* 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. 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). Once we start sampling the context, grouping of counters becomes irrelevant (and a hindrance) and static frequency sampling becomes an inferior method of sampling. ( The highest quality statistical approach is the kind of multi-counter sampling model you can see implemented in KernelTop for example, where the counters are independently sampled. Can go on in great detail about this if you are interested - this is the far more interesting usecase in practice. ) Ingo -- 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
