Formulas in perf stat

Question

I am wondering about the formulas used in perf stat to calculate figures from the raw data.

perf stat -e task-clock,cycles,instructions,cache-references,cache-misses ./myapp

    1080267.226401      task-clock (msec)         #   19.062 CPUs utilized          
 1,592,123,216,789      cycles                    #    1.474 GHz                      (50.00%)
   871,190,006,655      instructions              #    0.55  insn per cycle           (75.00%)
     3,697,548,810      cache-references          #    3.423 M/sec                    (75.00%)
       459,457,321      cache-misses              #   12.426 % of all cache refs      (75.00%)

In this context, how do you calculate M/sec from cache-references?

Not sure I get the question right. It's just `cache-references` / `task-clock`, isn't it? — Zulan, Sep 27 '17 at 13:57
@Zulan Duh! Of course, it is... I was thinking it would be much more complicated — Manolete, Sep 27 '17 at 14:07
No worries ;-). The complicated part is the counter multiplexing indicated by `(75%)`, but that's hidden behind the curtain. — Zulan, Sep 27 '17 at 16:19

score 3 · Accepted Answer · answered Oct 05 '17 at 00:00

Formulas are seems not to be implemented in the builtin-stat.c (where default event sets for perf stat are defined), but they are probably calculated (and averaged with stddev) in perf_stat__print_shadow_stats() (and some stats are collected into arrays in perf_stat__update_shadow_stats()):

http://elixir.free-electrons.com/linux/v4.13.4/source/tools/perf/util/stat-shadow.c#L626

When HW_INSTRUCTIONS is counted: "Instructions per clock" = HW_INSTRUCTIONS / HW_CPU_CYCLES; "stalled cycles per instruction" = HW_STALLED_CYCLES_FRONTEND / HW_INSTRUCTIONS

if (perf_evsel__match(evsel, HARDWARE, HW_INSTRUCTIONS)) {
    total = avg_stats(&runtime_cycles_stats[ctx][cpu]);
    if (total) {
        ratio = avg / total;
        print_metric(ctxp, NULL, "%7.2f ",
                "insn per cycle", ratio);
    } else {
        print_metric(ctxp, NULL, NULL, "insn per cycle", 0);
    }

Branch misses are from print_branch_misses as HW_BRANCH_MISSES / HW_BRANCH_INSTRUCTIONS

There are several cache miss ratio calculations in perf_stat__print_shadow_stats() too like HW_CACHE_MISSES / HW_CACHE_REFERENCES and some more detailed (perf stat -d mode).

Stalled percents are computed as HW_STALLED_CYCLES_FRONTEND / HW_CPU_CYCLES and HW_STALLED_CYCLES_BACKEND / HW_CPU_CYCLES

GHz is computed as HW_CPU_CYCLES / runtime_nsecs_stats, where runtime_nsecs_stats was updated from any of software events task-clock or cpu-clock (SW_TASK_CLOCK & SW_CPU_CLOCK, We still know no exact difference between them two since 2010 in LKML and 2014 at SO)

if (perf_evsel__match(counter, SOFTWARE, SW_TASK_CLOCK) ||
    perf_evsel__match(counter, SOFTWARE, SW_CPU_CLOCK))
    update_stats(&runtime_nsecs_stats[cpu], count[0]);

There are also several formulas for transactions (perf stat -T mode).

"CPU utilized" is from task-clock or cpu-clock / walltime_nsecs_stats, where walltime is calculated by the perf stat itself (in userspace using clock from the wall (astronomic time, ):

static inline unsigned long long rdclock(void)
{
    struct timespec ts;

    clock_gettime(CLOCK_MONOTONIC, &ts);
    return ts.tv_sec * 1000000000ULL + ts.tv_nsec;
}

...

static int __run_perf_stat(int argc, const char **argv)
{    
...
    /*
     * Enable counters and exec the command:
     */
    t0 = rdclock();
    clock_gettime(CLOCK_MONOTONIC, &ref_time);
    if (forks) {
        ....
    }
    t1 = rdclock();

    update_stats(&walltime_nsecs_stats, t1 - t0);

There are also some estimations from the Top-Down methodology (Tuning Applications Using a Top-down Microarchitecture Analysis Method, Software Optimizations Become Simple with Top-Down Analysis .. Name Skylake, IDF2015, #22 in Gregg's Methodology List. Described in 2016 by Andi Kleen https://lwn.net/Articles/688335/ "Add top down metrics to perf stat" (perf stat --topdown -I 1000 cmd mode).

And finally, if there was no exact formula for the currently printing event, there is universal "%c/sec" (K/sec or M/sec) metric: http://elixir.free-electrons.com/linux/v4.13.4/source/tools/perf/util/stat-shadow.c#L845 Anything divided by runtime nsec (task-clock or cpu-clock events, if they were present in perf stat event set)

} else if (runtime_nsecs_stats[cpu].n != 0) {
    char unit = 'M';
    char unit_buf[10];

    total = avg_stats(&runtime_nsecs_stats[cpu]);

    if (total)
        ratio = 1000.0 * avg / total;
    if (ratio < 0.001) {
        ratio *= 1000;
        unit = 'K';
    }
    snprintf(unit_buf, sizeof(unit_buf), "%c/sec", unit);
    print_metric(ctxp, NULL, "%8.3f", unit_buf, ratio);
}

Formulas in perf stat

1 Answers1