Michael S. Tsirkin wrote: > On Tue, Mar 23, 2010 at 10:57:33AM -0700, Badari Pulavarty wrote: > >> Michael S. Tsirkin wrote: >> >>> On Mon, Mar 22, 2010 at 05:34:04PM -0700, Badari Pulavarty wrote: >>> >>> >>>> Write Results: >>>> ============== >>>> >>>> I see degraded IO performance when doing sequential IO write >>>> tests with vhost-blk compared to virtio-blk. >>>> >>>> # time dd of=/dev/vda if=/dev/zero bs=2M oflag=direct >>>> >>>> I get ~110MB/sec with virtio-blk, but I get only ~60MB/sec with >>>> vhost-blk. Wondering why ? >>>> >>>> >>> Try to look and number of interrupts and/or number of exits. >>> >>> >> I checked interrupts and IO exits - there is no major noticeable >> difference between >> vhost-blk and virtio-blk scenerios. >> >>> It could also be that you are overrunning some queue. >>> >>> I don't see any exit mitigation strategy in your patch: >>> when there are already lots of requests in a queue, it's usually >>> a good idea to disable notifications and poll the >>> queue as requests complete. That could help performance. >>> >>> >> Do you mean poll eventfd for new requests instead of waiting for new >> notifications ? >> Where do you do that in vhost-net code ? >> > > vhost_disable_notify does this. > > >> Unlike network socket, since we are dealing with a file, there is no >> ->poll support for it. >> So I can't poll for the data. And also, Issue I am having is on the >> write() side. >> > > Not sure I understand. > > >> I looked at it some more - I see 512K write requests on the >> virtio-queue in both vhost-blk and virtio-blk cases. Both qemu or >> vhost is doing synchronous writes to page cache (there is no write >> batching in qemu that is affecting this case). I still puzzled on >> why virtio-blk outperforms vhost-blk. >> >> Thanks, >> Badari >> > > If you say the number of requests is the same, we are left with: > - requests are smaller for some reason? > - something is causing retries? > No. IO requests sizes are exactly same (512K) in both cases. There are no retries or errors in both cases. One thing I am not clear is - for some reason guest kernel could push more data into virtio-ring in case of virtio-blk vs vhost-blk. Is this possible ? Does guest gets to run much sooner in virtio-blk case than vhost-blk ? Sorry, if its dumb question - I don't understand all the vhost details :( Thanks, Badari _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
