Re: [dpdk-dev] [PATCH] prefetch second cacheline of mbufs on alloc

[Olivier Matz <olivier.matz@6wind.com>]

On Wed, 15 Feb 2017 09:44:35 +0100, Thomas Monjalon
<thomas.monjalon@6wind.com> wrote:
> Do we need to discuss again the prefetch calls inside DPDK
> or can we definitely close this kind of request?
> 	mbuf: http://dpdk.org/patch/4678/
> 	ethdev: http://dpdk.org/patch/8867/
> 

About the mbuf prefetch, I suggest the topic can be closed:
The rte_pkt_mbuf_alloc() function is not necessarily called in a place
where we will write the second cache line, so the prefetch is not always
useful. Having prefetches in generic functions does not look to be a
good idea, especially in that case, knowing it's easy to do it in the
application.


Olivier


> 
> 2015-07-20 10:02, Olivier MATZ:
> > Hi Thomas,
> > 
> > 
> > On 07/20/2015 03:00 AM, Thomas Monjalon wrote:  
> > > Please Olivier,
> > > What is the status of this patch?  
> > 
> > From what I remember, the last mail was a comment from Konstantin
> > on another thread (but same topic):
> > http://dpdk.org/ml/archives/dev/2015-May/017633.html
> > 
> > 
> > Regards,
> > Olivier
> > 
> >   
> > > 
> > > 2015-05-12 01:15, Paul Emmerich:  
> > >> this improves the throughput of a simple tx-only application by
> > >> 11% in the full-featured ixgbe tx path and by 14% in the simple
> > >> tx path. ---
> > >>  lib/librte_mbuf/rte_mbuf.h | 1 +
> > >>  1 file changed, 1 insertion(+)
> > >>
> > >> diff --git a/lib/librte_mbuf/rte_mbuf.h
> > >> b/lib/librte_mbuf/rte_mbuf.h index ab6de67..f6895b4 100644
> > >> --- a/lib/librte_mbuf/rte_mbuf.h
> > >> +++ b/lib/librte_mbuf/rte_mbuf.h
> > >> @@ -538,6 +538,7 @@ static inline struct rte_mbuf
> > >> *__rte_mbuf_raw_alloc(struct rte_mempool *mp) if
> > >> (rte_mempool_get(mp, &mb) < 0) return NULL;
> > >>  	m = (struct rte_mbuf *)mb;
> > >> +	rte_prefetch0(&m->cacheline1);
> > >>  	RTE_MBUF_ASSERT(rte_mbuf_refcnt_read(m) == 0);
> > >>  	rte_mbuf_refcnt_set(m, 1);
> > >>  	return (m);
> > >>  
> > > 
> > >   
> 
>