On Mon, 7 Jul 2008, Artem Belevich wrote:
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.
There were some patches floating around for if_em to do a prefetch of the
first bit of packet data on packets before handing them up the stack. My
understanding is that they moved the hot spot earlier, but didn't make a huge
difference because it doesn't really take that long to get to the point where
you're processing the IP header in our current stack (a downside to
optimization...). However, that's a pretty anecdotal story, and a proper
study of the effects of prefetching would be most welcome. One thing that I'd
really like to see someone look at is whether, by doing a bit of appropriately
timed prefetching, we can move cache misses out from under hot locks that
don't really relate to the data being prefetched.
Robert N M Watson
Computer Laboratory
University of Cambridge
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