Re: [dpdk-dev] [PATCH RFC 05/11] mbuf: merge physaddr and buf_len in a bitfield

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

Hi Konstantin,

On 05/14/2014 04:07 PM, Ananyev, Konstantin wrote:
> Apart from performance impact, one more concern:
> As I know, theoretical limit for PA on Intel is 52 bits.
> I understand that these days no-one using more than 48 bits and it probably would stay like that for next few years.
> Though if we'll occupy these (MAXPHYADDR - 48) bits now, it can become a potential problem in future.
> After all the savings from that changes are not that big - only 2 bytes.
> As I understand you already save extra 7 bytes with other proposed modifications of mbuf.
> That's enough to add TSO related information into the mbuf.
> So my suggestion would be to keep phys_addr 64bit long.

I think that 2 bytes is a good save and will probably be useful in
a near future, as it allows us to store more info in one cache line of
64 bytes. On the other hand, the 48 bits limit (256 Tera bytes) will
probably be reached in several years (probably 10).

I agree the patch is not useful today, so I'm ok to remove it if nobody
feels it useful to keep 4 bytes for future use. In my opinion, these
4 bytes will soon be use for some new feature, looking at linux skb
shows that there is plenty of useful information that could go in
a network packet descriptor (timestamp, queue id, protocol, qos, ...).

Regards,
Olivier