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

[Joyce Kong <Joyce.Kong@arm.com>]

> -----Original Message-----
> From: Joyce Kong
> Sent: Thursday, May 19, 2022 3:00 PM
> To: Ferruh Yigit <ferruh.yigit@xilinx.com>; Jakub Grajciar <jgrajcia@cisco.com>
> Cc: Ruifeng Wang <Ruifeng.Wang@arm.com>; dev@dpdk.org; nd
> <nd@arm.com>
> Subject: RE: [PATCH v1 1/2] net/memif: add a Rx fast path
> 
> Hi Ferruh,
> 
> > -----Original Message-----
> > From: Ferruh Yigit <ferruh.yigit@xilinx.com>
> > Sent: Thursday, May 19, 2022 12:53 AM
> > To: Joyce Kong <Joyce.Kong@arm.com>; Jakub Grajciar
> > <jgrajcia@cisco.com>
> > Cc: Ruifeng Wang <Ruifeng.Wang@arm.com>; dev@dpdk.org; nd
> <nd@arm.com>
> > Subject: Re: [PATCH v1 1/2] net/memif: add a Rx fast path
> >
> > 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?
> >
> For memif zero-copy mode, only client runs 'eth_memif_rx_zc', and server still
> runs 'eth_memif_rx', so the patch would impacts zero-copy mode.
> 
> > 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?
> >
> From memif run to run, run.pkt_buffer_size would change, and
> cfg.pkt_buffer_size which is the reserved max buffer size would not change.
> For patch details, I use cfg.pkt_buffer_size to implement the 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?
> >
> Yes, I think the gain performance case is more common case, as the default
> memif buffer size equals to mbuf size. In theory, when memif buf size >= mbuf
> size, the Rx runs the original path, it would not lead to obvious impact.
> 
> >
> > 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?

Yes, the patch leads to the behavior change, and I will highlight more in the commit
log for next version.
This change is the same as zero-copy mode does, reducing complexed comparation
with more memory space. I am also looking forward to get some feedback from the
community about the tradeoff.

Thanks,
Joyce