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 don't think avoiding coroutines is possible without bypassing the block layer altogether because everything is really expecting to be run in coroutine context. Kevin
