On Wed, 2013-11-13 at 15:10 +0800, Jason Wang wrote:
> There's one concern with EWMA. How well does it handle multiple streams
> each with different packet size? E.g there may be two flows, one with
> 256 bytes each packet another is 64K. Looks like it can result we
> allocate PAGE_SIZE buffer for 256 (which is bad since the
> payload/truesize is low) bytes or 1500+ for 64K buffer (which is ok
> since we can do coalescing).
It's hard to predict the future ;)
256 bytes frames consume 2.5 KB anyway on a traditional NIC.
If it was a concern, we would have it already.
If you receive a mix of big and small frames, there is no win.
> > + if (page) {
> > + est_buffer_len = page_private(page);
> > + if (est_buffer_len > len) {
> > + u32 truesize_delta = est_buffer_len - len;
> > +
> > + curr_skb->truesize += truesize_delta;
> > + if (curr_skb != head_skb)
> > + head_skb->truesize += truesize_delta;
> > + }
>
> Is there a chance that est_buffer_len was smaller than or equal with len?
Yes, and in this case we do not really care, see below.
> > + }
> > + ewma_add(&rq->mrg_avg_pkt_len, head_skb->len);
> > return 0;
> > }
> >
> > @@ -382,16 +404,21 @@ static void receive_buf(struct receive_queue *rq,
> > void *buf, unsigned int len)
> > skb_trim(skb, len);
> > } else if (vi->mergeable_rx_bufs) {
> > struct page *page = virt_to_head_page(buf);
> > - int truesize = max_t(int, len, MERGE_BUFFER_LEN);
> > + /* Use an initial truesize of 'len' bytes for page_to_skb --
> > + * receive_mergeable will fixup the truesize of the last page
> > + * frag if the packet is non-linear (> GOOD_COPY_LEN bytes).
> > + */
> > skb = page_to_skb(rq, page,
> > (char *)buf - (char *)page_address(page),
> > - len, truesize);
> > + len, len);
> > if (unlikely(!skb)) {
> > dev->stats.rx_dropped++;
> > put_page(page);
> > return;
> > }
> > - if (receive_mergeable(rq, skb)) {
> > + if (!skb_is_nonlinear(skb))
> > + page = NULL;
> > + if (receive_mergeable(rq, skb, page)) {
> > dev_kfree_skb(skb);
> > return;
> > }
> > @@ -540,24 +567,29 @@ static int add_recvbuf_big(struct receive_queue *rq,
> > gfp_t gfp)
> > static int add_recvbuf_mergeable(struct receive_queue *rq, gfp_t gfp)
> > {
> > struct virtnet_info *vi = rq->vq->vdev->priv;
> > + const size_t hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
> > struct page_frag *alloc_frag;
> > char *buf;
> > - int err, len, hole;
> > + int err, hole;
> > + u32 buflen;
> >
> > + buflen = hdr_len + clamp_t(u32, ewma_read(&rq->mrg_avg_pkt_len),
> > + GOOD_PACKET_LEN, PAGE_SIZE - hdr_len);
> > + buflen = ALIGN(buflen, L1_CACHE_BYTES);
> > alloc_frag = (gfp & __GFP_WAIT) ? &vi->sleep_frag : &rq->atomic_frag;
> > - if (unlikely(!skb_page_frag_refill(MERGE_BUFFER_LEN, alloc_frag, gfp)))
> > + if (unlikely(!skb_page_frag_refill(buflen, alloc_frag, gfp)))
> > return -ENOMEM;
> > buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset;
> > get_page(alloc_frag->page);
> > - len = MERGE_BUFFER_LEN;
> > - alloc_frag->offset += len;
> > + alloc_frag->offset += buflen;
> > + set_page_private(alloc_frag->page, buflen);
>
> Not sure this is accurate, since buflen may change and several frags may
> share a single page. So the est_buffer_len we get in receive_mergeable()
> may not be the correct value.
skb->truesize has to be reasonably accurate.
For example, fast clone storage is not accounted for in TCP skbs stored
in socket write queues. Thats ~256 bytes per skb of 'missing'
accounting.
This is about 10% error when TSO/GSO is off.
With this EWMA using a factor of 64, the potential error will be much
less than 10%.
Small frames tend to be consumed quite fast (ACK messages, small UDP
frames) in most cases.
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