[dpdk-dev] [PATCH] net/i40e: add additional prefetch instructions for bulk rx

[Ananyev, Konstantin]

Hi Vladislav,

> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Vladyslav Buslov
> Sent: Monday, October 10, 2016 6:06 PM
> To: Wu, Jingjing <jingjing.wu at intel.com>; Yigit, Ferruh <ferruh.yigit at intel.com>; Zhang, Helin <helin.zhang at intel.com>
> Cc: dev at dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] net/i40e: add additional prefetch instructions for bulk rx
> 
> > -----Original Message-----
> > From: Wu, Jingjing [mailto:jingjing.wu at intel.com]
> > Sent: Monday, October 10, 2016 4:26 PM
> > To: Yigit, Ferruh; Vladyslav Buslov; Zhang, Helin
> > Cc: dev at dpdk.org
> > Subject: RE: [dpdk-dev] [PATCH] net/i40e: add additional prefetch
> > instructions for bulk rx
> >
> >
> >
> > > -----Original Message-----
> > > From: Yigit, Ferruh
> > > Sent: Wednesday, September 14, 2016 9:25 PM
> > > To: Vladyslav Buslov <vladyslav.buslov at harmonicinc.com>; Zhang, Helin
> > > <helin.zhang at intel.com>; Wu, Jingjing <jingjing.wu at intel.com>
> > > Cc: dev at dpdk.org
> > > Subject: Re: [dpdk-dev] [PATCH] net/i40e: add additional prefetch
> > > instructions for bulk rx
> > >
> > > On 7/14/2016 6:27 PM, Vladyslav Buslov wrote:
> > > > Added prefetch of first packet payload cacheline in
> > > > i40e_rx_scan_hw_ring Added prefetch of second mbuf cacheline in
> > > > i40e_rx_alloc_bufs
> > > >
> > > > Signed-off-by: Vladyslav Buslov <vladyslav.buslov at harmonicinc.com>
> > > > ---
> > > >  drivers/net/i40e/i40e_rxtx.c | 7 +++++--
> > > >  1 file changed, 5 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/net/i40e/i40e_rxtx.c
> > > > b/drivers/net/i40e/i40e_rxtx.c index d3cfb98..e493fb4 100644
> > > > --- a/drivers/net/i40e/i40e_rxtx.c
> > > > +++ b/drivers/net/i40e/i40e_rxtx.c
> > > > @@ -1003,6 +1003,7 @@ i40e_rx_scan_hw_ring(struct i40e_rx_queue
> > *rxq)
> > > >                 /* Translate descriptor info to mbuf parameters */
> > > >                 for (j = 0; j < nb_dd; j++) {
> > > >                         mb = rxep[j].mbuf;
> > > > +                       rte_prefetch0(RTE_PTR_ADD(mb->buf_addr,
> > > RTE_PKTMBUF_HEADROOM));
> >
> > Why did prefetch here? I think if application need to deal with packet, it is
> > more suitable to put it in application.
> >
> > > >                         qword1 = rte_le_to_cpu_64(\
> > > >                                 rxdp[j].wb.qword1.status_error_len);
> > > >                         pkt_len = ((qword1 &
> > > I40E_RXD_QW1_LENGTH_PBUF_MASK) >>
> > > > @@ -1086,9 +1087,11 @@ i40e_rx_alloc_bufs(struct i40e_rx_queue
> > *rxq)
> > > >
> > > >         rxdp = &rxq->rx_ring[alloc_idx];
> > > >         for (i = 0; i < rxq->rx_free_thresh; i++) {
> > > > -               if (likely(i < (rxq->rx_free_thresh - 1)))
> > > > +               if (likely(i < (rxq->rx_free_thresh - 1))) {
> > > >                         /* Prefetch next mbuf */
> > > > -                       rte_prefetch0(rxep[i + 1].mbuf);
> > > > +                       rte_prefetch0(&rxep[i + 1].mbuf->cacheline0);
> > > > +                       rte_prefetch0(&rxep[i + 1].mbuf->cacheline1);

I think there are rte_mbuf_prefetch_part1/part2 defined in rte_mbuf.h,
specially for that case.

> > > > +               }
> > Agree with this change. And when I test it by testpmd with iofwd, no
> > performance increase is observed but minor decrease.
> > Can you share will us when it will benefit the performance in your scenario ?
> >
> >
> > Thanks
> > Jingjing
> 
> Hello Jingjing,
> 
> Thanks for code review.
> 
> My use case: We have simple distributor thread that receives packets from port and distributes them among worker threads according to
> VLAN and MAC address hash.
> 
> While working on performance optimization we determined that most of CPU usage of this thread is in DPDK.
> As and optimization we decided to switch to rx burst alloc function, however that caused additional performance degradation compared to
> scatter rx mode.
> In profiler two major culprits were:
>   1. Access to packet data Eth header in application code. (cache miss)
>   2. Setting next packet descriptor field to NULL in DPDK i40e_rx_alloc_bufs code. (this field is in second descriptor cache line that was not
> prefetched)

I wonder what will happen if we'll remove any prefetches here?
Would it make things better or worse (and by how much)?

> After applying my fixes performance improved compared to scatter rx mode.
> 
> I assumed that prefetch of first cache line of packet data belongs to DPDK because it is done in scatter rx mode. (in
> i40e_recv_scattered_pkts)
> It can be moved to application side but IMO it is better to be consistent across all rx modes.

I would agree with Jingjing here, probably PMD should avoid to prefetch packet's data. 
Konstantin