On Fri, 18 Jun 2010, Lawrence Stewart wrote:
True but I figured on large SMP systems where the potential to process more
is likely, 32bit counters per cpu may be enough to avoid overflow but the
aggregate number of events may exceed a 32bit variable. I suspect you're
right though and that if there's a likely chance the aggregate could
overflow, then the DPCPU var should simply be made 64bit to also remove any
possibility of individual PCPU counters overflowing.
I'll commit the above version of the macro this evening (GMT+10) unless I
hear any objections. Thanks to all of you for your input.
The only reservation I have, really, is that 64-bit writes are non-atomic on
i386 and other 32-bit architectures (or, at least, I think they are). This
means DPCPU_SUM may encounter non-atomicity rather than just staleness in the
values it reads as it iterates. That said, we should probably use 64-bit
anyway, because 32-bit counters are gauche. :-)
Robert
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