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? > 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
