On Wed, Sep 07, 2022 at 02:08:18PM +0000, Parav Pandit wrote: > > > From: Michael S. Tsirkin <mst@xxxxxxxxxx> > > Sent: Wednesday, September 7, 2022 5:27 AM > > > > On Wed, Sep 07, 2022 at 04:08:54PM +0800, Gavin Li wrote: > > > > > > On 9/7/2022 1:31 PM, Michael S. Tsirkin wrote: > > > > External email: Use caution opening links or attachments > > > > > > > > > > > > On Thu, Sep 01, 2022 at 05:10:38AM +0300, Gavin Li wrote: > > > > > Currently add_recvbuf_big() allocates MAX_SKB_FRAGS segments for > > > > > big packets even when GUEST_* offloads are not present on the > > device. > > > > > However, if guest GSO is not supported, it would be sufficient to > > > > > allocate segments to cover just up the MTU size and no further. > > > > > Allocating the maximum amount of segments results in a large waste > > > > > of buffer space in the queue, which limits the number of packets > > > > > that can be buffered and can result in reduced performance. > > > > actually how does this waste space? Is this because your device does not > > have INDIRECT? > VQ is 256 entries deep. > Driver posted total of 256 descriptors. > Each descriptor points to a page of 4K. > These descriptors are chained as 4K * 16. So without indirect then? with indirect each descriptor can point to 16 entries. > So total packets that can be serviced are 256/16 = 16. > So effective queue depth = 16. > > So, when GSO is off, for 9K mtu, packet buffer needed = 3 pages. (12k). > So, 13 descriptors (= 13 x 4K =52K) per packet buffer is wasted. > > After this improvement, these 13 descriptors are available, increasing the effective queue depth = 256/3 = 85. > > [..] > > > > > > > > > > MTU(Bytes)/Bandwidth (Gbit/s) > > > > > Before After > > > > > 1500 22.5 22.4 > > > > > 9000 12.8 25.9 > > > > > > is this buffer space? > Above performance numbers are showing improvement in bandwidth. In Gbps/sec. > > > just the overhead of allocating/freeing the buffers? > > of using INDIRECT? > The effective queue depth is so small, device cannot receive all the packets at given bw-delay product. > > > > > > > > > Which configurations were tested? > > > I tested it with DPDK vDPA + qemu vhost. Do you mean the feature set > > > of the VM? > > > The configuration of interest is mtu, not the backend. > Which is different mtu as shown in above perf numbers. > > > > > Did you test devices without VIRTIO_NET_F_MTU ? > > > No. It will need code changes. > No. It doesn't need any code changes. This is misleading/vague. > > This patch doesn't have any relation to a device which doesn't offer VIRTIO_NET_F_MTU. > Just the code restructuring is touching this area, that may require some existing tests. > I assume virtio tree will have some automation tests for such a device? I have some automated tests but I also expect developer to do testing. > > > > > > > > > > @@ -3853,12 +3866,10 @@ static int virtnet_probe(struct > > > > > virtio_device *vdev) > > > > > > > > > > dev->mtu = mtu; > > > > > dev->max_mtu = mtu; > > > > > - > > > > > - /* TODO: size buffers correctly in this case. */ > > > > > - if (dev->mtu > ETH_DATA_LEN) > > > > > - vi->big_packets = true; > > > > > } > > > > > > > > > > + virtnet_set_big_packets_fields(vi, mtu); > > > > > + > > > > If VIRTIO_NET_F_MTU is off, then mtu is uninitialized. > > > > You should move it to within if () above to fix. > > > mtu was initialized to 0 at the beginning of probe if VIRTIO_NET_F_MTU > > > is off. > > > > > > In this case, big_packets_num_skbfrags will be set according to guest gso. > > > > > > If guest gso is supported, it will be set to MAX_SKB_FRAGS else > > > zero---- do you > > > > > > think this is a bug to be fixed? > > > > > > yes I think with no mtu this should behave as it did historically. > > > Michael is right. > It should behave as today. There is no new bug introduced by this patch. > dev->mtu and dev->max_mtu is set only when VIRTIO_NET_F_MTU is offered with/without this patch. > > Please have mtu related fix/change in different patch. > > > > > > > > > > if (vi->any_header_sg) > > > > > dev->needed_headroom = vi->hdr_len; > > > > > > > > > > -- > > > > > 2.31.1 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
