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@xxxxxxxxx> 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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html
Attachment:
signature.asc
Description: PGP signature
