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    SubjectRe: Epic regression in throughput since v2.6.23
    Date
    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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