From: Maxime Coquelin <maxime.coquelin@redhat.com>
To: Tiwei Bie <tiwei.bie@intel.com>
Cc: dev@dpdk.org, i.maximets@samsung.com, zhihong.wang@intel.com,
jfreiman@redhat.com, mst@redhat.com
Subject: Re: [dpdk-dev] [PATCH v2] vhost: batch used descs chains write-back with packed ring
Date: Thu, 20 Dec 2018 10:27:46 +0100 [thread overview]
Message-ID: <543cf4ff-4712-da50-8a26-d51d6dfaa8d7@redhat.com> (raw)
In-Reply-To: <c3a2f456-1e53-21ec-e55c-0a2143620607@redhat.com>
On 12/20/18 9:49 AM, Maxime Coquelin wrote:
>
>
> On 12/20/18 5:44 AM, Tiwei Bie wrote:
>> On Wed, Dec 19, 2018 at 10:29:52AM +0100, Maxime Coquelin wrote:
>>> Instead of writing back descriptors chains in order, let's
>>> write the first chain flags last in order to improve batching.
>>>
>>> With Kernel's pktgen benchmark, ~3% performance gain is measured.
>>>
>>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
>>> ---
>>>
>>> V2:
>>> Revert back to initial implementation to have a write
>>> barrier before every descs flags store, but still
>>> store first desc flags last. (Missing barrier reported
>>> by Ilya)
>>>
>>>
>>> lib/librte_vhost/virtio_net.c | 19 ++++++++++++++++---
>>> 1 file changed, 16 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/lib/librte_vhost/virtio_net.c
>>> b/lib/librte_vhost/virtio_net.c
>>> index 8c657a101..de436af79 100644
>>> --- a/lib/librte_vhost/virtio_net.c
>>> +++ b/lib/librte_vhost/virtio_net.c
>>> @@ -97,6 +97,8 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
>>> {
>>> int i;
>>> uint16_t used_idx = vq->last_used_idx;
>>> + uint16_t head_idx = vq->last_used_idx;
>>> + uint16_t head_flags = 0;
>>> /* Split loop in two to save memory barriers */
>>> for (i = 0; i < vq->shadow_used_idx; i++) {
>>> @@ -126,12 +128,17 @@ flush_shadow_used_ring_packed(struct virtio_net
>>> *dev,
>>> flags &= ~VRING_DESC_F_AVAIL;
>>> }
>>> - vq->desc_packed[vq->last_used_idx].flags = flags;
>>> + if (i > 0) {
>>> + vq->desc_packed[vq->last_used_idx].flags = flags;
>>> - vhost_log_cache_used_vring(dev, vq,
>>> + vhost_log_cache_used_vring(dev, vq,
>>> vq->last_used_idx *
>>> sizeof(struct vring_packed_desc),
>>> sizeof(struct vring_packed_desc));
>>> + } else {
>>> + head_idx = vq->last_used_idx;
>>> + head_flags = flags;
>>> + }
>>> vq->last_used_idx += vq->shadow_used_packed[i].count;
>>> if (vq->last_used_idx >= vq->size) {
>>> @@ -140,7 +147,13 @@ flush_shadow_used_ring_packed(struct virtio_net
>>> *dev,
>>> }
>>> }
>>> - rte_smp_wmb();
>>> + vq->desc_packed[head_idx].flags = head_flags;
>>> +
>>> + vhost_log_cache_used_vring(dev, vq,
>>> + vq->last_used_idx *
>>
>> Should be head_idx.
>
> Oh yes, thanks for spotting this.
>
>>
>>> + sizeof(struct vring_packed_desc),
>>> + sizeof(struct vring_packed_desc));
>>> +
>>> vq->shadow_used_idx = 0;
>>
>> A wmb() is needed before log_cache_sync?
>
> I think you're right, I was wrong but thought we had a barrier in cache
> sync function.
> That's not very important for x86, but I think it should be preferable
> to do it in vhost_log_cache_sync(), if logging is enabled.
>
> What do you think?
I'll keep it in this function for now, as I think we cannot remove the
one in the split variant so it would mean having two barriers in that
case.
>>> vhost_log_cache_sync(dev, vq);
>>> }
>>> --
>>> 2.17.2
>>>
next prev parent reply other threads:[~2018-12-20 9:27 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-19 9:29 Maxime Coquelin
2018-12-19 16:43 ` Michael S. Tsirkin
2018-12-20 4:44 ` Tiwei Bie
2018-12-20 8:49 ` Maxime Coquelin
2018-12-20 9:27 ` Maxime Coquelin [this message]
2018-12-20 14:03 ` Michael S. Tsirkin
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