On 5/28/24 10:50 AM, Pavel Begunkov wrote: > On 5/28/24 15:34, Jens Axboe wrote: >> On 5/28/24 7:31 AM, Pavel Begunkov wrote: >>> On 5/24/24 23:58, Jens Axboe wrote: >>>> Hi, >>>> >>>> A ring setup with with IORING_SETUP_SINGLE_ISSUER, which is required to >>> >>> IORING_SETUP_SINGLE_ISSUER has nothing to do with it, it's >>> specifically an IORING_SETUP_DEFER_TASKRUN optimisation. >> >> Right, I should change that in the commit message. It's task_complete >> driving it, which is tied to DEFER_TASKRUN. >> >>>> use IORING_SETUP_DEFER_TASKRUN, will need two round trips through >>>> generic task_work. This isn't ideal. This patchset attempts to rectify >>>> that, taking a new approach rather than trying to use the io_uring >>>> task_work infrastructure to handle it as in previous postings. >>> >>> Not sure why you'd want to piggyback onto overflows, it's not >>> such a well made and reliable infra, whereas the DEFER_TASKRUN >>> part of the task_work approach was fine. >> >> It's not right now, because it's really a "don't get into this >> condition, if you do, things are slower". And this series doesn't really >> change that, and honestly it doesn't even need to. It's still way better >> than what we had before in terms of DEFER_TASKRUN and messages. > > Better than how it is now or comparing to the previous attempt? > I think the one using io_uring's tw infra was better, which is > where all wake ups and optimisations currently consolidate. Better than both - I haven't tested with the previous version, but I can certainly do that. The reason why I think it'll be better is that it avoids the double roundtrips. Yes v1 was using normal task_work which is better, but it didn't solve what I think is the fundamental issue here. I'll forward port it and give it a spin, then we'll know. >> We could certainly make the messages a subset of real overflow if we >> wanted, but honestly it looks decent enough as-is with the changes that >> I'm not hugely motivated to do that. > > Not sure what you mean here, but not really suggesting refactoring > overflows. Taking the tw based patches should be easy, it only > needs killing !DEFER_TASKRUN changes from there. Sorry wasn't clear, the refactoring was purely my suggestion to reduce a bit of the code duplication. >>> The completion path doesn't usually look at the overflow list >>> but on cached cqe pointers showing the CQ is full, that means >>> after you queue an overflow someone may post a CQE in the CQ >>> in the normal path and you get reordering. Not that bad >>> considering it's from another ring, but a bit nasty and surely >>> will bite us back in the future, it always does. >> >> This is true, but generally true as well as completions come in async. >> You don't get to control the exact order on a remote ring. Messages >> themselves will be ordered when posted, which should be the important >> aspect here. Order with locally posted completions I don't see why you'd >> care, that's a timing game that you cannot control. > > True for a single request, but in a more complex system > sender's ordering will affect the order on the receiving side. > > ring1: msg_ring(); write(pipe) > ring2: read(pipe) > > The user would definitely think that the other ring will > first get a msg_ring CQE and then an CQE from the read, but as > always it's hard to predict all repercussions. Nobody should be making assumptions like that, and that will in fact already not be the case. If the msg_ring fails to lock the remote ring, then it may very well end up in the hands of io-wq. And then you could get either result validly, msg CQE first or last. >>> That's assuming you decide io_msg_need_remote() solely based >>> ->task_complete and remove >>> >>> return current != target_ctx->submitter_task; >>> >>> otherwise you can get two linked msg_ring target CQEs reordered. >> >> Good point, since it'd now be cheap enough, would probably make sense to >> simply gate it on task_complete alone. I even toyed with the idea of >> just using this approach for any ring type and kill some code in there, >> but didn't venture that far yet. > > That task check doesn't make any real difference. If it's the > same thread you can skip io_uring all together. Yeah agree, I did add a patch in between after the last email to just remove the task check. It's not really a useful case to attempt to cater to in particular. >>> It's also duplicating that crappy overflow code nobody cares >>> much about, and it's still a forced wake up with no batching, >>> circumventing the normal wake up path, i.e. io_uring tw. >> >> Yes, since this is v1 I didn't bother to integrate more tightly with the >> generic overflows, that