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SubjectRe: zero-copy TCP fileserving
   From: Greg Ganger <ganger@gauss.ece.cmu.edu>
Date: Fri, 4 Jun 1999 16:25:32 -0400 (EDT)

As a result, those in industry who build these kinds of products
do, in fact, understand the importance of copy elimination, and
they spend significant energy to achieve it.

Guess where that energy goes? It goes into your latency, and that's a
fact. Take a look at the latency most of them get, it sucks.

Ask yourself, why can't such networking stacks move a byte through the
TCP stack, end to end, on the order of 100usec? It's because they
have all of the complexity in there to provide a zero copy framework.
For them it does cost something, even when you don't use it.

I have a hard time just blindly consuming the "increasing
computational speed vs. memory speed" argument, because that logic
leads just to bolting on more crap to the system and thus detracting
from the latency reduction which we should be realizing due to the
increased CPU power.

Furthermore, make no mistake, for transmit we will at some point have
a zero copy scheme available. But when we get it, it will be done
cheaply and in a well thought out manner. And you can be certain that
when it does happen, the end to end latency will not suffer like it
does on other systems for the cases where zero copy makes no sense at
all.

Linus knows what he is talking about, latency and simplicity are two
extremely important qualities to preserve.

Later,
David S. Miller
davem@redhat.com

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