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    SubjectAnalysis of sched_mc_power_savings
    Hi,

    The following experiments were conducted on a two socket dual core
    intel processor based machine in order to understand the impact of
    sched_mc_power_savings scheduler heuristics.

    Kernel linux-2.6.24-rc6:

    CONFIG_SCHED_SMT=y
    CONFIG_SCHED_MC=y
    CONFIG_TICK_ONESHOT=y
    CONFIG_NO_HZ=y
    CONFIG_HIGH_RES_TIMERS=y
    CONFIG_GENERIC_CLOCKEVENTS_BUILD=y
    CONFIG_HPET_TIMER=y
    CONFIG_HPET_EMULATE_RTC=y

    tick-sched.c has been instrumented to collect idle entry and exit time
    stamps.

    Instrumentation patch:

    Instrument tick-sched nohz code and generate time stamp trace data.

    Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
    ---
    kernel/time/tick-sched.c | 8 +++++++-
    1 file changed, 7 insertions(+), 1 deletion(-)

    --- linux-2.6.24-rc6.orig/kernel/time/tick-sched.c
    +++ linux-2.6.24-rc6/kernel/time/tick-sched.c
    @@ -20,6 +20,7 @@
    #include <linux/profile.h>
    #include <linux/sched.h>
    #include <linux/tick.h>
    +#include <linux/ktrace.h>

    #include <asm/irq_regs.h>

    @@ -200,7 +201,10 @@ void tick_nohz_stop_sched_tick(void)
    if (ts->tick_stopped) {
    delta = ktime_sub(now, ts->idle_entrytime);
    ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
    - }
    + ktrace_log2(KT_FUNC_tick_nohz_stop_sched_tick, KT_EVENT_INFO1,
    + ktime_to_ns(now), ts->idle_calls);
    >>>>>>>>>>>>>>>>Tracepoint A

    + } else
    + ktrace_log2(KT_FUNC_tick_nohz_stop_sched_tick,
    KT_EVENT_FUNC_ENTER, ktime_to_ns(now), 0);

    >>>>>>>>>>>>>>>>Tracepoint B

    ts->idle_entrytime = now;
    ts->idle_calls++;
    @@ -391,6 +395,8 @@ void tick_nohz_restart_sched_tick(void)
    tick_do_update_jiffies64(now);
    now = ktime_get();
    }
    + ktrace_log2(KT_FUNC_tick_nohz_restart_sched_tick, KT_EVENT_FUNC_EXIT,
    + ktime_to_ns(now), ts->idle_calls);
    >>>>>>>>>>>>>>>>Tracepoint C
    local_irq_enable();
    }

    The idle time collected are time stamp at (C) minus (B). This is the
    time interval between stopping ticks and restarting ticks in an idle
    system.
    Complete patch series:
    http://svaidy.googlepages.com/1-klog.patch
    http://svaidy.googlepages.com/1-trace-sched.patch

    Userspace program to extract trace data:
    http://svaidy.googlepages.com/1-klog.c

    Python script to post process binary trace data:
    http://svaidy.googlepages.com/1-sched-stats.py

    Gnuplot scripts that was used to generate the graphs:
    http://svaidy.googlepages.com/1-multiplot.gp

    The scheduler heuristics for multi core system
    /sys/devices/system/cpu/sched_mc_power_savings should ideally extend
    the cpu tickless idle time atleast on few CPU in an SMP machine.

    However in the experiment it was found that turning on
    sched_mc_power_savings marginally increased the idle time in only some
    of the CPUs.

    Experiment 1:
    -------------
    Setup:

    * yum-updated and irqbalance daemon was stopped to reduce the idle
    wakeup rate
    * All irqs manually routed to CPU0 only (hoping this will keep other
    CPUs idle) (http://svaidy.googlepages.com/1-irq-config.txt)
    * Powertop shows around 35 wakeups per second during idle
    (http://svaidy.googlepages.com/1-powertop-screen.txt)
    * The trace of idle time stamps was collected for 120 seconds with the
    system in idle state

    Results:

    There are 4 png files that plots the idle time for each CPU in the
    system.

    Please get the graphs from the following URLs

    http://svaidy.googlepages.com/1-idle-cpu0.png
    http://svaidy.googlepages.com/1-idle-cpu1.png
    http://svaidy.googlepages.com/1-idle-cpu2.png
    http://svaidy.googlepages.com/1-idle-cpu3.png

    Each png file has 4 graphs plotted that is relevant to one CPU

    * Right-top plot is the idle time sample obtained during the
    experiment
    * Left-top graph is histogram of right top plot
    * The bottom graphs corresponding to idle times when
    sched_mc_power_savings=1

    Observations with sched_mc_power_savings=1:

    * No major impact of sched_mc_power_savings on CPU0 and CPU1
    * Significant idle time improvement on CPU2
    * However, significant idle time reduction on CPU3

    Experiment 2:
    -------------
    Setup:

    * USB stopped
    * Most daemons like yum-updatesd, hal, autofs, syslog, crond, irqbalance,
    sendmail, pcscd were stopped
    * Interrupt routing left to default but irqbalance daemon stopped
    * Powertop shows around 4 wakeups per second during idle
    (http://svaidy.googlepages.com/2-powertop-screen.txt)
    * The trace of idle time stamps was collected for 120 seconds with the
    system in idle state

    Results:

    There are 4 png files that plots the idle time for each CPU in the
    system.

    http://svaidy.googlepages.com/2-idle-cpu0.png
    http://svaidy.googlepages.com/2-idle-cpu1.png
    http://svaidy.googlepages.com/2-idle-cpu2.png
    http://svaidy.googlepages.com/2-idle-cpu3.png

    The details of the plot are same as the previous experiment.

    Observations with sched_mc_power_savings=1:

    * No major impact of sched_mc_power_savings on CPU0 and CPU1
    * Good idle time improvement on CPU2 and CPU3

    Please review the experiment and comment on how the effectiveness of
    sched_mc_power_savings can be analysed.

    At very very low wakeup count of ~4 per second on 4 CPU system gave
    good idle time result when sched_mc_power_savings is enabled.

    However the results are not as expected even at a marginal wakeup
    count of ~35 per second.

    Please let us know your comments and suggestions on the experiment or
    results.

    Do we have similar analysis and data on scheduler heuristics for power
    savings.

    Thanks,
    Vaidy



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