Ingo Molnar writes: > * Paul Mackerras <paulus@xxxxxxxxx> wrote: > > > > Things like: "kerneltop would not be as accurate with: ..., to the > > > level of adding 5% of extra noise.". Would that work for you? > > > > OK, here's an example. I have an application whose execution has > > several different phases, and I want to measure the L1 Icache hit rate > > and the L1 Dcache hit rate as a function of time and make a graph. So > > I need counters for L1 Icache accesses, L1 Icache misses, L1 Dcache > > accesses, and L1 Dcache misses. I want to sample at 1ms intervals. The > > CPU I'm running on has two counters. > > > > With your current proposal, I don't see any way to make sure that the > > counter scheduler counts L1 Dcache accesses and L1 Dcache misses at the > > same time, then schedules L1 Icache accesses and L1 Icache misses. I > > could end up with L1 Dcache accesses and L1 Icache accesses, then L1 > > Dcache misses and L1 Icache misses - and get a nonsensical situation > > like the misses being greater than the accesses. > > yes, agreed, this is a valid special case of simple counter readout - > we'll add support to couple counters like that. This is an example of a sampling problem, but one where the thing being sampled is a derived statistic from two counter values. I don't agree that this is really a "special case". There are lots of derived statistics that are interesting for performance analysis, starting with CPI (cycles per instruction), proportions of various instructions in the code, cache hit/miss rates for various different caches, etc., etc. > Note that this issue does not impact use of multiple counters in > profilers. (i.e. anything that is not a pure readout of the counter, > along linear time, as your example above suggests). Well, that's the sampling vs. counting distinction that I made in my other email. We need to do both well. As far as I can see, my "counter set" proposal does everything yours does (since a counter set can be just a single counter), and also cleanly accommodates what's needed for counting and for sampling derived statistics. No? Paul. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
