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SubjectRe: Epic regression in throughput since v2.6.23
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Ingo Molnar <mingo@elte.hu> writes:

> Could still compare -j5 to -j4 on -tip, to see why -j4 is 3% short
> of -j5's throughput.
>
> (Plus maybe the NEW_FAIR_SLEEPERS change in -tip fixes the 3% drop.)

I've hacked the "perf" tool a bit (see the attached patch) and obtained
the following results with "perf stat -r9 make -j4 -- make clean":

--------------------------------------------------------------------------------
2.6.31-tip-01381-gf4c92b6:
Performance counter stats for 'make -j4' (9 runs):

94730.871493 task-clock-msecs # 3.503 CPUs ( +- 0.022% )
17499 context-switches # 0.000 M/sec ( +- 0.270% )
1127 CPU-migrations # 0.000 M/sec ( +- 1.159% )
4601950 page-faults # 0.049 M/sec ( +- 0.001% )
237853044260 cycles # 2510.829 M/sec ( +- 0.022% )
200554338937 instructions # 0.843 IPC ( +- 0.000% )
70615226021 cache-references # 745.430 M/sec ( +- 0.002% )
2268068445 cache-misses # 23.942 M/sec ( +- 0.020% )

27.046390652 seconds time elapsed ( +- 0.085% )


Performance counter stats for 'make -j5' (9 runs):

95283.766024 task-clock-msecs # 3.541 CPUs ( +- 0.024% )
20612 context-switches # 0.000 M/sec ( +- 0.297% )
1644 CPU-migrations # 0.000 M/sec ( +- 0.711% )
4601733 page-faults # 0.048 M/sec ( +- 0.001% )
239234709890 cycles # 2510.760 M/sec ( +- 0.024% )
200562340756 instructions # 0.838 IPC ( +- 0.000% )
70575215911 cache-references # 740.685 M/sec ( +- 0.004% )
2266361085 cache-misses # 23.785 M/sec ( +- 0.017% )

26.909772228 seconds time elapsed ( +- 0.041% )


Performance counter stats for 'make -j6' (9 runs):

95664.586804 task-clock-msecs # 3.568 CPUs ( +- 0.020% )
22358 context-switches # 0.000 M/sec ( +- 0.194% )
1835 CPU-migrations # 0.000 M/sec ( +- 0.659% )
4601777 page-faults # 0.048 M/sec ( +- 0.001% )
240188102235 cycles # 2510.732 M/sec ( +- 0.020% )
200569036210 instructions # 0.835 IPC ( +- 0.000% )
70565619805 cache-references # 737.636 M/sec ( +- 0.006% )
2265714056 cache-misses # 23.684 M/sec ( +- 0.017% )

26.808072908 seconds time elapsed ( +- 0.063% )
--------------------------------------------------------------------------------
2.6.31-tip-01385-g1ca1afc:
Performance counter stats for 'make -j4' (9 runs):

94873.128287 task-clock-msecs # 3.422 CPUs ( +- 0.020% )
13196 context-switches # 0.000 M/sec ( +- 0.181% )
1777 CPU-migrations # 0.000 M/sec ( +- 0.664% )
4601784 page-faults # 0.049 M/sec ( +- 0.001% )
238126976192 cycles # 2509.952 M/sec ( +- 0.020% )
200545291785 instructions # 0.842 IPC ( +- 0.000% )
70607104279 cache-references # 744.227 M/sec ( +- 0.005% )
2266980390 cache-misses # 23.895 M/sec ( +- 0.022% )

27.723375828 seconds time elapsed ( +- 0.134% )


Performance counter stats for 'make -j5' (9 runs):

95363.595017 task-clock-msecs # 3.519 CPUs ( +- 0.015% )
15003 context-switches # 0.000 M/sec ( +- 0.294% )
1880 CPU-migrations # 0.000 M/sec ( +- 1.017% )
4601648 page-faults # 0.048 M/sec ( +- 0.001% )
239351549481 cycles # 2509.884 M/sec ( +- 0.015% )
200552379266 instructions # 0.838 IPC ( +- 0.000% )
70576977633 cache-references # 740.083 M/sec ( +- 0.006% )
2265294365 cache-misses # 23.754 M/sec ( +- 0.012% )

27.096710006 seconds time elapsed ( +- 0.074% )


Performance counter stats for 'make -j6' (9 runs):

95739.941233 task-clock-msecs # 3.568 CPUs ( +- 0.024% )
16327 context-switches # 0.000 M/sec ( +- 0.230% )
1863 CPU-migrations # 0.000 M/sec ( +- 0.623% )
4601715 page-faults # 0.048 M/sec ( +- 0.002% )
240292969273 cycles # 2509.851 M/sec ( +- 0.024% )
200554772612 instructions # 0.835 IPC ( +- 0.000% )
70566092747 cache-references # 737.060 M/sec ( +- 0.004% )
2264738533 cache-misses # 23.655 M/sec ( +- 0.021% )

26.836474777 seconds time elapsed ( +- 0.181% )
--------------------------------------------------------------------------------
Note that the disabling NEW_FAIR_SLEEPERS doesn't fix 3% regression from
v2.6.23, but instead makes "make -j4" runtime another 2% worse (27.05 -> 27.72).

---
tools/perf/builtin-stat.c | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)
Index: linux/tools/perf/builtin-stat.c
===================================================================
--- linux.orig/tools/perf/builtin-stat.c
+++ linux/tools/perf/builtin-stat.c
@@ -44,6 +44,7 @@
#include "util/parse-events.h"
#include "util/event.h"
#include "util/debug.h"
+#include "util/run-command.h"

#include <sys/prctl.h>
#include <math.h>
@@ -458,7 +459,7 @@ static const struct option options[] = {

int cmd_stat(int argc, const char **argv, const char *prefix __used)
{
- int status;
+ int status, j, cleanup, cleanup_argc;

argc = parse_options(argc, argv, options, stat_usage,
PARSE_OPT_STOP_AT_NON_OPTION);
@@ -467,6 +468,19 @@ int cmd_stat(int argc, const char **argv
if (run_count <= 0)
usage_with_options(stat_usage, options);

+ // quick ugly hack: if a "--" appears in the command, treat is as
+ // a delimiter and use remaining part as a "cleanup command",
+ // not affecting performance counters.
+ cleanup = cleanup_argc = 0;
+ for (j = 1; j < (argc-1); j ++) {
+ if (!strcmp (argv[j], "--")) {
+ cleanup = j + 1;
+ cleanup_argc = argc - j - 1;
+ argv[j] = NULL;
+ argc = j;
+ }
+ }
+
/* Set attrs and nr_counters if no event is selected and !null_run */
if (!null_run && !nr_counters) {
memcpy(attrs, default_attrs, sizeof(default_attrs));
@@ -493,6 +507,8 @@ int cmd_stat(int argc, const char **argv
if (run_count != 1 && verbose)
fprintf(stderr, "[ perf stat: executing run #%d ... ]\n", run_idx + 1);
status = run_perf_stat(argc, argv);
+ if (cleanup)
+ run_command_v_opt (&argv [cleanup], 0);
}

print_stat(argc, argv);
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