rizzo at iet.unipi.it
Fri May 4 06:26:02 UTC 2012
On Fri, May 04, 2012 at 09:44:15AM +1000, Andrew Reilly wrote:
> On Wed, May 02, 2012 at 08:25:57PM +0200, Luigi Rizzo wrote:
> > as part of my netmap investigations, i was looking at how
> > expensive are memory copies, and here are a couple of findings
> > (first one is obvious, the second one less so)
> Most C compilers (well, the ones I regularly use) inline small,
> constant-length memcpy operations of the sort you're describing
> here. I would expect techniques like that to beat any amount of
> hand-tuning in a elf-linkage bcopy subroutine.
> Sure, you want a good implementation for your variable-length
> copies, and data layout and alignment is tremendously important
> these days, so there's no single silver bullet here.
The two things i was addressing on my message cannot be solved by
a compiler: the memcpy/bcopy has variable length in the places i
was looking at, and the compiler cannot infer that it is allowed
to extend the copy to full words or cache lines instead of stopping
at the exact boundary.
I don't even dare anymore to hand-optimize code: too many
times i have been beaten by the compiler.
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