Hartmut,
I have patched a 3.2 kernel for a OMAP compatible processor (ARM
Cortex A8) to be used on an Gumstix Overo with the CONFIG_PREEMPT_RT
patch.
I have noticed that the latency of the patched kernel on this platform
is<300usec (using cyclictest), similar to what is reported for the
ARM9 platforms at
https://rt.wiki.kernel.org/index.php/CONFIG_PREEMPT_RT_Patch
Is this the best performance that can be expected from ARM7/9 platforms?
No, some of them have latencies below 100 us.
For a comparison of the various ARM platforms, revisions and versions,
you may refer to the OSADL QA Farm.
Relatively long latencies (this is a know CPU cache issue):
- Rack #2, Slot #1
ARM926EJ-S rev 4 (v5l), Phytec/phyCARD-i.MX27
Latency plot -> https://www.osadl.org/?id=990
- Rack #2, Slot #2
ARMv7 rev 3 (v7l), Phytec/phyCARD-L OMAP3525
Latency plot -> https://www.osadl.org/?id=988
Short latencies:
- Rack #0, Slot #5
ARMv7 rev 5 (v7l), Freescale/MX53 Quickstart Board (LOCO)
Latency plot -> https://www.osadl.org/?id=1351
- Rack #1, Slot #7
ARMv6-compatible rev 3 (v6l), (undisclosed)
Latency plot -> https://www.osadl.org/?id=1335
- Rack #2, Slot #3
ARMv7 rev 7 (v7l), Texas Instruments/OMAP3517/AM3517 EVM
Latency plot -> https://www.osadl.org/?id=887
- Rack #2, Slot #8
ARMv6-compatible rev 3 (v6l), Phytec/PhyCARD-M pca101
Latency plot -> https://www.osadl.org/?id=920
- Rack #4, Slot #4
ARMv7 Processor rev 10 (v7l)x2, Texas Instruments OMAP4/Pandaboard
Latency plot -> https://www.osadl.org/?id=911
- Rack #5, Slot #5
Feroceon 88FR131 rev 1 (v5l), Marvell/SheevaPlug
Latency plot -> https://www.osadl.org/?id=881
Could it be improved by using different kernel config settings - e.g.
CONFIG_LATENCY_TRACE=n -
https://rt.wiki.kernel.org/index.php/Cyclictest mentions that latency
tracing adds significant kernel overhead?
CONFIG_LATENCY_TRACE no longer is a valid kernel configuration item. But
even if you configure a valid tracing configuration, this will not lead
to a relevant increase of the system's latencies, since tracing must
explicitly be enabled to become effective, such as
# echo <your tracer here> >/sys/kernel/debug/tracing/current_tracer
or using cyclictest's trace options.
Of course, a particular kernel config may lead to long latencies. To
check your kernel configuration, you may compare it to the kernel
configurations of the OSADL QA Farm systems that are given at the bottom
of the system profiles, the overview is here ->
https://www.osadl.org/?id=1084.
Hope this helps,
-Carsten.
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