Using TopDown metrics in user space ----------------------------------- Intel CPUs (since Sandy Bridge and Silvermont) support a TopDown methology to break down CPU pipeline execution into 4 bottlenecks: frontend bound, backend bound, bad speculation, retiring. For more details on Topdown see [1][5] Traditionally this was implemented by events in generic counters and specific formulas to compute the bottlenecks. perf stat --topdown implements this. Full Top Down includes more levels that can break down the bottlenecks further. This is not directly implemented in perf, but available in other tools that can run on top of perf, such as toplev[2] or vtune[3] New Topdown features in Ice Lake =============================== With Ice Lake CPUs the TopDown metrics are directly available as fixed counters and do not require generic counters. This allows to collect TopDown always in addition to other events. % perf stat -a --topdown -I1000 # time retiring bad speculation frontend bound backend bound 1.001281330 23.0% 15.3% 29.6% 32.1% 2.003009005 5.0% 6.8% 46.6% 41.6% 3.004646182 6.7% 6.7% 46.0% 40.6% 4.006326375 5.0% 6.4% 47.6% 41.0% 5.007991804 5.1% 6.3% 46.3% 42.3% 6.009626773 6.2% 7.1% 47.3% 39.3% 7.011296356 4.7% 6.7% 46.2% 42.4% 8.012951831 4.7% 6.7% 47.5% 41.1% ... This also enables measuring TopDown per thread/process instead of only per core. Using TopDown through RDPMC in applications on Ice Lake ====================================================== For more fine grained measurements it can be useful to access the new directly from user space. This is more complicated, but drastically lowers overhead. On Ice Lake, there is a new fixed counter 3: SLOTS, which reports "pipeline SLOTS" (cycles multiplied by core issue width) and a metric register that reports slots ratios for the different bottleneck categories. The metrics counter is CPU model specific and is not available on older CPUs. Example code ============ Library functions to do the functionality described below is also available in libjevents [4] The application opens a group with fixed counter 3 (SLOTS) and any metric event, and allow user programs to read the performance counters. Fixed counter 3 is mapped to a pseudo event event=0x00, umask=04, so the perf_event_attr structure should be initialized with { .config = 0x0400, .type = PERF_TYPE_RAW } The metric events are mapped to the pseudo event event=0x00, umask=0x8X. For example, the perf_event_attr structure can be initialized with { .config = 0x8000, .type = PERF_TYPE_RAW } for Retiring metric event The Fixed counter 3 must be the leader of the group. #include #include #include /* Provide own perf_event_open stub because glibc doesn't */ __attribute__((weak)) int perf_event_open(struct perf_event_attr *attr, pid_t pid, int cpu, int group_fd, unsigned long flags) { return syscall(__NR_perf_event_open, attr, pid, cpu, group_fd, flags); } /* Open slots counter file descriptor for current task. */ struct perf_event_attr slots = { .type = PERF_TYPE_RAW, .size = sizeof(struct perf_event_attr), .config = 0x400, .exclude_kernel = 1, }; int slots_fd = perf_event_open(&slots, 0, -1, -1, 0); if (slots_fd < 0) ... error ... /* * Open metrics event file descriptor for current task. * Set slots event as the leader of the group. */ struct perf_event_attr metrics = { .type = PERF_TYPE_RAW, .size = sizeof(struct perf_event_attr), .config = 0x8000, .exclude_kernel = 1, }; int metrics_fd = perf_event_open(&metrics, 0, -1, slots_fd, 0); if (metrics_fd < 0) ... error ... The RDPMC instruction (or _rdpmc compiler intrinsic) can now be used to read slots and the topdown metrics at different points of the program: #include #include #define RDPMC_FIXED (1 << 30) /* return fixed counters */ #define RDPMC_METRIC (1 << 29) /* return metric counters */ #define FIXED_COUNTER_SLOTS 3 #define METRIC_COUNTER_TOPDOWN_L1 0 static inline uint64_t read_slots(void) { return _rdpmc(RDPMC_FIXED | FIXED_COUNTER_SLOTS); } static inline uint64_t read_metrics(void) { return _rdpmc(RDPMC_METRIC | METRIC_COUNTER_TOPDOWN_L1); } Then the program can be instrumented to read these metrics at different points. It's not a good idea to do this with too short code regions, as the parallelism and overlap in the CPU program execution will cause too