Avi Kivity wrote: > >Perhaps. This raises another point about AIO vs. threads: > > > >If I submit sequential O_DIRECT reads with aio_read(), will they enter > >the device read queue in the same order, and reach the disk in that > >order (allowing for reordering when worthwhile by the elevator)? > > Yes, unless the implementation in the kernel (or glibc) is threaded.
> > >With threads this isn't guaranteed and scheduling makes it quite > >likely to issue the parallel synchronous reads out of order, and for > >them to reach the disk out of order because the elevator doesn't see > >them simultaneously. > > If the disk is busy, it doesn't matter. The requests will queue and the > elevator will sort them out. So it's just the first few requests that > may get to disk out of order. There's two cases where it matters to a read-streaming app: 1. Disk isn't busy with anything else, maximum streaming performance is desired. 2. Disk is busy with unrelated things, but you're using I/O priorities to give the streaming app near-absolute priority. Then you need to maintain overlapped streaming requests, otherwise disk is given to a lower priority I/O. If that happens often, you lose, priority is ineffective. Because one of the streaming requests is usually being serviced, elevator has similar limitations as for a disk which is not busy with anything else. > I haven't considered tape, but this is a good point indeed. I expect it > doesn't make much of a difference for a loaded disk. Yes, as long as it's loaded with unrelated requests at the same I/O priority, the elevator has time to sort requests and hide thread scheduling artifacts. Btw, regarding QEMU: QEMU gets requests _after_ sorting by the guest's elevator, then submits them to the host's elevator. If the guest and host elevators are both configured 'anticipatory', do the anticipatory delays add up? -- Jamie ------------------------------------------------------------------------- This SF.net email is sponsored by the 2008 JavaOne(SM) Conference Don't miss this year's exciting event. There's still time to save $100. Use priority code J8TL2D2. http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel