From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH] virtio_blk: merge S/G list entries by default Date: Sun, 7 Sep 2014 14:41:53 +0300 Message-ID: <20140907114153.GB26569@redhat.com> References: <1410044994-14943-1-git-send-email-hch@lst.de> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <1410044994-14943-1-git-send-email-hch@lst.de> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Christoph Hellwig Cc: Jens Axboe , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org 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 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