Re: [RFC PATCH] net/virtio: Align Virtio-net header on cache line in receive path

-----Original Message-----
From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Yuanhan Liu
Sent: Wednesday, February 22, 2017 9:38 AM
To: Maxime Coquelin <redacted>
Cc: Liang, Cunming <redacted>; Tan, Jianfeng
[off-list ref]; dev@dpdk.org
Subject: Re: [dpdk-dev] [RFC PATCH] net/virtio: Align Virtio-net header on
cache line in receive path

On Tue, Feb 21, 2017 at 06:32:43PM +0100, Maxime Coquelin wrote:

quoted

This patch aligns the Virtio-net header on a cache-line boundary to
optimize cache utilization, as it puts the Virtio-net header (which is
always accessed) on the same cache line as the packet header.

For example with an application that forwards packets at L2 level, a
single cache-line will be accessed with this patch, instead of two
before.

I'm assuming you were testing pkt size <= (64 - hdr_size)?

quoted

In case of multi-buffers packets, next segments will be aligned on a
cache-line boundary, instead of cache-line boundary minus size of vnet
header before.

The another thing is, this patch always makes the pkt data cache unaligned
for the first packet, which makes Zhihong's optimization on memcpy (for big
packet) useless.

Why not could  we keep pkt data starting always on  the cache-line boundary?
In case of multi-buffer, the first remains unchanged, next segments can do as
Maxime said that.

Thanks
Zhiyong