Re: [dpdk-dev] [Qemu-devel] [PATCH 1/2] vhost: enable any layout feature

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 09/29/2016 11:23 PM, Maxime Coquelin wrote:
>
>
> On 09/29/2016 10:21 PM, Michael S. Tsirkin wrote:
>> On Thu, Sep 29, 2016 at 10:05:22PM +0200, Maxime Coquelin wrote:
>>>
>>>
>>> On 09/29/2016 07:57 PM, Michael S. Tsirkin wrote:
>>>> On Thu, Sep 29, 2016 at 05:30:53PM +0200, Maxime Coquelin wrote:
>>> ...
>>>>>
>>>>> Before enabling anything by default, we should first optimize the 1
>>>>> slot
>>>>> case. Indeed, micro-benchmark using testpmd in txonly[0] shows ~17%
>>>>> perf regression for 64 bytes case:
>>>>>  - 2 descs per packet: 11.6Mpps
>>>>>  - 1 desc per packet: 9.6Mpps
>>>>>
>>>>> This is due to the virtio header clearing in virtqueue_enqueue_xmit().
>>>>> Removing it, we get better results than with 2 descs (1.20Mpps).
>>>>> Since the Virtio PMD doesn't support offloads, I wonder whether we can
>>>>> just drop the memset?
>>>>
>>>> What will happen? Will the header be uninitialized?
>>> Yes..
>>> I didn't look closely at the spec, but just looked at DPDK's and Linux
>>> vhost implementations. IIUC, the header is just skipped in the two
>>> implementations.
>>
>> In linux guest skbs are initialized AFAIK. See virtio_net_hdr_from_skb
>> first thing it does is
>>         memset(hdr, 0, sizeof(*hdr));
>
> I meant in vhost-net linux implementation, the header is just skipped:
>
> static void handle_tx(struct vhost_net *net)
> {
> ...
>         /* Skip header. TODO: support TSO. */
>         len = iov_length(vq->iov, out);
>         iov_iter_init(&msg.msg_iter, WRITE, vq->iov, out, len);
>         iov_iter_advance(&msg.msg_iter, hdr_size);
>
> And the same is done is done in DPDK:
>
> static inline int __attribute__((always_inline))
> copy_desc_to_mbuf(struct virtio_net *dev, struct vring_desc *descs,
>           uint16_t max_desc, struct rte_mbuf *m, uint16_t desc_idx,
>           struct rte_mempool *mbuf_pool)
> {
> ...
>     /*
>      * A virtio driver normally uses at least 2 desc buffers
>      * for Tx: the first for storing the header, and others
>      * for storing the data.
>      */
>     if (likely((desc->len == dev->vhost_hlen) &&
>            (desc->flags & VRING_DESC_F_NEXT) != 0)) {
>         desc = &descs[desc->next];
>         if (unlikely(desc->flags & VRING_DESC_F_INDIRECT))
>             return -1;
>
>         desc_addr = gpa_to_vva(dev, desc->addr);
>         if (unlikely(!desc_addr))
>             return -1;
>
>         rte_prefetch0((void *)(uintptr_t)desc_addr);
>
>         desc_offset = 0;
>         desc_avail  = desc->len;
>         nr_desc    += 1;
>
>         PRINT_PACKET(dev, (uintptr_t)desc_addr, desc->len, 0);
>     } else {
>         desc_avail  = desc->len - dev->vhost_hlen;
>         desc_offset = dev->vhost_hlen;
>     }

Actually, the header is parsed in DPDK vhost implementation.
But as Virtio PMD provides a zero'ed header, we could just parse
the header only if VIRTIO_NET_F_NO_TX_HEADER is not negotiated.

>>
>>
>>
>>>>
>>>> The spec says:
>>>>     The driver can send a completely checksummed packet. In this
>>>> case, flags
>>>>     will be zero, and gso_type
>>>>     will be VIRTIO_NET_HDR_GSO_NONE.
>>>>
>>>> and
>>>>     The driver MUST set num_buffers to zero.
>>>>     If VIRTIO_NET_F_CSUM is not negotiated, the driver MUST set
>>>> flags to
>>>>     zero and SHOULD supply a fully
>>>>     checksummed packet to the device.
>>>>
>>>> and
>>>>     If none of the VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO options have
>>>> been
>>>>     negotiated, the driver MUST
>>>>     set gso_type to VIRTIO_NET_HDR_GSO_NONE.
>>>>
>>>> so doing this unconditionally would be a spec violation, but if you see
>>>> value in this, we can add a feature bit.
>>> Right it would be a spec violation, so it should be done conditionally.
>>> If a feature bit is to be added, what about VIRTIO_NET_F_NO_TX_HEADER?
>>> It would imply VIRTIO_NET_F_CSUM not set, and no GSO features set.
>>> If negotiated, we wouldn't need to prepend a header.
>>
>> Yes but two points.
>>
>> 1. why is this memset expensive? Is the test completely skipping looking
>>    at the packet otherwise?
> Yes.
>>
>> 2. As long as we are doing this, see
>>     Alignment vs. Networking
>>     ========================
>> in Documentation/unaligned-memory-access.txt
> Thanks, I'll have a look tomorrow.

I did a rough prototype which removes Tx headers unconditionally, to
see what gain we could expect. I expect the results to be a little lower
with no headers in full implementation, as some more checks will have
to be done.

For PVP use-case with 0.05% acceptable packets loss:
  - Original (with headers): 9.43Mpps
  - Indirect descs: 9.36 Mpps
  - Prototype (no headers): 10.65Mpps

For PVP use-case with 0% acceptable packets loss:
  - Original (with headers): 5.23Mpps
  - Indirect descs: 7.13 Mpps
  - Prototype (no headers): 7.92Mpps

Maxime