Re: [PATCH] mbuf: reduce pktmbuf init cycles

[Jerin Jacob <jerin.jacob@caviumnetworks.com>]

-----Original Message-----
> Date: Fri, 23 Jun 2017 11:42:30 +0200
> From: Olivier Matz <olivier.matz@6wind.com>
> To: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] mbuf: reduce pktmbuf init cycles
> X-Mailer: Claws Mail 3.14.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu)
> 
> On Mon,  5 Jun 2017 22:08:07 +0530, Jerin Jacob <jerin.jacob@caviumnetworks.com> wrote:
> > There is no need for initializing the complete
> > packet buffer with zero as the packet data area will be
> > overwritten by the NIC Rx HW anyway.
> > 
> > The testpmd configures the packet mempool
> > with around 180k buffers with
> > 2176B size. In existing scheme, the init routine
> > needs to memset around ~370MB vs the proposed scheme
> > requires only around ~44MB on 128B cache aligned system.
> > 
> > Useful in running DPDK in HW simulators/emulators,
> > where millions of cycles have an impact on boot time.
> > 
> > Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> > ---
> >  lib/librte_mbuf/rte_mbuf.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/lib/librte_mbuf/rte_mbuf.c b/lib/librte_mbuf/rte_mbuf.c
> > index 0e3e36a58..1d5ce7816 100644
> > --- a/lib/librte_mbuf/rte_mbuf.c
> > +++ b/lib/librte_mbuf/rte_mbuf.c
> > @@ -131,8 +131,7 @@ rte_pktmbuf_init(struct rte_mempool *mp,
> >  	RTE_ASSERT(mp->elt_size >= mbuf_size);
> >  	RTE_ASSERT(buf_len <= UINT16_MAX);
> >  
> > -	memset(m, 0, mp->elt_size);
> > -
> > +	memset(m, 0, mbuf_size + RTE_PKTMBUF_HEADROOM);
> >  	/* start of buffer is after mbuf structure and priv data */
> >  	m->priv_size = priv_size;
> >  	m->buf_addr = (char *)m + mbuf_size;
> 
> Yes, I don't foresee any risk to do that.
> 
> I'm just wondering why RTE_PKTMBUF_HEADROOM should be zeroed.
> For example, rte_pktmbuf_free() does not touch the data either, so
> after some packets processing, we also have garbage data in the
> headroom.

Yes. Headroom can be garbage as application pull the packet offset up and writes
new header on encapsulation use case.

I will the send v2 with clearing only mbuf area.

> 
> Olivier