On Wed, Aug 6, 2014 at 4:48 PM, Kevin Wolf <kw...@redhat.com> wrote: > Am 06.08.2014 um 07:33 hat Ming Lei geschrieben: >> Hi Kevin, >> >> On Tue, Aug 5, 2014 at 10:47 PM, Kevin Wolf <kw...@redhat.com> wrote: >> > Am 05.08.2014 um 15:48 hat Stefan Hajnoczi geschrieben: >> >> I have been wondering how to prove that the root cause is the ucontext >> >> coroutine mechanism (stack switching). Here is an idea: >> >> >> >> Hack your "bypass" code path to run the request inside a coroutine. >> >> That way you can compare "bypass without coroutine" against "bypass with >> >> coroutine". >> >> >> >> Right now I think there are doubts because the bypass code path is >> >> indeed a different (and not 100% correct) code path. So this approach >> >> might prove that the coroutines are adding the overhead and not >> >> something that you bypassed. >> > >> > My doubts aren't only that the overhead might not come from the >> > coroutines, but also whether any coroutine-related overhead is really >> > unavoidable. If we can optimise coroutines, I'd strongly prefer to do >> > just that instead of introducing additional code paths. >> >> OK, thank you for taking look at the problem, and hope we can >> figure out the root cause, :-) >> >> > >> > Another thought I had was this: If the performance difference is indeed >> > only coroutines, then that is completely inside the block layer and we >> > don't actually need a VM to test it. We could instead have something >> > like a simple qemu-img based benchmark and should be observing the same. >> >> Even it is simpler to run a coroutine-only benchmark, and I just >> wrote a raw one, and looks coroutine does decrease performance >> a lot, please see the attachment patch, and thanks for your template >> to help me add the 'co_bench' command in qemu-img. > > Yes, we can look at coroutines microbenchmarks in isolation. I actually > did do that yesterday with the yield test from tests/test-coroutine.c. > And in fact profiling immediately showed something to optimise: > pthread_getspecific() was quite high, replacing it by __thread on > systems where it works is more efficient and helped the numbers a bit. > Also, a lot of time seems to be spent in pthread_mutex_lock/unlock (even > in qemu-img bench), maybe there's even something that can be done here.
The lock/unlock in dataplane is often from memory_region_find(), and Paolo should have done lots of work on that. > > However, I just wasn't sure whether a change on this level would be > relevant in a realistic environment. This is the reason why I wanted to > get a benchmark involving the block layer and some I/O. > >> From the profiling data in below link: >> >> http://pastebin.com/YwH2uwbq >> >> With coroutine, the running time for same loading is increased >> ~50%(1.325s vs. 0.903s), and dcache load events is increased >> ~35%(693M vs. 512M), insns per cycle is decreased by ~50%( >> 1.35 vs. 1.63), compared with bypassing coroutine(-b parameter). >> >> The bypass code in the benchmark is very similar with the approach >> used in the bypass patch, since linux-aio with O_DIRECT seldom >> blocks in the the kernel I/O path. >> >> Maybe the benchmark is a bit extremely, but given modern storage >> device may reach millions of IOPS, and it is very easy to slow down >> the I/O by coroutine. > > I think in order to optimise coroutines, such benchmarks are fair game. > It's just not guaranteed that the effects are exactly the same on real > workloads, so we should take the results with a grain of salt. > > Anyhow, the coroutine version of your benchmark is buggy, it leaks all > coroutines instead of exiting them, so it can't make any use of the > coroutine pool. On my laptop, I get this (where fixed coroutine is a > version that simply removes the yield at the end): > > | bypass | fixed coro | buggy coro > ----------------+---------------+---------------+-------------- > time | 1.09s | 1.10s | 1.62s > L1-dcache-loads | 921,836,360 | 932,781,747 | 1,298,067,438 > insns per cycle | 2.39 | 2.39 | 1.90 > > Begs the question whether you see a similar effect on a real qemu and > the coroutine pool is still not big enough? With correct use of > coroutines, the difference seems to be barely measurable even without > any I/O involved. When I comment qemu_coroutine_yield(), looks result of bypass and fixed coro is very similar as your test, and I am just wondering if stack is always switched in qemu_coroutine_enter() without calling qemu_coroutine_yield(). Without the yield, the benchmark can't emulate coroutine usage in bdrv_aio_readv/writev() path any more, and bypass in the patchset skips two qemu_coroutine_enter() and one qemu_coroutine_yield() for each bdrv_aio_readv/writev(). > >> > I played a bit with the following, I hope it's not too naive. I couldn't >> > see a difference with your patches, but at least one reason for this is >> > probably that my laptop SSD isn't fast enough to make the CPU the >> > bottleneck. Haven't tried ramdisk yet, that would probably be the next >> > thing. (I actually wrote the patch up just for some profiling on my own, >> > not for comparing throughput, but it should be usable for that as well.) >> >> This might not be good for the test since it is basically a sequential >> read test, which can be optimized a lot by kernel. And I always use >> randread benchmark. > > Yes, I shortly pondered whether I should implement random offsets > instead. But then I realised that a quicker kernel operation would only > help the benchmark because we want it to test the CPU consumption in > userspace. So the faster the kernel gets, the better for us, because it > should make the impact of coroutines bigger. OK, I will compare coroutine vs. bypass-co with the benchmark. Thanks,
