Re: [dpdk-dev] [memnic PATCH v2 6/7] pmd: add branch hint in recv/xmit

[Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>]

Hi,

> Subject: Re: [dpdk-dev] [memnic PATCH v2 6/7] pmd: add branch hint in recv/xmit
> 
> On Tue, Sep 30, 2014 at 11:14:40AM +0000, Hiroshi Shimamoto wrote:
> > From: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
> >
> > To reduce instruction cache miss, add branch condition hints into
> > recv/xmit functions. This improves a bit performance.
> >
> > We can see performance improvements with memnic-tester.
> > Using Xeon E5-2697 v2 @ 2.70GHz, 4 vCPU.
> >  size |  before  |  after
> >    64 | 5.54Mpps | 5.55Mpps
> >   128 | 5.46Mpps | 5.44Mpps
> >   256 | 5.21Mpps | 5.22Mpps
> >   512 | 4.50Mpps | 4.52Mpps
> >  1024 | 3.71Mpps | 3.73Mpps
> >  1280 | 3.21Mpps | 3.22Mpps
> >  1518 | 2.92Mpps | 2.93Mpps
> >
> > Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
> > Reviewed-by: Hayato Momma <h-momma@ce.jp.nec.com>
> > ---
> >  pmd/pmd_memnic.c | 18 +++++++++---------
> >  1 file changed, 9 insertions(+), 9 deletions(-)
> >
> > diff --git a/pmd/pmd_memnic.c b/pmd/pmd_memnic.c
> > index 7fc3093..875d3ea 100644
> > --- a/pmd/pmd_memnic.c
> > +++ b/pmd/pmd_memnic.c
> > @@ -289,26 +289,26 @@ static uint16_t memnic_recv_pkts(void *rx_queue,
> >  	int idx, next;
> >  	struct rte_eth_stats *st = &adapter->stats[rte_lcore_id()];
> >
> > -	if (!adapter->nic->hdr.valid)
> > +	if (unlikely(!adapter->nic->hdr.valid))
> >  		return 0;
> >
> >  	pkts = bytes = errs = 0;
> >  	idx = adapter->up_idx;
> >  	for (nr = 0; nr < nb_pkts; nr++) {
> >  		p = &data->packets[idx];
> > -		if (p->status != MEMNIC_PKT_ST_FILLED)
> > +		if (unlikely(p->status != MEMNIC_PKT_ST_FILLED))
> >  			break;
> >  		/* prefetch the next area */
> >  		next = idx;
> > -		if (++next >= MEMNIC_NR_PACKET)
> > +		if (unlikely(++next >= MEMNIC_NR_PACKET))
> >  			next = 0;
> >  		rte_prefetch0(&data->packets[next]);
> > -		if (p->len > framesz) {
> > +		if (unlikely(p->len > framesz)) {
> >  			errs++;
> >  			goto drop;
> >  		}
> >  		mb = rte_pktmbuf_alloc(adapter->mp);
> > -		if (!mb)
> > +		if (unlikely(!mb))
> >  			break;
> >
> >  		rte_memcpy(rte_pktmbuf_mtod(mb, void *), p->data, p->len);
> > @@ -350,7 +350,7 @@ static uint16_t memnic_xmit_pkts(void *tx_queue,
> >  	uint64_t pkts, bytes, errs;
> >  	uint32_t framesz = adapter->framesz;
> >
> > -	if (!adapter->nic->hdr.valid)
> > +	if (unlikely(!adapter->nic->hdr.valid))
> >  		return 0;
> >
> >  	pkts = bytes = errs = 0;
> > @@ -360,7 +360,7 @@ static uint16_t memnic_xmit_pkts(void *tx_queue,
> >  		struct rte_mbuf *sg;
> >  		void *ptr;
> >
> > -		if (pkt_len > framesz) {
> > +		if (unlikely(pkt_len > framesz)) {
> >  			errs++;
> >  			break;
> >  		}
> > @@ -379,7 +379,7 @@ retry:
> >  			goto retry;
> >  		}
> >
> > -		if (idx != ACCESS_ONCE(adapter->down_idx)) {
> > +		if (unlikely(idx != ACCESS_ONCE(adapter->down_idx))) {
> Why are you using ACCESS_ONCE here?  Or for that matter, anywhere else in this
> PMD?  The whole idea of the ACCESS_ONCE macro is to assign a value to a variable
> once and prevent it from getting reloaded from memory at a later time, this is
> exactly contrary to that, both in the sense that you're explicitly reloading the
> same variable multiple times, and that you're using it as part of a comparison
> operation, rather than an asignment operation

ACCESS_ONCE prevents compiler optimization and ensures load from memory.
There could be multiple threads which read/write that index.
We should compare the value previous and the current value in memory.
In that reason, I use ACCESS_ONCE macro to get value in the memory.

thanks,
Hiroshi

> 
> Neil