On 08/12, Martin Karsten wrote: > On 2024-08-12 21:54, Stanislav Fomichev wrote: > > On 08/12, Martin Karsten wrote: > > > On 2024-08-12 19:03, Stanislav Fomichev wrote: > > > > On 08/12, Martin Karsten wrote: > > > > > On 2024-08-12 16:19, Stanislav Fomichev wrote: > > > > > > On 08/12, Joe Damato wrote: > > > > > > > Greetings: > > > > > > > > > > > > > > Martin Karsten (CC'd) and I have been collaborating on some ideas about > > > > > > > ways of reducing tail latency when using epoll-based busy poll and we'd > > > > > > > love to get feedback from the list on the code in this series. This is > > > > > > > the idea I mentioned at netdev conf, for those who were there. Barring > > > > > > > any major issues, we hope to submit this officially shortly after RFC. > > > > > > > > > > > > > > The basic idea for suspending IRQs in this manner was described in an > > > > > > > earlier paper presented at Sigmetrics 2024 [1]. > > > > > > > > > > > > Let me explicitly call out the paper. Very nice analysis! > > > > > > > > > > Thank you! > > > > > > > > > > [snip] > > > > > > > > > > > > Here's how it is intended to work: > > > > > > > - An administrator sets the existing sysfs parameters for > > > > > > > defer_hard_irqs and gro_flush_timeout to enable IRQ deferral. > > > > > > > > > > > > > > - An administrator sets the new sysfs parameter irq_suspend_timeout > > > > > > > to a larger value than gro-timeout to enable IRQ suspension. > > > > > > > > > > > > Can you expand more on what's the problem with the existing gro_flush_timeout? > > > > > > Is it defer_hard_irqs_count? Or you want a separate timeout only for the > > > > > > perfer_busy_poll case(why?)? Because looking at the first two patches, > > > > > > you essentially replace all usages of gro_flush_timeout with a new variable > > > > > > and I don't see how it helps. > > > > > > > > > > gro-flush-timeout (in combination with defer-hard-irqs) is the default irq > > > > > deferral mechanism and as such, always active when configured. Its static > > > > > periodic softirq processing leads to a situation where: > > > > > > > > > > - A long gro-flush-timeout causes high latencies when load is sufficiently > > > > > below capacity, or > > > > > > > > > > - a short gro-flush-timeout causes overhead when softirq execution > > > > > asynchronously competes with application processing at high load. > > > > > > > > > > The shortcomings of this are documented (to some extent) by our experiments. > > > > > See defer20 working well at low load, but having problems at high load, > > > > > while defer200 having higher latency at low load. > > > > > > > > > > irq-suspend-timeout is only active when an application uses > > > > > prefer-busy-polling and in that case, produces a nice alternating pattern of > > > > > application processing and networking processing (similar to what we > > > > > describe in the paper). This then works well with both low and high load. > > > > > > > > So you only want it for the prefer-busy-pollingc case, makes sense. I was > > > > a bit confused by the difference between defer200 and suspend200, > > > > but now I see that defer200 does not enable busypoll. > > > > > > > > I'm assuming that if you enable busypool in defer200 case, the numbers > > > > should be similar to suspend200 (ignoring potentially affecting > > > > non-busypolling queues due to higher gro_flush_timeout). > > > > > > defer200 + napi busy poll is essentially what we labelled "busy" and it does > > > not perform as well, since it still suffers interference between application > > > and softirq processing. > > > > With all your patches applied? Why? Userspace not keeping up? > > Note our "busy" case does not utilize our patches. Great, thanks for confirming, that makes sense! > As illustrated by our performance numbers, its performance is better than > the base case, but at the cost of higher cpu utilization and it's still not > as good as suspend20. > > Explanation (conjecture): > > It boils down to having to set a particular static value for > gro-flush-timeout that is then always active. > > If busy-poll + application processing takes longer than this timeout, the > next softirq runs while the application is still active, which causes > interference. > > Once a softirq runs, the irq-loop (Loop 2) takes control. When the > application thread comes back to epoll_wait, it already finds data, thus > ep_poll does not run napi_busy_poll at all, thus the irq-loop stays in > control. > > This continues until by chance the application finds no readily available > data when calling epoll_wait and ep_poll runs another napi_busy_poll. Then > the system switches back to busy-polling mode. > > So essentially the system non-deterministically alternates between > busy-polling and irq deferral. irq deferral determines the high-order tail > latencies, but there is still enough interference to make a difference. It's > not as bad as in the base case, but not as good as properly controlled irq > suspension. > > > > > > > Maybe