Re: vmstat's entries type

Brian Candler wrote:
 > John Baldwin wrote:
 > > 	lock incl counter
 > > 	jnc 1f
 > > 	lock incl counter+4
 > > 1:
 > 
 > That looks safe to me. How expensive is a forward jump like that, i.e. do
 > you get a pipeline bubble?

The jump is not executed only once out of 4 billion times,
so the processor's branch prediction should be handle to
optimize away it pretty well.  As far as I can tell, the
"lock" prefixes are _much_ more to worry about.

 > The 'polling' argument says just do
 > 
 >     lock incl counter
 > 
 > and poll all counters every 5 minutes, looking for a wrap. I think that's
 > almost certainly going to be cheaper, as long as you can keep track of where
 > all these counters are located.

It's not much cheaper (it still seems to need a "lock"),
and it's ugly, and there are situations where it can break,
e.g. if the counter is incremented fast enough to overflow
twice within the polling interval.

Best regards
   Oliver

-- 
Oliver Fromme,  secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing
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