From: Yuanhan Liu <yuanhan.liu@linux.intel.com>
To: "Xie, Huawei" <huawei.xie@intel.com>
Cc: "dev@dpdk.org" <dev@dpdk.org>, "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [dpdk-dev] [PATCH 1/3] vhost: pre update used ring for Tx and Rx
Date: Wed, 1 Jun 2016 14:55:57 +0800 [thread overview]
Message-ID: <20160601065557.GB10038@yliu-dev.sh.intel.com> (raw)
In-Reply-To: <C37D651A908B024F974696C65296B57B4C7C01AF@SHSMSX101.ccr.corp.intel.com>
On Wed, Jun 01, 2016 at 06:40:41AM +0000, Xie, Huawei wrote:
> > /* 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.
I would assume pre-updating gives better performance gain, as we are
fiddling with avail and used ring together, which would be more cache
friendly.
> 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.
I doubt it's worthwhile. One fact is that vhost_log_used_ring is
a non operation in most time: it will take action only in the short
gap of during live migration.
And FYI, I even tried with all vhost_log_xxx being removed, it showed
no performance boost at all. Therefore, it's not a factor that will
impact performance.
--yliu
next prev parent reply other threads:[~2016-06-01 6:53 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-03 0:46 [dpdk-dev] [PATCH 0/3] [RFC] vhost: micro vhost optimization Yuanhan Liu
2016-05-03 0:46 ` [dpdk-dev] [PATCH 1/3] vhost: pre update used ring for Tx and Rx Yuanhan Liu
2016-06-01 6:40 ` Xie, Huawei
2016-06-01 6:55 ` Yuanhan Liu [this message]
2016-06-03 8:18 ` Xie, Huawei
2016-06-01 13:05 ` Michael S. Tsirkin
2016-05-03 0:46 ` [dpdk-dev] [PATCH 2/3] vhost: optimize dequeue for small packets Yuanhan Liu
2016-06-01 6:24 ` Xie, Huawei
2016-06-01 6:44 ` Yuanhan Liu
2016-06-03 7:42 ` Xie, Huawei
2016-06-03 7:43 ` Xie, Huawei
2016-05-03 0:46 ` [dpdk-dev] [PATCH 3/3] vhost: arrange virtio_net fields for better cache sharing Yuanhan Liu
2016-06-01 6:42 ` Xie, Huawei
2016-05-10 21:49 ` [dpdk-dev] [PATCH 0/3] [RFC] vhost: micro vhost optimization Rich Lane
2016-05-10 22:08 ` Yuanhan Liu
2016-06-14 12:42 ` Yuanhan Liu
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