Subject: Re: [PATCH net-next 4/4] virtio-net: auto-tune mergeable rx buffer size for improved performance

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 13 Nov 2013 19:42:45 +0200

On Wed, Nov 13, 2013 at 03:10:20PM +0800, Jason Wang wrote:
> On 11/13/2013 06:21 AM, Michael Dalton wrote:
> > Commit 2613af0ed18a ("virtio_net: migrate mergeable rx buffers to page frag
> > allocators") changed the mergeable receive buffer size from PAGE_SIZE to
> > MTU-size, introducing a single-stream regression for benchmarks with large
> > average packet size. There is no single optimal buffer size for all workloads.
> > For workloads with packet size <= MTU bytes, MTU + virtio-net header-sized
> > buffers are preferred as larger buffers reduce the TCP window due to SKB
> > truesize. However, single-stream workloads with large average packet sizes
> > have higher throughput if larger (e.g., PAGE_SIZE) buffers are used.
> >
> > This commit auto-tunes the mergeable receiver buffer packet size by choosing
> > the packet buffer size based on an EWMA of the recent packet sizes for the
> > receive queue. Packet buffer sizes range from MTU_SIZE + virtio-net header
> > len to PAGE_SIZE. This improves throughput for large packet workloads, as
> > any workload with average packet size >= PAGE_SIZE will use PAGE_SIZE
> > buffers.
> 
> Hi Michael:
> 
> 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).
> >
> > These optimizations interact positively with recent commit
> > ba275241030c ("virtio-net: coalesce rx frags when possible during rx"),
> > which coalesces adjacent RX SKB fragments in virtio_net. The coalescing
> > optimizations benefit buffers of any size.
> >
> > Benchmarks taken from an average of 5 netperf 30-second TCP_STREAM runs
> > between two QEMU VMs on a single physical machine. Each VM has two VCPUs
> > with all offloads & vhost enabled. All VMs and vhost threads run in a
> > single 4 CPU cgroup cpuset, using cgroups to ensure that other processes
> > in the system will not be scheduled on the benchmark CPUs. Trunk includes
> > SKB rx frag coalescing.
> >
> > net-next trunk w/ virtio_net before 2613af0ed18a (PAGE_SIZE bufs): 14642.85Gb/s
> > net-next trunk (MTU-size bufs):  13170.01Gb/s
> > net-next trunk + auto-tune: 14555.94Gb/s
> 
> Do you have perf numbers that just without this patch? We need to know
> how much EWMA help exactly.

Yes I'm curious too.

> >
> > Signed-off-by: Michael Dalton <[email-masked]>
> > ---
> >  drivers/net/virtio_net.c | 73 +++++++++++++++++++++++++++++++++++-------------
> >  1 file changed, 53 insertions(+), 20 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 0c93054..b1086e0 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -27,6 +27,7 @@
> >  #include <linux/if_vlan.h>
> >  #include <linux/slab.h>
> >  #include <linux/cpu.h>
> > +#include <linux/average.h>
> >  
> >  static int napi_weight = NAPI_POLL_WEIGHT;
> >  module_param(napi_weight, int, 0444);
> > @@ -37,10 +38,8 @@ module_param(gso, bool, 0444);
> >  
> >  /* FIXME: MTU in config. */
> >  #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
> > -#define MERGE_BUFFER_LEN (ALIGN(GOOD_PACKET_LEN + \
> > -                                sizeof(struct virtio_net_hdr_mrg_rxbuf), \
> > -                                L1_CACHE_BYTES))
> >  #define GOOD_COPY_LEN	128
> > +#define RECEIVE_AVG_WEIGHT 64
> 
> Maybe we can make this as a module parameter.

I'm not sure it's useful - no one is likely to tune it in practice.
But how about a comment explaining how was the number chosen?

