On 5/3/2016 8:42 AM, Yuanhan Liu wrote:
> Pre update and update used ring in batch for Tx and Rx at the stage
> while fetching all avail desc idx. This would reduce some cache misses
> and hence, increase the performance a bit.
>
> Pre update would be feasible as guest driver will not start processing
> those entries as far as we don't update "used->idx". (I'm not 100%
> certain I don't miss anything, though).
>
> Cc: Michael S. Tsirkin <mst at redhat.com>
> Signed-off-by: Yuanhan Liu <yuanhan.liu at linux.intel.com>
> ---
> lib/librte_vhost/vhost_rxtx.c | 58
> +++++++++++++++++++++----------------------
> 1 file changed, 28 insertions(+), 30 deletions(-)
>
> diff --git a/lib/librte_vhost/vhost_rxtx.c b/lib/librte_vhost/vhost_rxtx.c
> index c9cd1c5..2c3b810 100644
> --- a/lib/librte_vhost/vhost_rxtx.c
> +++ b/lib/librte_vhost/vhost_rxtx.c
> @@ -137,7 +137,7 @@ copy_virtio_net_hdr(struct virtio_net *dev, uint64_t
> desc_addr,
>
> static inline int __attribute__((always_inline))
> copy_mbuf_to_desc(struct virtio_net *dev, struct vhost_virtqueue *vq,
> - struct rte_mbuf *m, uint16_t desc_idx, uint32_t *copied)
> + struct rte_mbuf *m, uint16_t desc_idx)
> {
> uint32_t desc_avail, desc_offset;
> uint32_t mbuf_avail, mbuf_offset;
> @@ -161,7 +161,6 @@ copy_mbuf_to_desc(struct virtio_net *dev, struct
> vhost_virtqueue *vq,
> desc_offset = dev->vhost_hlen;
> desc_avail = desc->len - dev->vhost_hlen;
>
> - *copied = rte_pktmbuf_pkt_len(m);
> mbuf_avail = rte_pktmbuf_data_len(m);
> mbuf_offset = 0;
> while (mbuf_avail != 0 || m->next != NULL) {
> @@ -262,6 +261,7 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t queue_id,
> struct vhost_virtqueue *vq;
> uint16_t res_start_idx, res_end_idx;
> uint16_t desc_indexes[MAX_PKT_BURST];
> + uint16_t used_idx;
> uint32_t i;
>
> LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__);
> @@ -285,27 +285,29 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t queue_id,
> /* Retrieve all of the desc indexes first to avoid caching issues. */
> rte_prefetch0(&vq->avail->ring[res_start_idx & (vq->size - 1)]);
> for (i = 0; i < count; i++) {
> - desc_indexes[i] = vq->avail->ring[(res_start_idx + i) &
> - (vq->size - 1)];
> + used_idx = (res_start_idx + i) & (vq->size - 1);
> + desc_indexes[i] = vq->avail->ring[used_idx];
> + vq->used->ring[used_idx].id = desc_indexes[i];
> + vq->used->ring[used_idx].len = pkts[i]->pkt_len +
> + dev->vhost_hlen;
> + vhost_log_used_vring(dev, vq,
> + offsetof(struct vring_used, ring[used_idx]),
> + sizeof(vq->used->ring[used_idx]));
> }
>
> rte_prefetch0(&vq->desc[desc_indexes[0]]);
> for (i = 0; i < count; i++) {
> uint16_t desc_idx = desc_indexes[i];
> - uint16_t used_idx = (res_start_idx + i) & (vq->size - 1);
> - uint32_t copied;
> int err;
>
> - err = copy_mbuf_to_desc(dev, vq, pkts[i], desc_idx, &copied);
> -
> - vq->used->ring[used_idx].id = desc_idx;
> - if (unlikely(err))
> + err = copy_mbuf_to_desc(dev, vq, pkts[i], desc_idx);
> + if (unlikely(err)) {
> + used_idx = (res_start_idx + i) & (vq->size - 1);
> vq->used->ring[used_idx].len = dev->vhost_hlen;
> - else
> - vq->used->ring[used_idx].len = copied + dev->vhost_hlen;
> - vhost_log_used_vring(dev, vq,
> - offsetof(struct vring_used, ring[used_idx]),
> - sizeof(vq->used->ring[used_idx]));
> + vhost_log_used_vring(dev, vq,
> + offsetof(struct vring_used, ring[used_idx]),
> + sizeof(vq->used->ring[used_idx]));
> + }
>
> if (i + 1 < count)
> rte_prefetch0(&vq->desc[desc_indexes[i+1]]);
> @@ -879,6 +881,7 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
> /* Prefetch available ring to retrieve head indexes. */
> used_idx = vq->last_used_idx & (vq->size - 1);
> rte_prefetch0(&vq->avail->ring[used_idx]);
> + rte_prefetch0(&vq->used->ring[used_idx]);
>
> count = RTE_MIN(count, MAX_PKT_BURST);
> count = RTE_MIN(count, free_entries);
> @@ -887,22 +890,23 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
>
> /* Retrieve all of the head indexes first to avoid caching issues. */
> for (i = 0; i < count; i++) {
> - desc_indexes[i] = vq->avail->ring[(vq->last_used_idx + i) &
> - (vq->size - 1)];
> + used_idx = (vq->last_used_idx + i) & (vq->size - 1);
> + desc_indexes[i] = vq->avail->ring[used_idx];
> +
> + vq->used->ring[used_idx].id = desc_indexes[i];
> + vq->used->ring[used_idx].len = 0;
> + vhost_log_used_vring(dev, vq,
> + offsetof(struct vring_used, ring[used_idx]),
> + sizeof(vq->used->ring[used_idx]));
> }
>
> /* Prefetch descriptor index. */
> rte_prefetch0(&vq->desc[desc_indexes[0]]);
> - rte_prefetch0(&vq->used->ring[vq->last_used_idx & (vq->size - 1)]);
> -
> for (i = 0; i < count; i++) {
> int err;
>
> - if (likely(i + 1 < count)) {
> + if (likely(i + 1 < count))
> rte_prefetch0(&vq->desc[desc_indexes[i + 1]]);
> - rte_prefetch0(&vq->used->ring[(used_idx + 1) &
> - (vq->size - 1)]);
> - }
>
> pkts[i] = rte_pktmbuf_alloc(mbuf_pool);
> if (unlikely(pkts[i] == NULL)) {
> @@ -916,18 +920,12 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
> rte_pktmbuf_free(pkts[i]);
> break;
> }
> -
> - used_idx = vq->last_used_idx++ & (vq->size - 1);
> - vq->used->ring[used_idx].id = desc_indexes[i];
> - vq->used->ring[used_idx].len = 0;
> - vhost_log_used_vring(dev, vq,
> - offsetof(struct vring_used, ring[used_idx]),
> - sizeof(vq->used->ring[used_idx]));
> }
Had tried post-updating used ring in batch, but forget the perf change.
One optimization would be on vhost_log_used_ring.
I have two ideas,
a) In QEMU side, we always assume use ring will be changed. so that we
don't need to log used ring in VHOST.
Michael: feasible in QEMU? comments on this?
b) We could always mark the total used ring modified rather than entry
by entry.
>
> rte_smp_wmb();
> rte_smp_rmb();
> vq->used->idx += i;
> + vq->last_used_idx += i;
> vhost_log_used_vring(dev, vq, offsetof(struct vring_used, idx),
> sizeof(vq->used->idx));
>
