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SubjectRe: zero-copy TCP fileserving

I think I agree with you here -- in fact, had I known that copy
avoidance via IO-lite like buffer management was forthcoming, I
would not have chimed in. I like Linux's growth and potential,
and I was told that influential people were arguing against copy
avoidance on the grounds that it doesn't matter. The current
code base and the message to which I reacted both fairly strongly
supported that impression.

If what was really being said is simply that copy avoidance matters
most in the important (but certainly not all-encompassing) class
of lightly-processed (per-byte) data movement activities, then I
agree and I look forward to the new copy-eliminating buffer
management support.


> > While I applaud Linus for sticking to his philosophical guns, I hope
> > that few people are compelled to ignore 20 years of networking and
> > OS research (and practice) based on his misinformed commentary.
> A while ago I also did the research. Linus is right for most cases, and the
> cases that matter otherwise are streaming existing data - ie sendfile. Thats
> all you really need to be zero copy. That and message passing, which is a
> different game (there its almost pure latency not bandwidth) but with the
> same needs.
> > server -- although it did not support CGI scripts (much like NetApp's
> > servers don't), it did in fact do HTTP/1.0 for real. Far more importantly,
> > though, web service can in fact benefit significantly from zero-copy
> > techniques. If you choose to ignore the lessons taught by the exokernel
> Static web serving is shipping canned responses off disk (or hopefully out
> of disk cache). NFS is canned responses, video streaming is canned
> responses. All of this is 'slap on a header and dump the disk to the
> network card'. Everything else is cache optimisation.
> Thus Im not actually sure you are disagreeing, just your definition
> of 'canned' is different.
> > work, perhaps you will be more compelled by the more recent Rice work
> > (IO-lite, which won Best Paper at the recent OSDI'99 conference).
> IO-lite type stuff is on the 2.3 plan - thats why Stephen is working on the
> kiovec stuff.
> > Further, other domains (e.g., IPC, high-speed I/O, cluster computing)
> > benefit significantly from zero-copy cross-domain transfers. There
> > are any number of research projects (e.g., U-net, Fbufs) and industry
> > efforts (e.g., VIA, SiliconTCP) that clearly demonstrate the importance
> > of copy avoidance.
> They are almost entirely based on message passing. Anyone can do zero
> copy message passing with clever hardware. Thats what VI architecture is
> (dont call it VIA, they are a chip manufacturer and quite fed up of being
> confused)
> Alan

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