It's customary to use a 64-bit integer to count events in NetBSD because we don't expect for the count to roll over in the lifetime of a box running NetBSD.
I've been thinking about what these wide integers do to the cache footprint of a system and wondering if we shouldn't make a couple of changes: 1) Cram just as many counters into each cacheline as possible. Extend/replace evcnt(9) to allow the caller to provide the storage for the integer. On a multiprocessor box, you don't want CPUs sharing counter cachelines if you can help it, but do cram together each individual CPU's counters. 2) Split all counters into two parts: high-order 32 bits, low-order 32 bits. It's only necessary to touch the high-order part when the low-order part rolls over, so in effect you split the counters into write-often (hot) and write-rarely (cold) parts. Cram together the cold parts in cachelines. Cram together the hot parts in cachelines. Only the hot parts change that often, so the ordinary footprint of counters in the cache is cut almost in half. I suppose you could split counters into four or more parts of 16 or fewer bits each, and in that shrink the footprint even further, but it seems that you would reach a point of diminishing returns very quickly. Perhaps this has been tried before and found to (not) work reasonably well? Dave -- David Young [email protected] Urbana, IL (217) 721-9981
