* Pekka Enberg <penberg@xxxxxxxxxx> wrote: > Hi Ingo, > > On Thu, Jun 16, 2011 at 10:24 AM, Ingo Molnar <mingo@xxxxxxx> wrote: > > - executing AIO in the vcpu thread eats up precious vcpu execution > > time: combined QCOW2 throughput would be limited by a single > > core's performance, and any time spent on QCOW2 processing would > > not be spent running the guest CPU. (In such a model we certainly > > couldnt do more intelligent, CPU-intense storage solutions like on > > the fly compress/decompress of QCOW2 data.) > > Most image formats have optional on-the-fly > compression/decompression so we'd need to keep the current I/O > thread scheme anyway. Yeah - although high-performance setups will probably not use that. > > I'd only consider KAIO it if it provides some *real* measurable > > performance advantage of at least 10% in some important usecase. > > A few percent probably wouldnt be worth it. > > I've only been following AIO kernel development from the sidelines > but I really haven't seen any reports of significant gains over > read()/write() from a thread pool. Are there any such reports? I've measured such gains myself a couple of years ago, using an Oracle DB and a well-known OLTP benchmark, on a 64-way system. I also profiled+tuned the kernel-side AIO implementation to be more scalable so i'm reasonably certain that the gains exist, and they were above 10%. So the kaio gains existed back then but they needed sane userspace (POSIX AIO with signal notification sucks) and needed a well-tuned in-kernel implementation as well. (the current AIO code might have bitrotted) Also, synchronous read()/write() [and scheduler() :-)] scalability improvements have not stopped in the past few years so the performance picture might have shifted in favor of a thread pool. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
