1Using TopDown metrics 2--------------------- 3 4TopDown metrics break apart performance bottlenecks. Starting at level 51 it is typical to get metrics on retiring, bad speculation, frontend 6bound, and backend bound. Higher levels provide more detail in to the 7level 1 bottlenecks, such as at level 2: core bound, memory bound, 8heavy operations, light operations, branch mispredicts, machine 9clears, fetch latency and fetch bandwidth. For more details see [1][2][3]. 10 11perf stat --topdown implements this using available metrics that vary 12per architecture. 13 14% perf stat -a --topdown -I1000 15# time % tma_retiring % tma_backend_bound % tma_frontend_bound % tma_bad_speculation 16 1.001141351 11.5 34.9 46.9 6.7 17 2.006141972 13.4 28.1 50.4 8.1 18 3.010162040 12.9 28.1 51.1 8.0 19 4.014009311 12.5 28.6 51.8 7.2 20 5.017838554 11.8 33.0 48.0 7.2 21 5.704818971 14.0 27.5 51.3 7.3 22... 23 24New Topdown features in Intel Ice Lake 25====================================== 26 27With Ice Lake CPUs the TopDown metrics are directly available as 28fixed counters and do not require generic counters. This allows 29to collect TopDown always in addition to other events. 30 31Using TopDown through RDPMC in applications on Intel Ice Lake 32============================================================= 33 34For more fine grained measurements it can be useful to 35access the new directly from user space. This is more complicated, 36but drastically lowers overhead. 37 38On Ice Lake, there is a new fixed counter 3: SLOTS, which reports 39"pipeline SLOTS" (cycles multiplied by core issue width) and a 40metric register that reports slots ratios for the different bottleneck 41categories. 42 43The metrics counter is CPU model specific and is not available on older 44CPUs. 45 46Example code 47============ 48 49Library functions to do the functionality described below 50is also available in libjevents [4] 51 52The application opens a group with fixed counter 3 (SLOTS) and any 53metric event, and allow user programs to read the performance counters. 54 55Fixed counter 3 is mapped to a pseudo event event=0x00, umask=04, 56so the perf_event_attr structure should be initialized with 57{ .config = 0x0400, .type = PERF_TYPE_RAW } 58The metric events are mapped to the pseudo event event=0x00, umask=0x8X. 59For example, the perf_event_attr structure can be initialized with 60{ .config = 0x8000, .type = PERF_TYPE_RAW } for Retiring metric event 61The Fixed counter 3 must be the leader of the group. 62 63#include <linux/perf_event.h> 64#include <sys/mman.h> 65#include <sys/syscall.h> 66#include <unistd.h> 67 68/* Provide own perf_event_open stub because glibc doesn't */ 69__attribute__((weak)) 70int perf_event_open(struct perf_event_attr *attr, pid_t pid, 71 int cpu, int group_fd, unsigned long flags) 72{ 73 return syscall(__NR_perf_event_open, attr, pid, cpu, group_fd, flags); 74} 75 76/* Open slots counter file descriptor for current task. */ 77struct perf_event_attr slots = { 78 .type = PERF_TYPE_RAW, 79 .size = sizeof(struct perf_event_attr), 80 .config = 0x400, 81 .exclude_kernel = 1, 82}; 83 84int slots_fd = perf_event_open(&slots, 0, -1, -1, 0); 85if (slots_fd < 0) 86 ... error ... 87 88/* Memory mapping the fd permits _rdpmc calls from userspace */ 89void *slots_p = mmap(0, getpagesize(), PROT_READ, MAP_SHARED, slots_fd, 0); 90if (!slot_p) 91 .... error ... 92 93/* 94 * Open metrics event file descriptor for current task. 95 * Set slots event as the leader of the group. 96 */ 97struct perf_event_attr metrics = { 98 .type = PERF_TYPE_RAW, 99 .size = sizeof(struct perf_event_attr), 100 .config = 0x8000, 101 .exclude_kernel = 1, 102}; 103 104int metrics_fd = perf_event_open(&metrics, 0, -1, slots_fd, 0); 105if (metrics_fd < 0) 106 ... error ... 107 108/* Memory mapping the fd permits _rdpmc calls from userspace */ 109void *metrics_p = mmap(0, getpagesize(), PROT_READ, MAP_SHARED, metrics_fd, 0); 110if (!metrics_p) 111 ... error ... 