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

[Olivier MATZ <olivier.matz-pdR9zngts4EAvxtiuMwx3w@public.gmane.org>]

Hi Jeff,

Thank you for your comment.

On 05/09/2014 05:39 PM, Shaw, Jeffrey B wrote:
> have you tested this patch to see if there is a negative impact to
> performance?

Yes, but not with testpmd. I passed our internal non-regression
performance tests and it shows no difference (or below the error
margin), even with low overhead processing like forwarding whatever
the number of cores I use.

> Wouldn't the processor have to mask the high bytes of the physical
> address when it is used, for example, to populate descriptors with
> buffer addresses?  When compute bound, this could steal CPU cycles
> away from packet processing.  I think we should understand the
> performance trade-off in order to save these 2 bytes.

I would naively say that the cost is negligible: accessing to the
length is the same as before (it's a 16 bits field) and accessing
the physical address is just a mask or a shift, which should not
be very long on an Intel processor (1 cycle?). This is to be
compared with the number of cycles per packet in io-fwd mode,
which is probably around 150 or 200.

> It would be interesting to see how throughput is impacted when the
> workload is core-bound.  This could be accomplished by running testpmd
> in io-fwd mode across 4x 10G ports.

I agree, this is something we could check. If you agree, let's first
wait for some other comments and see if we find a consensus on the
patches.

Regards,
Olivier