| Message ID | 20090904204442.GA30941@lst.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Christoph Hellwig <hch@lst.de> writes: > It seems like the addition of QUEUE_FLAG_VIRT caueses major performance > regressions for Fedora users: > > https://bugzilla.redhat.com/show_bug.cgi?id=509383 > https://bugzilla.redhat.com/show_bug.cgi?id=505695 > > while I can't reproduce those extreme regressions myself I think the flag > is wrong. > > Rationale: > > QUEUE_FLAG_VIRT expands to QUEUE_FLAG_NONROT which casus the queue > unplugged immediately. This is not a good behaviour for at least > qemu and kvm where we do have significant overhead for every > I/O operations. Even with all the latested speeups (native AIO, > MSI support, zero copy) we can only get native speed for up to 128kb > I/O requests we already are down to 66% of native performance for 4kb > requests even on my laptop running the Intel X25-M SSD for which the > QUEUE_FLAG_NONROT was designed. > If we ever get virtio-blk overhead low enough that this flag makes > sense it should only be set based on a feature flag set by the host. I agree with that rationale. Acked-by: Jeff Moyer <jmoyer@redhat.com> -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Index: linux-2.6/drivers/block/virtio_blk.c =================================================================== --- linux-2.6.orig/drivers/block/virtio_blk.c 2009-09-04 17:33:48.802523987 -0300 +++ linux-2.6/drivers/block/virtio_blk.c 2009-09-04 17:33:56.186522158 -0300 @@ -314,7 +314,6 @@ static int __devinit virtblk_probe(struc } vblk->disk->queue->queuedata = vblk; - queue_flag_set_unlocked(QUEUE_FLAG_VIRT, vblk->disk->queue); if (index < 26) { sprintf(vblk->disk->disk_name, "vd%c", 'a' + index % 26);
It seems like the addition of QUEUE_FLAG_VIRT caueses major performance regressions for Fedora users: https://bugzilla.redhat.com/show_bug.cgi?id=509383 https://bugzilla.redhat.com/show_bug.cgi?id=505695 while I can't reproduce those extreme regressions myself I think the flag is wrong. Rationale: QUEUE_FLAG_VIRT expands to QUEUE_FLAG_NONROT which casus the queue unplugged immediately. This is not a good behaviour for at least qemu and kvm where we do have significant overhead for every I/O operations. Even with all the latested speeups (native AIO, MSI support, zero copy) we can only get native speed for up to 128kb I/O requests we already are down to 66% of native performance for 4kb requests even on my laptop running the Intel X25-M SSD for which the QUEUE_FLAG_NONROT was designed. If we ever get virtio-blk overhead low enough that this flag makes sense it should only be set based on a feature flag set by the host. Signed-off-by: Christoph Hellwig <hch@lst.de> -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html