Re: [dpdk-dev] [PATCH] mbuf: reduce pktmbuf init cycles

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

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.

Olivier