ipv4: Optimize flow initialization in output route lookup.

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We burn a lot of useless cycles, cpu store buffer traffic, and
memory operations memset()'ing the on-stack flow used to perform
output route lookups in __ip_route_output_key().

Only the first half of the flow object members even matter for
output route lookups in this context, specifically:

FIB rules matching cares about:

	dst, src, tos, iif, oif, mark

FIB trie lookup cares about:

	dst

FIB semantic match cares about:

	tos, scope, oif

Therefore only initialize these specific members and elide the
memset entirely.

On Niagara2 this kills about ~300 cycles from the output route
lookup path.

Likely, we can take things further, since all callers of output
route lookups essentially throw away the on-stack flow they use.
So they don't care if we use it as a scratch-pad to compute the
final flow key.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>

...

This commit is contained in:

David S. Miller 2011-03-04 21:24:47 -08:00

parent 65e8354ec1

commit 44713b67db

1 changed files with 10 additions and 8 deletions

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net/ipv4/route.c

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