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    SubjectRe: [patch] Performance Counters for Linux, v2

    * Paul Mackerras <> 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. )


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