257 lines
9.1 KiB
Plaintext
257 lines
9.1 KiB
Plaintext
Using TopDown metrics in user space
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-----------------------------------
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Intel CPUs (since Sandy Bridge and Silvermont) support a TopDown
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methology to break down CPU pipeline execution into 4 bottlenecks:
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frontend bound, backend bound, bad speculation, retiring.
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For more details on Topdown see [1][5]
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Traditionally this was implemented by events in generic counters
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and specific formulas to compute the bottlenecks.
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perf stat --topdown implements this.
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Full Top Down includes more levels that can break down the
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bottlenecks further. This is not directly implemented in perf,
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but available in other tools that can run on top of perf,
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such as toplev[2] or vtune[3]
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New Topdown features in Ice Lake
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===============================
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With Ice Lake CPUs the TopDown metrics are directly available as
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fixed counters and do not require generic counters. This allows
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to collect TopDown always in addition to other events.
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% perf stat -a --topdown -I1000
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# time retiring bad speculation frontend bound backend bound
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1.001281330 23.0% 15.3% 29.6% 32.1%
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2.003009005 5.0% 6.8% 46.6% 41.6%
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3.004646182 6.7% 6.7% 46.0% 40.6%
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4.006326375 5.0% 6.4% 47.6% 41.0%
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5.007991804 5.1% 6.3% 46.3% 42.3%
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6.009626773 6.2% 7.1% 47.3% 39.3%
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7.011296356 4.7% 6.7% 46.2% 42.4%
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8.012951831 4.7% 6.7% 47.5% 41.1%
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...
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This also enables measuring TopDown per thread/process instead
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of only per core.
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Using TopDown through RDPMC in applications on Ice Lake
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======================================================
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For more fine grained measurements it can be useful to
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access the new directly from user space. This is more complicated,
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but drastically lowers overhead.
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On Ice Lake, there is a new fixed counter 3: SLOTS, which reports
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"pipeline SLOTS" (cycles multiplied by core issue width) and a
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metric register that reports slots ratios for the different bottleneck
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categories.
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The metrics counter is CPU model specific and is not available on older
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CPUs.
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Example code
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============
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Library functions to do the functionality described below
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is also available in libjevents [4]
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The application opens a group with fixed counter 3 (SLOTS) and any
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metric event, and allow user programs to read the performance counters.
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Fixed counter 3 is mapped to a pseudo event event=0x00, umask=04,
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so the perf_event_attr structure should be initialized with
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{ .config = 0x0400, .type = PERF_TYPE_RAW }
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The metric events are mapped to the pseudo event event=0x00, umask=0x8X.
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For example, the perf_event_attr structure can be initialized with
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{ .config = 0x8000, .type = PERF_TYPE_RAW } for Retiring metric event
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The Fixed counter 3 must be the leader of the group.
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#include <linux/perf_event.h>
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#include <sys/syscall.h>
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#include <unistd.h>
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/* Provide own perf_event_open stub because glibc doesn't */
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__attribute__((weak))
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int perf_event_open(struct perf_event_attr *attr, pid_t pid,
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int cpu, int group_fd, unsigned long flags)
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{
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return syscall(__NR_perf_event_open, attr, pid, cpu, group_fd, flags);
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}
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/* Open slots counter file descriptor for current task. */
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struct perf_event_attr slots = {
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.type = PERF_TYPE_RAW,
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.size = sizeof(struct perf_event_attr),
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.config = 0x400,
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.exclude_kernel = 1,
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};
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int slots_fd = perf_event_open(&slots, 0, -1, -1, 0);
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if (slots_fd < 0)
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... error ...
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/*
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* Open metrics event file descriptor for current task.
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* Set slots event as the leader of the group.
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*/
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struct perf_event_attr metrics = {
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.type = PERF_TYPE_RAW,
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.size = sizeof(struct perf_event_attr),
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.config = 0x8000,
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.exclude_kernel = 1,
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};
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int metrics_fd = perf_event_open(&metrics, 0, -1, slots_fd, 0);
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if (metrics_fd < 0)
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... error ...
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The RDPMC instruction (or _rdpmc compiler intrinsic) can now be used
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to read slots and the topdown metrics at different points of the program:
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#include <stdint.h>
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#include <x86intrin.h>
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#define RDPMC_FIXED (1 << 30) /* return fixed counters */
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#define RDPMC_METRIC (1 << 29) /* return metric counters */
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#define FIXED_COUNTER_SLOTS 3
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#define METRIC_COUNTER_TOPDOWN_L1 0
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static inline uint64_t read_slots(void)
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{
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return _rdpmc(RDPMC_FIXED | FIXED_COUNTER_SLOTS);
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}
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static inline uint64_t read_metrics(void)
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{
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return _rdpmc(RDPMC_METRIC | METRIC_COUNTER_TOPDOWN_L1);
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}
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Then the program can be instrumented to read these metrics at different
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points.
