Re: Still not sexy! (Re: sendfile+zerocopy: fairly sexy (nothing todowith ECN)

[jamal <hadi@cyberus.ca>]

On Tue, 30 Jan 2001, Rick Jones wrote:

> > > How does ZC/SG change the nature of the packets presented to the NIC?
> >
> > what do you mean? I am _sure_ you know how SG/ZC work. So i am suspecting
> > more than socratic view on life here. Could be influence from Aristotle;->
>
> Well, I don't know  the specifics of Linux, but I gather from what I've
> read on the list thusfar, that prior to implementing SG support, Linux
> NIC drivers would copy packets into single contiguous buffers that were
> then sent to the NIC yes?
>

yes.

> If so, the implication is with SG going, that copy no longer takes
> place, and so a chain of buffers is given to the NIC.
>

yes.

> Also, if one is fully ZC :) pesky things like protocol headers can
> naturally end-up in separate buffers.
>

yes.

> So, now you have to ask how well any given NIC follows chains of
> buffers. At what number of buffers is the overhead in the NIC of
> following the chains enough to keep it from achieving link-rate?
>

hmmm... not sure how you would enforce this today or why you would
want that. Alexey, Dave?
The kernel should be able to break it into two buffers(with netperf,
for example -- header + data).
Ok, probably with tux-http 3 (header, data, trailler).

> One way to try and deduce that would be to meld some of the SG and preSG
> behaviours and copy packets into varying numbers of buffers per packet
> and measure the resulting impact on throughput through the NIC.
>

If only time were on my hands i'd love to do this. Alas.
NOTE also, that effect would also be an effect of the specif NIC.

> rick jones
>
> As time marches on, the orders of magnitude of the constants may change,
> but basic concepts still remain, and the "lessons" learned in the past
> by one generation tend to get relearned in the next :) for example -
> there is no such a thing as a free lunch... :)

;->
BTW, i am reading one of your papers (circa 1993 ;->, "we go fast with a
little help from your apps")  in which you make an interesting
observation. That (figure 2) there is "a considerable increase in
efficiency but not a considerable increase in throughput" .... I "scanned"
to the end of the paper and dont see an explanation.
I've made a somehow similar observation with the current zc patches and
infact observed that throughput goes down with the linux zc patches.
[This is being contested but no-one else is testing at gigE, so my word is
the only truth].
Of course your paper doesnt talk about sendfile rather the page pinning +
COW tricks (which are considered taboo in Linux) but i do sense a
relationship.

cheers,
jamal

PS:- I dont have "my" machines yet and i have a feeling it will be a while
before i re-run the tests; however, i have created a patch for
linux-sendfile with netperf. Please take a look at it at:
http://www.cyberus.ca/~hadi/patch-nperf-sfile-linux.gz
tell me if is missing anything and if it is ok, could you please merge in
your tree?



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