[dpdk-dev] mbuf next field belongs in the first cacheline

[Bruce Richardson]

On Tue, Jun 15, 2021 at 02:16:27PM +0200, Morten Brørup wrote:
> MBUF and MLX5 maintainers,
> 
> I'm picking up an old discussion, which you might consider pursuing. Feel free to ignore, if you consider this discussion irrelevant or already closed and done with.
> 
> The Techboard has previously discussed the organization of the mbuf fields. Ref: http://mails.dpdk.org/archives/dev/2020-November/191859.html
> 
> It was concluded that there was no measured performance difference if the "pool" or "next" field was in the first cacheline, so it was decided to put the "pool" field in the first cacheline. And further optimizing the mbuf field organization could be reconsidered later.
> 
> I have been looking at it. In theory it should not be required to touch the "pool" field at RX. But the "next" field must be written for segmented packets.
> 
Question: are there cases where segmented packets are used, but they aren't
big packets, and so need a high packets-per-second value? The thinking when
designing the mbuf was that any application which could handle high packets
per second for medium/small packets would be fine with a few extra cycles
penalty for big ones, since the overall PPS for the driver would be much
lower.

/Bruce