> >  
> >  #define VIRTNET_DRIVER_VERSION "1.0.0"
> >  
> > @@ -79,6 +78,9 @@ struct receive_queue {
> >  	/* Chain pages by the private ptr. */
> >  	struct page *pages;
> >  
> > +	/* Average packet length for mergeable receive buffers. */
> > +	struct ewma mrg_avg_pkt_len;
> > +
> >  	/* Page frag for GFP_ATOMIC packet buffer allocation. */
> >  	struct page_frag atomic_frag;
> >  
> > @@ -302,14 +304,17 @@ static struct sk_buff *page_to_skb(struct receive_queue *rq,
> >  	return skb;
> >  }
> >  
> > -static int receive_mergeable(struct receive_queue *rq, struct sk_buff *head_skb)
> > +static int receive_mergeable(struct receive_queue *rq, struct sk_buff *head_skb,
> > +			     struct page *head_page)
> >  {
> >  	struct skb_vnet_hdr *hdr = skb_vnet_hdr(head_skb);
> >  	struct sk_buff *curr_skb = head_skb;
> > +	struct page *page = head_page;
> >  	char *buf;
> > -	struct page *page;
> > -	int num_buf, len, offset, truesize;
> > +	int num_buf, len, offset;
> > +	u32 est_buffer_len;
> >  
> > +	len = head_skb->len;
> >  	num_buf = hdr->mhdr.num_buffers;
> >  	while (--num_buf) {
> >  		int num_skb_frags = skb_shinfo(curr_skb)->nr_frags;
> > @@ -320,7 +325,6 @@ static int receive_mergeable(struct receive_queue *rq, struct sk_buff *head_skb)
> >  			head_skb->dev->stats.rx_length_errors++;
> >  			return -EINVAL;
> >  		}
> > -		truesize = max_t(int, len, MERGE_BUFFER_LEN);
> >  		if (unlikely(num_skb_frags == MAX_SKB_FRAGS)) {
> >  			struct sk_buff *nskb = alloc_skb(0, GFP_ATOMIC);
> >  			if (unlikely(!nskb)) {
> > @@ -338,20 +342,38 @@ static int receive_mergeable(struct receive_queue *rq, struct sk_buff *head_skb)
> >  		if (curr_skb != head_skb) {
> >  			head_skb->data_len += len;
> >  			head_skb->len += len;
> > -			head_skb->truesize += truesize;
> > +			head_skb->truesize += len;
> >  		}
> >  		page = virt_to_head_page(buf);
> >  		offset = buf - (char *)page_address(page);
> >  		if (skb_can_coalesce(curr_skb, num_skb_frags, page, offset)) {
> >  			put_page(page);
> >  			skb_coalesce_rx_frag(curr_skb, num_skb_frags - 1,
> > -					     len, truesize);
> > +					     len, len);
> >  		} else {
> >  			skb_add_rx_frag(curr_skb, num_skb_frags, page,
> > -					offset, len, truesize);
> > +					offset, len, len);
> >  		}
> >  		--rq->num;
> >  	}
> > +	/* All frags before the last frag are fully used -- for those frags,
> > +	 * truesize = len. Use the size of the most recent buffer allocation
> > +	 * from the last frag's page to estimate the truesize of the last frag.
> > +	 * EWMA with a weight of 64 makes the size adjustments quite small in
> > +	 * the frags allocated on one page (even a order-3 one), and truesize
> > +	 * doesn't need to be 100% accurate.
> > +	 */
> > +	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?
> > +	}
> > +	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.
> >  	hole = alloc_frag->size - alloc_frag->offset;
> > -	if (hole < MERGE_BUFFER_LEN) {
> > -		len += hole;
> > +	if (hole < buflen) {
> > +		buflen += hole;
> >  		alloc_frag->offset += hole;
> >  	}
> >  
> > -	sg_init_one(rq->sg, buf, len);
> > +	sg_init_one(rq->sg, buf, buflen);
> >  	err = virtqueue_add_inbuf(rq->vq, rq->sg, 1, buf, gfp);
> >  	if (err < 0)
> >  		put_page(virt_to_head_page(buf));
> > @@ -1475,6 +1507,7 @@ static int virtnet_alloc_queues(struct virtnet_info *vi)
> >  			       napi_weight);
> >  
> >  		sg_init_table(vi->rq[i].sg, ARRAY_SIZE(vi->rq[i].sg));
> > +		ewma_init(&vi->rq[i].mrg_avg_pkt_len, 1, RECEIVE_AVG_WEIGHT);
> >  		sg_init_table(vi->sq[i].sg, ARRAY_SIZE(vi->sq[i].sg));
> >  	}
> >