Re: [PATCH] mbuf: reduce pktmbuf init cycles

-----Original Message-----

Date: Fri, 23 Jun 2017 11:42:30 +0200
From: Olivier Matz <redacted>
To: Jerin Jacob <redacted>
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 [off-list ref] wrote:

quoted

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 <redacted>
---
 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