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It's not a good idea to do this with too short code regions,
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as the parallelism and overlap in the CPU program execution will
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cause too much measurement inaccuracy. For example instrumenting
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individual basic blocks is definitely too fine grained.
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Decoding metrics values
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=======================
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The value reported by read_metrics() contains four 8 bit fields
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that represent a scaled ratio that represent the Level 1 bottleneck.
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All four fields add up to 0xff (= 100%)
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The binary ratios in the metric value can be converted to float ratios:
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#define GET_METRIC(m, i) (((m) >> (i*8)) & 0xff)
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#define TOPDOWN_RETIRING(val) ((float)GET_METRIC(val, 0) / 0xff)
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#define TOPDOWN_BAD_SPEC(val) ((float)GET_METRIC(val, 1) / 0xff)
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#define TOPDOWN_FE_BOUND(val) ((float)GET_METRIC(val, 2) / 0xff)
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#define TOPDOWN_BE_BOUND(val) ((float)GET_METRIC(val, 3) / 0xff)
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and then converted to percent for printing.
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The ratios in the metric accumulate for the time when the counter
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is enabled. For measuring programs it is often useful to measure
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specific sections. For this it is needed to deltas on metrics.
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This can be done by scaling the metrics with the slots counter
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read at the same time.
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Then it's possible to take deltas of these slots counts
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measured at different points, and determine the metrics
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for that time period.
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slots_a = read_slots();
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metric_a = read_metrics();
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... larger code region ...
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slots_b = read_slots()
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metric_b = read_metrics()
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# compute scaled metrics for measurement a
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retiring_slots_a = GET_METRIC(metric_a, 0) * slots_a
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bad_spec_slots_a = GET_METRIC(metric_a, 1) * slots_a
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fe_bound_slots_a = GET_METRIC(metric_a, 2) * slots_a
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be_bound_slots_a = GET_METRIC(metric_a, 3) * slots_a
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# compute delta scaled metrics between b and a
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retiring_slots = GET_METRIC(metric_b, 0) * slots_b - retiring_slots_a
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bad_spec_slots = GET_METRIC(metric_b, 1) * slots_b - bad_spec_slots_a
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fe_bound_slots = GET_METRIC(metric_b, 2) * slots_b - fe_bound_slots_a
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be_bound_slots = GET_METRIC(metric_b, 3) * slots_b - be_bound_slots_a
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Later the individual ratios for the measurement period can be recreated
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from these counts.
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slots_delta = slots_b - slots_a
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retiring_ratio = (float)retiring_slots / slots_delta
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bad_spec_ratio = (float)bad_spec_slots / slots_delta
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fe_bound_ratio = (float)fe_bound_slots / slots_delta
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be_bound_ratio = (float)be_bound_slots / slota_delta
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printf("Retiring %.2f%% Bad Speculation %.2f%% FE Bound %.2f%% BE Bound %.2f%%\n",
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retiring_ratio * 100.,
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bad_spec_ratio * 100.,
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fe_bound_ratio * 100.,
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be_bound_ratio * 100.);
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Resetting metrics counters
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==========================
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Since the individual metrics are only 8bit they lose precision for
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short regions over time because the number of cycles covered by each
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fraction bit shrinks. So the counters need to be reset regularly.
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When using the kernel perf API the kernel resets on every read.
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So as long as the reading is at reasonable intervals (every few
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seconds) the precision is good.
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When using perf stat it is recommended to always use the -I option,
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with no longer interval than a few seconds
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perf stat -I 1000 --topdown ...
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For user programs using RDPMC directly the counter can
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be reset explicitly using ioctl:
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ioctl(perf_fd, PERF_EVENT_IOC_RESET, 0);
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This "opens" a new measurement period.
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A program using RDPMC for TopDown should schedule such a reset
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regularly, as in every few seconds.
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Limits on Ice Lake
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==================
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Four pseudo TopDown metric events are exposed for the end-users,
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topdown-retiring, topdown-bad-spec, topdown-fe-bound and topdown-be-bound.
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They can be used to collect the TopDown value under the following
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rules:
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- All the TopDown metric events must be in a group with the SLOTS event.
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- The SLOTS event must be the leader of the group.
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- The PERF_FORMAT_GROUP flag must be applied for each TopDown metric
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events
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The SLOTS event and the TopDown metric events can be counting members of
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a sampling read group. Since the SLOTS event must be the leader of a TopDown
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group, the second event of the group is the sampling event.
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For example, perf record -e '{slots, $sampling_event, topdown-retiring}:S'
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[1] https://software.intel.com/en-us/top-down-microarchitecture-analysis-method-win
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[2] https://github.com/andikleen/pmu-tools/wiki/toplev-manual
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[3] https://software.intel.com/en-us/intel-vtune-amplifier-xe
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[4] https://github.com/andikleen/pmu-tools/tree/master/jevents
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[5] https://sites.google.com/site/analysismethods/yasin-pubs
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