretty sure is impossible: > > - M (possibly 1) thread(s) sitting on recv() in its own ring > - N client handling worker threads with independent rings on the same WQ > - SQE's with writes to the same backend FD are serialized by a magical > unicorn. > > Then: > - worker with a request for 100 keys makes and submits the SQE's itself, > then moves on to the next client connection. > - recv() thread gathers responses and signals worker when the batch is > complete. > > If I can avoid issues with short/colliding writes I can still make this > work as my protocol can allow for out of order responses, but it's not the > default mode so I need both to work anyway. > > Apologies if this isn't clear or was answered recently; I did try to read > archives/code/etc. -- Pavel Begunkov
