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SubjectRe: zero-copy TCP


On Sun, 3 Sep 2000, Jamie Lokier wrote:
>
> Nice point! Only valid for TCP & UDP though.

Yeah. But "we need oxygen" is only a valid point for carbon-based
life-forms. You might as well argue that oxygen is not avalid criteria for
being livable, because it's only valid for the particular kind of
creatures we are.

Basically, only TCP and UDP really matter. Decnet, IPX, etc don't really
make a big selling point any more.

> When people want _real_ low latency, they don't use TCP or UDP, and they
> certainly don't put data checksums at the start. They still aim for
> zero copies. That pass, even over cached data, is still significant.

I disagree.

Look at history.

Exercise 1: name a protocol that did something like that
(yes, I know, there are multiple).

Exercise 2: name one of them that is still relevant today.

See? Performance, in the end, is very much secondary. It doesn't matter
one whit if you perform better than everybody else, if you cannot _talk_
to everybody else.

I think the RISC vendors found that out. And I think most network vendors
find that out.

(Yes, I know, you're probably talking about things like the networking
protocols for clusters etc. I'm just saying that historically such
special-purpose stuff always tends to end up being not as good as the
"real thing".)

> Fair enough. Please read my description of a zero-copy scheme that
> doesn't require much intelligence on the card though. I think it's a
> neat kernel trick that might just pay off. Sometimes, maybe.

We could certainly try to do better. But some of the scemes I've seen have
implied a lot of complexity for gains that aren't actually real in the end
(eg playing expensive games with memory mapping in order to avoid a copy
that ends up happening anyway because the particular card you're using
doesn't do scatter-gather: you'd perform a lot better if you just did the
copy outright and forgot about the expensive games - which is what Linux
does).

Linus

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