112 113Note: the file descriptors returned by the perf_event_open calls must be memory 114mapped to permit calls to the _rdpmd instruction. Permission may also be granted 115by writing the /sys/devices/cpu/rdpmc sysfs node. 116 117The RDPMC instruction (or _rdpmc compiler intrinsic) can now be used 118to read slots and the topdown metrics at different points of the program: 119 120#include <stdint.h> 121#include <x86intrin.h> 122 123#define RDPMC_FIXED (1 << 30) /* return fixed counters */ 124#define RDPMC_METRIC (1 << 29) /* return metric counters */ 125 126#define FIXED_COUNTER_SLOTS 3 127#define METRIC_COUNTER_TOPDOWN_L1_L2 0 128 129static inline uint64_t read_slots(void) 130{ 131 return _rdpmc(RDPMC_FIXED | FIXED_COUNTER_SLOTS); 132} 133 134static inline uint64_t read_metrics(void) 135{ 136 return _rdpmc(RDPMC_METRIC | METRIC_COUNTER_TOPDOWN_L1_L2); 137} 138 139Then the program can be instrumented to read these metrics at different 140points. 141 142It's not a good idea to do this with too short code regions, 143as the parallelism and overlap in the CPU program execution will 144cause too much measurement inaccuracy. For example instrumenting 145individual basic blocks is definitely too fine grained. 146 147_rdpmc calls should not be mixed with reading the metrics and slots counters 148through system calls, as the kernel will reset these counters after each system 149call. 150 151Decoding metrics values 152======================= 153 154The value reported by read_metrics() contains four 8 bit fields 155that represent a scaled ratio that represent the Level 1 bottleneck. 156All four fields add up to 0xff (= 100%) 157 158The binary ratios in the metric value can be converted to float ratios: 159 160#define GET_METRIC(m, i) (((m) >> (i*8)) & 0xff) 161 162/* L1 Topdown metric events */ 163#define TOPDOWN_RETIRING(val) ((float)GET_METRIC(val, 0) / 0xff) 164#define TOPDOWN_BAD_SPEC(val) ((float)GET_METRIC(val, 1) / 0xff) 165#define TOPDOWN_FE_BOUND(val) ((float)GET_METRIC(val, 2) / 0xff) 166#define TOPDOWN_BE_BOUND(val) ((float)GET_METRIC(val, 3) / 0xff) 167 168/* 169 * L2 Topdown metric events. 170 * Available on Sapphire Rapids and later platforms. 171 */ 172#define TOPDOWN_HEAVY_OPS(val) ((float)GET_METRIC(val, 4) / 0xff) 173#define TOPDOWN_BR_MISPREDICT(val) ((float)GET_METRIC(val, 5) / 0xff) 174#define TOPDOWN_FETCH_LAT(val) ((float)GET_METRIC(val, 6) / 0xff) 175#define TOPDOWN_MEM_BOUND(val) ((float)GET_METRIC(val, 7) / 0xff) 176 177and then converted to percent for printing. 178 179The ratios in the metric accumulate for the time when the counter 180is enabled. For measuring programs it is often useful to measure 181specific sections. For this it is needed to deltas on metrics. 182 183This can be done by scaling the metrics with the slots counter 184read at the same time. 185 186Then it's possible to take deltas of these slots counts 187measured at different points, and determine the metrics 188for that time period. 189 190 slots_a = read_slots(); 191 metric_a = read_metrics(); 192 193 ... larger code region ... 194 195 slots_b = read_slots() 196 metric_b = read_metrics() 197 198 # compute scaled metrics for measurement a 199 retiring_slots_a = GET_METRIC(metric_a, 0) * slots_a 200 bad_spec_slots_a = GET_METRIC(metric_a, 1) * slots_a 201 fe_bound_slots_a = GET_METRIC(metric_a, 2) * slots_a 202 be_bound_slots_a = GET_METRIC(metric_a, 3) * slots_a 203 204 # compute delta scaled metrics between b and a 205 retiring_slots = GET_METRIC(metric_b, 0) * slots_b - retiring_slots_a 206 bad_spec_slots = GET_METRIC(metric_b, 1) * slots_b - bad_spec_slots_a 207 fe_bound_slots = GET_METRIC(metric_b, 2) * slots_b - fe_bound_slots_a 208 be_bound_slots = GET_METRIC(metric_b, 3) * slots_b - be_bound_slots_a 209 210Later the individual ratios of L1 metric events for the measurement period can 211be recreated from these counts. 212 213 slots_delta = slots_b - slots_a 214 retiring_ratio = (float)retiring_slots / slots_delta 215 bad_spec_ratio = (float)bad_spec_slots / slots_delta 216 fe_bound_ratio = (float)fe_bound_slots / slots_delta 217 be_bound_ratio = (float)be_bound_slots / slota_delta 218 219 printf("Retiring %.2f%% Bad Speculation %.2f%% FE Bound %.2f%% BE Bound %.2f%%\n", 220 retiring_ratio * 100., 221 bad_spec_ratio * 100., 222 fe_bound_ratio * 100., 223 be_bound_ratio * 100.); 224 225The individual ratios of L2 metric events for the measurement period can be 226recreated from L1 and L2 metric counters. (Available on Sapphire Rapids and 227later platforms) 228 229 # compute scaled metrics for measurement a 230 heavy_ops_slots_a = GET_METRIC(metric_a, 4) * slots_a 231 br_mispredict_slots_a = GET_METRIC(metric_a, 5) * slots_a 232 fetch_lat_slots_a = GET_METRIC(metric_a, 6) * slots_a 233 mem_bound_slots_a = GET_METRIC(metric_a, 7) * slots_a 234 235 # compute delta scaled metrics between b and a 236 heavy_ops_slots = GET_METRIC(metric_b, 4) * slots_b - heavy_ops_slots_a 237 br_mispredict_slots = GET_METRIC(metric_b, 5) * slots_b - br_mispredict_slots_a 238 fetch_lat_slots = GET_METRIC(metric_b, 6) * slots_b - fetch_lat_slots_a 239 mem_bound_slots = GET_METRIC(metric_b, 7) * slots_b - mem_bound_slots_a 240 241 slots_delta = slots_b - slots_a 242 heavy_ops_ratio = (float)heavy_ops_slots / slots_delta 243 light_ops_ratio = retiring_ratio - heavy_ops_ratio; 244 245 br_mispredict_ratio = (float)br_mispredict_slots / slots_delta 246 machine_clears_ratio = bad_spec_ratio - br_mispredict_ratio; 247 248 fetch_lat_ratio = (float)fetch_lat_slots / slots_delta 249 fetch_bw_ratio = fe_bound_ratio - fetch_lat_ratio; 250 251 mem_bound_ratio = (float)mem_bound_slots / slota_delta 252 core_bound_ratio = be_bound_ratio - mem_bound_ratio; 253 254 printf("Heavy Operations %.2f%% Light Operations %.2f%% " 255 "Branch Mispredict %.2f%% Machine Clears %.2f%% " 256 "Fetch Latency %.2f%% Fetch Bandwidth %.2f%% " 257 "Mem Bound %.2f%% Core Bound %.2f%%\n", 258 heavy_ops_ratio * 100., 259 light_ops_ratio * 100., 260 br_mispredict_ratio * 100., 261 machine_clears_ratio * 100., 262 fetch_lat_ratio * 100., 263 fetch_bw_ratio * 100., 264 mem_bound_ratio * 100., 265 core_bound_ratio * 100.); 266 267Resetting metrics counters 268========================== 269 270Since the individual metrics are only 8bit they lose precision for 271short regions over time because the number of cycles covered by each 272fraction bit shrinks. So the counters need to be reset regularly. 273 274When using the kernel perf API the kernel resets on every read. 275So as long as the reading is at reasonable intervals (every few 276seconds) the precision is good. 277 278When using perf stat it is recommended to always use the -I option, 279with no longer interval than a few seconds 280 281 perf stat -I 1000 --topdown ... 282 283For user programs using RDPMC directly the counter can 284be reset explicitly using ioctl: 285 286 ioctl(perf_fd, PERF_EVENT_IOC_RESET, 0); 287 288This "opens" a new measurement period. 289 290A program using RDPMC for TopDown should schedule such a reset 291regularly, as in every few seconds. 292 293Limits on Intel Ice Lake 294======================== 295 296Four pseudo TopDown metric events are exposed for the end-users, 297topdown-retiring, topdown-bad-spec, topdown-fe-bound and topdown-be-bound. 298They can be used to collect the TopDown value under the following 299rules: 300- All the TopDown metric events must be in a group with the SLOTS event. 301- The SLOTS event must be the leader of the group. 302- The PERF_FORMAT_GROUP flag must be applied for each TopDown metric 303 events 304 305The SLOTS event and the TopDown metric events can be counting members of 306a sampling read group. Since the SLOTS event must be the leader of a TopDown 307group, the second event of the group is the sampling event. 308For example, perf record -e '{slots, $sampling_event, topdown-retiring}:S' 309 310Extension on Intel Sapphire Rapids Server 311========================================= 312The metrics counter is extended to support TMA method level 2 metrics. 313The lower half of the register is the TMA level 1 metrics (legacy). 314The upper half is also divided into four 8-bit fields for the new level 2 315metrics. Four more TopDown metric events are exposed for the end-users, 316topdown-heavy-ops, topdown-br-mispredict, topdown-fetch-lat and 317topdown-mem-bound. 318 319Each of the new level 2 metrics in the upper half is a subset of the 320corresponding level 1 metric in the lower half. Software can deduce the 321other four level 2 metrics by subtracting corresponding metrics as below. 322 323 Light_Operations = Retiring - Heavy_Operations 324 Machine_Clears = Bad_Speculation - Branch_Mispredicts 325 Fetch_Bandwidth = Frontend_Bound - Fetch_Latency 326 Core_Bound = Backend_Bound - Memory_Bound 327 328 329[1] https://software.intel.com/en-us/top-down-microarchitecture-analysis-method-win 330[2] https://sites.google.com/site/analysismethods/yasin-pubs 331[3] https://perf.wiki.kernel.org/index.php/Top-Down_Analysis 332[4] https://github.com/andikleen/pmu-tools/tree/master/jevents 333