using prefetch

[Valdis.Kletnieks at vt.edu]

On Fri, 15 Feb 2013 12:16:02 +0200, Kevin Wilson said:

> Is the prefetch operation synchronous ? I mean, after calling it, are
> we gauranteed that the variable is
> indeed in the cache ?

No, the whole *point* is that it's asynchronous.  You issue the prefetch
several lines of code before you need it to be in cache, so that you
can get several lines of hopefully not data-dependent code to run while
the cache line fetch happens, rather than take a stall when you reference
the variable.  The prefetch may in fact not complete in time, but at
worst you end up just stalling for a cache miss the same as you would have
otherwise.

> According to this logic, anywhere that we want to call skb_shinfo(skb)
> we better do a prefetch before.

No, because most references to skb will be cache-hot because you're in the
middle of the IP stack, which touches the skb struct all over the place, and
therefor it's probably in L2 already.

> In fact, if we prefetch any variable that we want to use then we end up
> with performance boost.

Nope. Not as true as you might think.  If you play around with the 'perf'
command you'll find out that on modern processors you'll see a 98% or so
hit rate on the L2 cache - so 98% of the time you'll *waste* a cycle
issuing the opcode needlessly.

If you look carefully at some of the other structs in the net/ subtree,
you'll see where they've put variables together so that once you reference
one field of the struct, all/most of the needed stuff gets sucked in on
the same cache line.  That's probably more productive than trying to add
prefetch calls all over the place.

> So - any hints, what are the guidlines for using prefetch()?

Only use it if you have good reason to believe that you *will* need
that variable (in other words, it's not in the unlikely half of an if
statement or somehting) *and* there's a good chance that the
variable/memory is cache-cold.
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