much measurement inaccuracy. For example instrumenting individual basic blocks is definitely too fine grained. Decoding metrics values ======================= The value reported by read_metrics() contains four 8 bit fields that represent a scaled ratio that represent the Level 1 bottleneck. All four fields add up to 0xff (= 100%) The binary ratios in the metric value can be converted to float ratios: #define GET_METRIC(m, i) (((m) >> (i*8)) & 0xff) #define TOPDOWN_RETIRING(val) ((float)GET_METRIC(val, 0) / 0xff) #define TOPDOWN_BAD_SPEC(val) ((float)GET_METRIC(val, 1) / 0xff) #define TOPDOWN_FE_BOUND(val) ((float)GET_METRIC(val, 2) / 0xff) #define TOPDOWN_BE_BOUND(val) ((float)GET_METRIC(val, 3) / 0xff) and then converted to percent for printing. The ratios in the metric accumulate for the time when the counter is enabled. For measuring programs it is often useful to measure specific sections. For this it is needed to deltas on metrics. This can be done by scaling the metrics with the slots counter read at the same time. Then it's possible to take deltas of these slots counts measured at different points, and determine the metrics for that time period. slots_a = read_slots(); metric_a = read_metrics(); ... larger code region ... slots_b = read_slots() metric_b = read_metrics() # compute scaled metrics for measurement a retiring_slots_a = GET_METRIC(metric_a, 0) * slots_a bad_spec_slots_a = GET_METRIC(metric_a, 1) * slots_a fe_bound_slots_a = GET_METRIC(metric_a, 2) * slots_a be_bound_slots_a = GET_METRIC(metric_a, 3) * slots_a # compute delta scaled metrics between b and a retiring_slots = GET_METRIC(metric_b, 0) * slots_b - retiring_slots_a bad_spec_slots = GET_METRIC(metric_b, 1) * slots_b - bad_spec_slots_a fe_bound_slots = GET_METRIC(metric_b, 2) * slots_b - fe_bound_slots_a be_bound_slots = GET_METRIC(metric_b, 3) * slots_b - be_bound_slots_a Later the individual ratios for the measurement period can be recreated from these counts. slots_delta = slots_b - slots_a retiring_ratio = (float)retiring_slots / slots_delta bad_spec_ratio = (float)bad_spec_slots / slots_delta fe_bound_ratio = (float)fe_bound_slots / slots_delta be_bound_ratio = (float)be_bound_slots / slota_delta printf("Retiring %.2f%% Bad Speculation %.2f%% FE Bound %.2f%% BE Bound %.2f%%\n", retiring_ratio * 100., bad_spec_ratio * 100., fe_bound_ratio * 100., be_bound_ratio * 100.); Resetting metrics counters ========================== Since the individual metrics are only 8bit they lose precision for short regions over time because the number of cycles covered by each fraction bit shrinks. So the counters need to be reset regularly. When using the kernel perf API the kernel resets on every read. So as long as the reading is at reasonable intervals (every few seconds) the precision is good. When using perf stat it is recommended to always use the -I option, with no longer interval than a few seconds perf stat -I 1000 --topdown ... For user programs using RDPMC directly the counter can be reset explicitly using ioctl: ioctl(perf_fd, PERF_EVENT_IOC_RESET, 0); This "opens" a new measurement period. A program using RDPMC for TopDown should schedule such a reset regularly, as in every few seconds. Limits on Ice Lake ================== Four pseudo TopDown metric events are exposed for the end-users, topdown-retiring, topdown-bad-spec, topdown-fe-bound and topdown-be-bound. They can be used to collect the TopDown value under the following rules: - All the TopDown metric events must be in a group with the SLOTS event. - The SLOTS event must be the leader of the group. - The PERF_FORMAT_GROUP flag must be applied for each TopDown metric events The SLOTS event and the TopDown metric events can be counting members of a sampling read group. Since the SLOTS event must be the leader of a TopDown group, the second event of the group is the sampling event. For example, perf record -e '{slots, $sampling_event, topdown-retiring}:S' [1] https://software.intel.com/en-us/top-down-microarchitecture-analysis-method-win [2] https://github.com/andikleen/pmu-tools/wiki/toplev-manual [3] https://software.intel.com/en-us/intel-vtune-amplifier-xe [4] https://github.com/andikleen/pmu-tools/tree/master/jevents [5] https://sites.google.com/site/analysismethods/yasin-pubs