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Date: Mon, 23 Dec 2013 14:51:23 +0200
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Michael Dalton <mwdalton@...gle.com>
Cc: "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
Eric Dumazet <edumazet@...gle.com>,
Rusty Russell <rusty@...tcorp.com.au>,
Jason Wang <jasowang@...hat.com>,
virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH net-next 3/3] net: auto-tune mergeable rx buffer size for
improved performance
On Mon, Dec 16, 2013 at 04:16:29PM -0800, 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.
>
> 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 w/ virtio_net before 2613af0ed18a (PAGE_SIZE bufs): 14642.85Gb/s
> net-next (MTU-size bufs): 13170.01Gb/s
> net-next + auto-tune: 14555.94Gb/s
>
> Signed-off-by: Michael Dalton <mwdalton@...gle.com>
OK so a high level benchmark shows it's worth it,
but how well does the logic work?
I think we should make the buffer size accessible in sysfs
or debugfs, and look at it, otherwise we don't really know.
> ---
> drivers/net/virtio_net.c | 63 +++++++++++++++++++++++++++++++++++-------------
> 1 file changed, 46 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index d38d130..904af37 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -26,6 +26,7 @@
> #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);
> @@ -36,11 +37,15 @@ 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
>
> +/* Weight used for the RX packet size EWMA. The average packet size is used to
> + * determine the packet buffer size when refilling RX rings. As the entire RX
> + * ring may be refilled at once, the weight is chosen so that the EWMA will be
> + * insensitive to short-term, transient changes in packet size.
> + */
> +#define RECEIVE_AVG_WEIGHT 64
> +
> #define VIRTNET_DRIVER_VERSION "1.0.0"
>
> struct virtnet_stats {
> @@ -78,6 +83,9 @@ 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;
>
> @@ -339,13 +347,11 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> int num_buf = hdr->mhdr.num_buffers;
> struct page *page = virt_to_head_page(buf);
> int offset = buf - page_address(page);
> - int truesize = max_t(int, len, MERGE_BUFFER_LEN);
> - struct sk_buff *head_skb = page_to_skb(rq, page, offset, len, truesize);
> + struct sk_buff *head_skb = page_to_skb(rq, page, offset, len, len);
> struct sk_buff *curr_skb = head_skb;
>
> if (unlikely(!curr_skb))
> goto err_skb;
> -
Don't like this chunk :)
> while (--num_buf) {
> int num_skb_frags;
>
> @@ -374,23 +380,40 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> head_skb->truesize += nskb->truesize;
> num_skb_frags = 0;
> }
> - truesize = max_t(int, len, MERGE_BUFFER_LEN);
> if (curr_skb != head_skb) {
> head_skb->data_len += len;
> head_skb->len += len;
> - head_skb->truesize += truesize;
> + head_skb->truesize += len;
> }
> offset = buf - 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);
> }
> }
>
> + /* 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.
I don't get the real motivation for this.
We have skbs A,B,C sharing a page, with chunk D being unused.
This randomly charges chunk D to an skb that ended up last
in the page.
Correct?
Why does this make sense?
> + * 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 the explanation for the above is that we don't care where D is
charged, let's not charge it to any skbs.
> + */
> + if (skb_is_nonlinear(head_skb)) {
> + u32 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;
> + }
> + }
> + ewma_add(&rq->mrg_avg_pkt_len, head_skb->len);
Why head_skb only? Why not full buffer size that comes from host?
This is simply len.
> return head_skb;
>
> err_skb:
> @@ -578,24 +601,29 @@ 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);
> 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));
> @@ -1516,6 +1544,7 @@ 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));
> }
>
> --
> 1.8.5.1
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