Kevin Wolf writes:
> Am 15.03.2019 um 16:33 hat Sergio Lopez geschrieben: >> >> Stefan Hajnoczi writes: >> >> > On Thu, Mar 14, 2019 at 06:31:34PM +0100, Sergio Lopez wrote: >> >> Our current AIO path does a great job at unloading the work from the VM, >> >> and combined with IOThreads provides a good performance in most >> >> scenarios. But it also comes with its costs, in both a longer execution >> >> path and the need of the intervention of the scheduler at various >> >> points. >> >> >> >> There's one particular workload that suffers from this cost, and that's >> >> when you have just 1 or 2 cores on the Guest issuing synchronous >> >> requests. This happens to be a pretty common workload for some DBs and, >> >> in a general sense, on small VMs. >> >> >> >> I did a quick'n'dirty implementation on top of virtio-blk to get some >> >> numbers. This comes from a VM with 4 CPUs running on an idle server, >> >> with a secondary virtio-blk disk backed by a null_blk device with a >> >> simulated latency of 30us. >> > >> > Can you describe the implementation in more detail? Does "synchronous" >> > mean that hw/block/virtio_blk.c makes a blocking preadv()/pwritev() call >> > instead of calling blk_aio_preadv/pwritev()? If so, then you are also >> > bypassing the QEMU block layer (coroutines, request tracking, etc) and >> > that might explain some of the latency. >> >> The first implementation, the one I've used for getting these numbers, >> it's just preadv/pwrite from virtio_blk.c, as you correctly guessed. I >> know it's unfair, but I wanted to take a look at the best possible >> scenario, and then measure the cost of the other layers. >> >> I'm working now on writing non-coroutine counterparts for >> blk_co_[preadv|pwrite], so we have SIO without bypassing the block layer. > > Maybe try to keep the change local to file-posix.c? I think you would > only have to modify raw_thread_pool_submit() so that it doesn't go > through the thread pool, but just calls func directly. I already tried something similar, but I'd like to explore the possibility of avoiding the coroutine/aio_poll dance to trim down another ~10us. If it's deemed to be too complex or hard to maintain, we can always fall back to something simpler. Thanks, Sergio.
