On Fri, 23 Sep 2016 21:22:23 +0300 "Michael S. Tsirkin" <mst at redhat.com> wrote:
> On Fri, Sep 23, 2016 at 08:16:09PM +0200, Maxime Coquelin wrote: > > > > > > On 09/23/2016 08:06 PM, Michael S. Tsirkin wrote: > > > On Fri, Sep 23, 2016 at 08:02:27PM +0200, Maxime Coquelin wrote: > > > > > > > > > > > > On 09/23/2016 05:49 PM, Michael S. Tsirkin wrote: > > > > > On Fri, Sep 23, 2016 at 10:28:23AM +0200, Maxime Coquelin wrote: > > > > > > Indirect descriptors are usually supported by virtio-net devices, > > > > > > allowing to dispatch a larger number of requests. > > > > > > > > > > > > When the virtio device sends a packet using indirect descriptors, > > > > > > only one slot is used in the ring, even for large packets. > > > > > > > > > > > > The main effect is to improve the 0% packet loss benchmark. > > > > > > A PVP benchmark using Moongen (64 bytes) on the TE, and testpmd > > > > > > (fwd io for host, macswap for VM) on DUT shows a +50% gain for > > > > > > zero loss. > > > > > > > > > > > > On the downside, micro-benchmark using testpmd txonly in VM and > > > > > > rxonly on host shows a loss between 1 and 4%.i But depending on > > > > > > the needs, feature can be disabled at VM boot time by passing > > > > > > indirect_desc=off argument to vhost-user device in Qemu. > > > > > > > > > > Even better, change guest pmd to only use indirect > > > > > descriptors when this makes sense (e.g. sufficiently > > > > > large packets). > > > > With the micro-benchmark, the degradation is quite constant whatever > > > > the packet size. > > > > > > > > For PVP, I could not test with larger packets than 64 bytes, as I don't > > > > have a 40G interface, > > > > > > Don't 64 byte packets fit in a single slot anyway? > > No, indirect is used. I didn't checked in details, but I think this is > > because there is no headroom reserved in the mbuf. > > > > This is the condition to meet to fit in a single slot: > > /* optimize ring usage */ > > if (vtpci_with_feature(hw, VIRTIO_F_ANY_LAYOUT) && > > rte_mbuf_refcnt_read(txm) == 1 && > > RTE_MBUF_DIRECT(txm) && > > txm->nb_segs == 1 && > > rte_pktmbuf_headroom(txm) >= hdr_size && > > rte_is_aligned(rte_pktmbuf_mtod(txm, char *), > > __alignof__(struct virtio_net_hdr_mrg_rxbuf))) > > can_push = 1; > > else if (vtpci_with_feature(hw, VIRTIO_RING_F_INDIRECT_DESC) && > > txm->nb_segs < VIRTIO_MAX_TX_INDIRECT) > > use_indirect = 1; > > > > I will check more in details next week. > > Two thoughts then > 1. so can some headroom be reserved? > 2. how about using indirect with 3 s/g entries, > but direct with 2 and down? The default mbuf allocator does keep headroom available. Sounds like a test bug.
