Re: cyclictest better values with system load than without (OMAP3530 target)

[Clark Williams <williams@redhat.com>]

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In my experience (on x86_64 mainly), that behavior (worse times when
not under load) is due to the overhead of coming out of power-save/idle
states. When you've got a big load on the system and all the cores are
active, then the power-save logic and/or the idle logic doesn't kick in
and devices aren't being powered down.

Do you know if your OMAP has power-save logic available? Alternatively
do you know how expensive the idle mechanism is? Have you tried booting
with idle=poll then measuring without a load?

Clark


On Tue, 26 Nov 2013 10:21:34 +0100
Stefan Roese <stefan.roese@gmail.com> wrote:

> Hi!
> 
> I'm running cylictest on a OMAP3530 target board and am a bit
> astonished about the results. Especially that the latency values
> are better on a system with system load (hackbench) than on one
> without system load. Here the values I get:
> 
> With system load (hackbench):
> -----------------------------
> # ./cyclictest -l 10000 -i 1000 -n -p 80 -q
> # /dev/cpu_dma_latency set to 0us
> T: 0 ( 1853) P:80 I:1000 C:  10000 Min:     36 Act:  156 Avg:  154 Max:
>     244
> 
> Idle system:
> ------------
> # ./cyclictest -l 10000 -i 1000 -n -p 80 -q
> # /dev/cpu_dma_latency set to 0us
> T: 0 ( 2332) P:80 I:1000 C:  10000 Min:     81 Act:  530 Avg:  484 Max:
>     602
> 
> 
> Some details to my test/system setup:
> - Linux v3.8.13
> - preempt-rt patch 3.8.13-rt14
> - HW: TI OMAP3530 CM_T35 board
> - Latest cyclictest from rt-tests git repository
> 
> 
> I might have misconfigured the system. So here some extracts from
> my .config:
> 
> ...
> CONFIG_TICK_ONESHOT=y
> CONFIG_NO_HZ=y
> CONFIG_HIGH_RES_TIMERS=y
> ...
> # CONFIG_CPU_FREQ is not set
> # CONFIG_CPU_IDLE is not set
> # CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED is not set
> ...
> CONFIG_PREEMPT_RT_FULL=y
> ...
> 
> With CONFIG_NO_HZ disabled I get slightly better results:
> 
> With system load (hackbench):
> -----------------------------
> # ./cyclictest -l 10000 -i 1000 -n -p 80 -q
> # /dev/cpu_dma_latency set to 0us
> T: 0 ( 1840) P:80 I:1000 C:  10000 Min:     30 Act:  153 Avg:  154 Max:
>     238
> 
> Idle system:
> -------------
> # ./cyclictest -l 10000 -i 1000 -n -p 80 -q
> # /dev/cpu_dma_latency set to 0us
> T: 0 ( 1371) P:80 I:1000 C:  10000 Min:     40 Act:  465 Avg:  435 Max:
>     502
> 
> 
> Any ideas/explanations are really appreciated.
> 
> Thanks,
> Stefan
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