Freebsd IP Forwarding performance (question, and some info) [7-stable, current, em, smp]

Julian Elischer julian at
Mon Jul 7 20:25:15 UTC 2008

Artem Belevich wrote:
> Hi,
> As was already mentioned, we can't avoid all cache misses as there's
> data that's recently been updated in memory via DMA and therefor
> kicked out of cache.
> However, we may hide some of the latency penalty by prefetching
> 'interesting' data early. I.e. we know that we want to access some
> ethernet headers, so we may start pulling relevant data into cache
> early. Ideally, by the time we need to access the field, it will
> already be in the cache. When we're counting nanoseconds per packet
> this may bring some performance gain.

Prefetching when you are waiting for the data isn't a help.
what you need is a speculative prefetch where you an tell teh 
processor "We will probably need the following address so start 
getting it while we go do other stuff".

As far as I know we have no capacity to do that..

> Just my $0.02.
> --Artem
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