should obviously be done by first adding a >> helper for this. I consider that pretty minor. > > My problem is not duplication of code base but rather > extending the internal user base of it. You can say that > msg_ring can easily become a hot path for some, and > then we'll be putting efforts both into overflows and > task_work when in essence they solve quite a similar > problem here. That's why I was tempted to remove the task_work path for messages on top of this. But I agree on the wakeup side, that's obviously something that doesn't currently work with msg ring. And while I don't see that as a hard problem to solve, it is a bit annoying to have multiple sources for that. Would naturally be better to retain just the task_work side. For one use case that I think is interesting with messages is work passing, eliminating a user side data structure (and lock) for that and side channel wakeups, I don't think the wakeup batching matters as you're generally not going to be batching receiving work. You're either already running work for processing, or sleeping waiting for one. >>> I don't think we should care about the request completion >>> latency (sender latency), people should be more interested >>> in the reaction speed (receiver latency), but if you care >>> about it for a reason, perhaps you can just as well allocate >>> a new request and complete the previous one right away. >> >> I know the numbers I posted was just sender side improvements on that >> particular box, however that isn't really the case for others. Here's on >> an another test box: >> >> axboe@r7525 ~> ./msg-lat >> init_flags=3000 >> Wait on startup >> 802775: my fd 3, other 4 >> 802776: my fd 4, other 3 >> Latencies for: Receiver >> percentiles (nsec): >> | 1.0000th=[ 4192], 5.0000th=[ 4320], 10.0000th=[ 4448], >> | 20.0000th=[ 4576], 30.0000th=[ 4704], 40.0000th=[ 4832], >> | 50.0000th=[ 4960], 60.0000th=[ 5088], 70.0000th=[ 5216], >> | 80.0000th=[ 5344], 90.0000th=[ 5536], 95.0000th=[ 5728], >> | 99.0000th=[ 6176], 99.5000th=[ 7136], 99.9000th=[19584], >> | 99.9500th=[20352], 99.9900th=[28288] >> Latencies for: Sender >> percentiles (nsec): >> | 1.0000th=[ 6560], 5.0000th=[ 6880], 10.0000th=[ 7008], >> | 20.0000th=[ 7264], 30.0000th=[ 7456], 40.0000th=[ 7712], >> | 50.0000th=[ 8032], 60.0000th=[ 8256], 70.0000th=[ 8512], >> | 80.0000th=[ 8640], 90.0000th=[ 8896], 95.0000th=[ 9152], >> | 99.0000th=[ 9792], 99.5000th=[11584], 99.9000th=[23168], >> | 99.9500th=[28032], 99.9900th=[40192] >> >> and after: >> >> axboe@r7525 ~> ./msg-lat 1.776s >> init_flags=3000 >> Wait on startup >> 3767: my fd 3, other 4 >> 3768: my fd 4, other 3 >> Latencies for: Sender >> percentiles (nsec): >> | 1.0000th=[ 740], 5.0000th=[ 748], 10.0000th=[ 756], >> | 20.0000th=[ 764], 30.0000th=[ 764], 40.0000th=[ 772], >> | 50.0000th=[ 772], 60.0000th=[ 780], 70.0000th=[ 780], >> | 80.0000th=[ 860], 90.0000th=[ 892], 95.0000th=[ 900], >> | 99.0000th=[ 1224], 99.5000th=[ 1368], 99.9000th=[ 1656], >> | 99.9500th=[ 1976], 99.9900th=[ 3408] >> Latencies for: Receiver >> percentiles (nsec): >> | 1.0000th=[ 2736], 5.0000th=[ 2736], 10.0000th=[ 2768], >> | 20.0000th=[ 2800], 30.0000th=[ 2800], 40.0000th=[ 2800], >> | 50.0000th=[ 2832], 60.0000th=[ 2832], 70.0000th=[ 2896], >> | 80.0000th=[ 2928], 90.0000th=[ 3024], 95.0000th=[ 3120], >> | 99.0000th=[ 4080], 99.5000th=[15424], 99.9000th=[18560], >> | 99.9500th=[21632], 99.9900th=[58624] >> >> Obivously some variation in runs in general, but it's most certainly >> faster in terms of receiving too. This test case is fixed at doing 100K >> messages per second, didn't do any peak testing. But I strongly suspect >> you'll see very nice efficiency gains here too, as doing two TWA_SIGNAL >> task_work is pretty sucky imho. > > Sure, you mentioned wins on the receiver side, I consider it > to be the main merit (latency and throughput) Actually I think both are interesting - not because the sender side is latency sensitive for on receving the CQE for it, but because it goes hand in hand with a reduction in cycles spent sending that work. That's the win on the sender side, more so than the latency win. The latter is just gravy on top. >> You could just make it io_kiocb based, but I did not want to get into >> foreign requests on remote rings. What would you envision with that >> approach, using our normal ring task_work for this instead? That would > > It was buggy in the !DEFER_TASKRUN path. Fortunately, you don't care > about it because it just does it all under ->completion_lock, which > is why you shouldn't have ever hit the problem in testing. I'll test the old approach and we'll see where we are at. -- Jens Axboe