expand more on what code paths are we trying to improve? Existing > > > > > > busy polling code is not super readable, so would be nice to simplify > > > > > > it a bit in the process (if possible) instead of adding one more tunable. > > > > > > > > > > There are essentially three possible loops for network processing: > > > > > > > > > > 1) hardirq -> softirq -> napi poll; this is the baseline functionality > > > > > > > > > > 2) timer -> softirq -> napi poll; this is deferred irq processing scheme > > > > > with the shortcomings described above > > > > > > > > > > 3) epoll -> busy-poll -> napi poll > > > > > > > > > > If a system is configured for 1), not much can be done, as it is difficult > > > > > to interject anything into this loop without adding state and side effects. > > > > > This is what we tried for the paper, but it ended up being a hack. > > > > > > > > > > If however the system is configured for irq deferral, Loops 2) and 3) > > > > > "wrestle" with each other for control. Injecting the larger > > > > > irq-suspend-timeout for 'timer' in Loop 2) essentially tilts this in favour > > > > > of Loop 3) and creates the nice pattern describe above. > > > > > > > > And you hit (2) when the epoll goes to sleep and/or when the userspace > > > > isn't fast enough to keep up with the timer, presumably? I wonder > > > > if need to use this opportunity and do proper API as Joe hints in the > > > > cover letter. Something over netlink to say "I'm gonna busy-poll on > > > > this queue / napi_id and with this timeout". And then we can essentially make > > > > gro_flush_timeout per queue (and avoid > > > > napi_resume_irqs/napi_suspend_irqs). Existing gro_flush_timeout feels > > > > too hacky already :-( > > > > > > If someone would implement the necessary changes to make these parameters > > > per-napi, this would improve things further, but note that the current > > > proposal gives strong performance across a range of workloads, which is > > > otherwise difficult to impossible to achieve. > > > > Let's see what other people have to say. But we tried to do a similar > > setup at Google recently and getting all these parameters right > > was not trivial. Joe's recent patch series to push some of these into > > epoll context are a step in the right direction. It would be nice to > > have more explicit interface to express busy poling preference for > > the users vs chasing a bunch of global tunables and fighting against softirq > > wakups. > > One of the goals of this patch set is to reduce parameter tuning and make > the parameter setting independent of workload dynamics, so it should make > things easier. This is of course notwithstanding that per-napi settings > would be even better. > > If you are able to share more details of your previous experiments (here or > off-list), I would be very interested. We went through a similar exercise of trying to get the tail latencies down. Starting with SO_BUSY_POLL, then switching to the per-epoll variant (except we went with a hard-coded napi_id argument instead of tracking) and trying to get a workable set of budget/timeout/gro_flush. We were fine with burning all cpu capacity we had and no sleep at all, so we ended up having a bunch of special cases in epoll loop to avoid the sleep. But we were trying to make a different model work (the one you mention in the paper as well) where the userspace busy-pollers are just running napi_poll on one cpu and the actual work is consumed by the userspace on a different cpu. (we had two epoll fds - one with napi_id=xxx and no sockets to drive napi_poll and another epoll fd with actual sockets for signaling). This mode has a different set of challenges with socket lock, socket rx queue and the backlog processing :-( > > > Note that napi_suspend_irqs/napi_resume_irqs is needed even for the sake of > > > an individual queue or application to make sure that IRQ suspension is > > > enabled/disabled right away when the state of the system changes from busy > > > to idle and back. > > > > Can we not handle everything in napi_busy_loop? If we can mark some napi > > contexts as "explicitly polled by userspace with a larger defer timeout", > > we should be able to do better compared to current NAPI_F_PREFER_BUSY_POLL > > which is more like "this particular napi_poll call is user busy polling". > > Then either the application needs to be polling all the time (wasting cpu > cycles) or latencies will be determined by the timeout. > > Only when switching back and forth between polling and interrupts is it > possible to get low latencies across a large spectrum of offered loads > without burning cpu cycles at 100%. Ah, I see what you're saying, yes, you're right. In this case ignore my comment about ep_suspend_napi_irqs/napi_resume_irqs. Let's see how other people feel about per-dev irq_suspend_timeout. Properly disabling napi during busy polling is super useful, but it would still be nice to plumb irq_suspend_timeout via epoll context or have it set on a per-napi basis imho.
