Il 07/09/2014 13:41, Michael S. Tsirkin ha scritto: > On Sat, Sep 06, 2014 at 04:09:54PM -0700, Christoph Hellwig wrote: >> Most virtio setups have a fairly limited number of ring entries available. > > Seems a bit vague: QEMU at least has pretty large queues. > Which hypervisor do you have in mind? > This could be a gain everywhere if you manage to make descriptors > completely linear, so they fit in a single s/g. > ATM __virtblk_add_req always adds an s/g for the header: > is there a chance linux can pre-allocate a bit of memory > in front of the buffer to stick the header in? Nope, the buffer usually comes directly from the page cache and will be page aligned. Paolo >> Enable S/G entry merging by default to fit into less of them. This restores >> the behavior at time of the virtio-blk blk-mq conversion, which was changed >> by commit "block: add queue flag for disabling SG merging" which made the >> behavior optional, but didn't update the existing drivers to keep their >> previous behavior. >> >> Signed-off-by: Christoph Hellwig <hch@xxxxxx> > > OK so this is an optimization patch right? > What kind of performance gain is observed with it? > >> --- >> drivers/block/virtio_blk.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c >> index 0a58140..311b857 100644 >> --- a/drivers/block/virtio_blk.c >> +++ b/drivers/block/virtio_blk.c >> @@ -634,7 +634,7 @@ static int virtblk_probe(struct virtio_device *vdev) >> vblk->tag_set.ops = &virtio_mq_ops; >> vblk->tag_set.queue_depth = virtblk_queue_depth; >> vblk->tag_set.numa_node = NUMA_NO_NODE; >> - vblk->tag_set.flags = BLK_MQ_F_SHOULD_MERGE; >> + vblk->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_SG_MERGE; >> vblk->tag_set.cmd_size = >> sizeof(struct virtblk_req) + >> sizeof(struct scatterlist) * sg_elems; >> -- >> 1.9.1 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
