Carsten Otte wrote: > >>> System call latency adds to the in-kernel approach here. >> I don't understand this. > What I meant to state was: If the host side of the block driver runs > in userspace, we have the extra latency to leave the kernel system > call context, compute on behalf of the user process, and do another > system call (to drive the IO). This extra overhead does not show when > handling IO requests from the guest in the kernel. >
Well, this argument seems to be in favor of not using an fd ;) Actually, an fd is usable when storing a disk in a raw file. But qemu supports non-raw (formatted) disk images, which have additional features like snapshots. The fd alone does not contain enough information. > >> I/O may be slow, but you can have a lot more disks than cpus. >> >> For example, if an I/O takes 1ms, and you have 100 disks, then you >> can issue 100K IOPS. With one hypercall per request, that's ~50% of >> a cpu (at about 5us per hypercall that goes all the way to >> userspace). That's not counting the overhead of calling io_submit(). > Even when a hypercall round-trip takes as long as 5us, and even if you > have 512byte per biovec only (we use 4k blocksize), I don't see how > this gets a performance problem: > With linear read/write you get 200.000 hypercalls per second with 128 > kbyte per hypercall. That's 25.6 GByte per second per CPU. With random > read (worst case: 512 byte per hypercall) you still get 100 MByte per > second per CPU. There are tighter bottlenecks in the IO hardware afaics. > If all you do is I/O, sure. If you want to limit I/O cpu overhead to 10%, the raw bandwidth becomes 10 MB/s/cpu. (bandwidth isn't a good measure for random I/O, IOPS is the interesting metric) Both the guest and host will favor batched requests. It's a shame to deny that because of a simplistic protocol. -- error compiling committee.c: too many arguments to function ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
