Re: Improving PACKET_{RX,TX}_RING documentation

[Carsten Andrich <carsten.andrich-hs6bpBdVsEZfm0AUMx9V0g@public.gmane.org>]

Willem de Bruijn schrieb:
>>> I would describe such points in a positive manner (optimization) as
>>> opposed to a negative (inferior performance).
>>
>> Using positive wording is always a good idea, but packet_mmap.txt
>> already tricked me into believing that PACKET_TX_RING should be faster
>> than plain sendto(). The user should be allowed to make an informed
>> decision,
> 
> Indeed. The document should not contain any simple statements about
> one option being faster than another, because this invariably depends on
> workload details (packet size, rate, threading, ...).
> 
> Instead, it should just explain the technical details and their implications:
> an mmapped ring reduces the number of system calls, as does
> recvmmsg/sendmmsg. It does not necessarily reduce the number of
> data copies (a common misconception). Etcetera.
> 
>> which requires the manpage to tell the (ugly) truth that
>> sendto() currently outperforms TX_RING.
> 
> I would not make such statements either way, then.

You're right. I'll just a note regarding the necessity of careful
performance considerations/evaluations :)
Maybe, eventually, some of Jesper's findings regarding *_RING
performance should end up in packet_mmap.txt.

>>>> Absolutely, perhaps explaining differences from TPACKET_V1 -> V3 API and the
>>>> like.
>>>
>>> That would be very interesting. The packet -> block batching mechanism
>>> likely was tested with small packet performance, but may have little
>>> benefit for larger packets. A discussion of the trade offs from a user
>>> point of view would be very interesting.
>>
>> Actually I intended to deal only with TPACKET_V2 for now, since it is
>> simpler than TPACKET_V3 and can be use for RX and TX. TPACKET_V3 can be
>> added later on or could remain in packet_mmap.txt.
> 
> Sure, let's leave that.
> 
> Your plan sounds good to me, Carsten.

Okay, it might take me a few weeks to come up with a first draft.

Cheers,
Carsten
--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html