Re: [PATCH v1 1/2] net/memif: add a Rx fast path

[Ferruh Yigit <ferruh.yigit@xilinx.com>]

On 5/17/2022 11:51 AM, Joyce Kong wrote:
> For memif non-zero-copy mode, there is a branch to compare
> the mbuf and memif buffer size during memory copying. Add
> a fast memory copy path by removing this branch with mbuf
> and memif buffer size defined at compile time. The removal
> of the branch leads to considerable performance uplift.
> 
> When memif <= buffer size, Rx chooses the fast memcpy path,
> otherwise it would choose the original path.
> 
> Test with 1p1q on Ampere Altra AArch64 server,
> --------------------------------------------
>    buf size  | memif <= mbuf | memif > mbuf |
> --------------------------------------------
> non-zc gain |     4.30%     |    -0.52%    |
> --------------------------------------------
>     zc gain  |     2.46%     |     0.70%    |
> --------------------------------------------
> 
> Test with 1p1q on Cascade Lake Xeon X86server,
> -------------------------------------------
>    buf size  | memif <= mbuf | memif > mbuf |
> -------------------------------------------
> non-zc gain |     2.13%     |    -1.40%    |
> -------------------------------------------
>     zc gain  |     0.18%     |     0.48%    |
> -------------------------------------------
> 


Hi Joyce,

I have multiple questions,

1) The patch updates only non-zero-copy mode Rx path ('eth_memif_rx'), 
why zero-copy path performance also impacted?

2) As far as I can see there is a behavior change, more details below

3) patch talking about memif buffer size being defined in compile time, 
is the big "memif <= mbuf" if block optimized out?
Since 'pkt_buffer_size' is a devarg, so it can change from run to run 
and it is not known in compile time, I doubt that it is optimized out.
Is having  'pkt_buffer_size' as devarg breaks your logic?

4) One option gains performance and other loose performance, do you 
think gain performance case is more common use case? Is there any data 
around it?


Jakub,

Do you want to test this patch first before progressing with it?

> Signed-off-by: Joyce Kong <joyce.kong@arm.com>
> ---
>   drivers/net/memif/rte_eth_memif.c | 124 ++++++++++++++++++++----------
>   1 file changed, 84 insertions(+), 40 deletions(-)
> 
> diff --git a/drivers/net/memif/rte_eth_memif.c b/drivers/net/memif/rte_eth_memif.c
> index 587ad45576..f55776ca46 100644
> --- a/drivers/net/memif/rte_eth_memif.c
> +++ b/drivers/net/memif/rte_eth_memif.c
> @@ -342,66 +342,111 @@ eth_memif_rx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
>   		goto refill;
>   	n_slots = last_slot - cur_slot;
>   
> -	while (n_slots && n_rx_pkts < nb_pkts) {
> -		mbuf_head = rte_pktmbuf_alloc(mq->mempool);
> -		if (unlikely(mbuf_head == NULL))
> -			goto no_free_bufs;
> -		mbuf = mbuf_head;
> -		mbuf->port = mq->in_port;
> +	if (likely(mbuf_size >= pmd->cfg.pkt_buffer_size)) {
> +		while (n_slots && n_rx_pkts < nb_pkts) {
> +			mbuf_head = rte_pktmbuf_alloc(mq->mempool);
> +			if (unlikely(mbuf_head == NULL))
> +				goto no_free_bufs;
> +			mbuf = mbuf_head;
> +			mbuf->port = mq->in_port;
> +
> +next_slot1:
> +			s0 = cur_slot & mask;
> +			d0 = &ring->desc[s0];
>   
> -next_slot:
> -		s0 = cur_slot & mask;
> -		d0 = &ring->desc[s0];
> +			cp_len = d0->length;
>   
> -		src_len = d0->length;
> -		dst_off = 0;
> -		src_off = 0;
> +			rte_pktmbuf_data_len(mbuf) = cp_len;
> +			rte_pktmbuf_pkt_len(mbuf) = cp_len;
> +			if (mbuf != mbuf_head)
> +				rte_pktmbuf_pkt_len(mbuf_head) += cp_len;
>   
> -		do {
> -			dst_len = mbuf_size - dst_off;
> -			if (dst_len == 0) {
> -				dst_off = 0;
> -				dst_len = mbuf_size;
> +			rte_memcpy(rte_pktmbuf_mtod(mbuf, void *),
> +				(uint8_t *)memif_get_buffer(proc_private, d0), cp_len);
> +
> +			cur_slot++;
> +			n_slots--;
>   
> -				/* store pointer to tail */
> +			if (d0->flags & MEMIF_DESC_FLAG_NEXT) {
>   				mbuf_tail = mbuf;
>   				mbuf = rte_pktmbuf_alloc(mq->mempool);
>   				if (unlikely(mbuf == NULL))
>   					goto no_free_bufs;
> -				mbuf->port = mq->in_port;
>   				ret = memif_pktmbuf_chain(mbuf_head, mbuf_tail, mbuf);
>   				if (unlikely(ret < 0)) {
>   					MIF_LOG(ERR, "number-of-segments-overflow");
>   					rte_pktmbuf_free(mbuf);
>   					goto no_free_bufs;
>   				}
> +				goto next_slot1;
>   			}

It is very hard to comment on the correct part of the patch, since it is 
mixed a lot, but
- previously when memif buffer is segmented, and its size is less than 
mbuf; mbuf is filled with as much memif data as possible and later 
switched to next mbuf, like:

   memif buffer
+-+  +-+  +-+  +-+
|a|->|b|->|c|->|d|
+-+  +-+  +-+  +-+

+---+  +---+
|abc|->|d  |
+---+  +---+
   mbuf


- Now each memif segment is a mbuf,

   memif buffer
+-+  +-+  +-+  +-+
|a|->|b|->|c|->|d|
+-+  +-+  +-+  +-+

+---+  +---+  +---+  +---+
|a  |->|b  |->|c  |->|d  |
+---+  +---+  +---+  +---+
   mbuf

Can you please confirm this behavior change? If so can you please 
highlight is more in the commit log?
And is this tradeoff something preferred?