Re: 2.6.24-rc8-rt1: Strange latencies on mpc5200 powerpc

Hi Fu,

Luotao Fu wrote:
Hi,

I took some time today and went through Wolfgangs scenarios partly. Now
some results from my side. I ran my tests on a 2.6.24-rt1

Wolfgang Grandegger wrote:
> I also did some more measurements and made, by chance, interesting
observations. I will summarize in more detail later on. Here are some
preliminary results. My high latencies of up to 570us (without latency
tracer) seem to be caused mainly by the following setting:

CONFIG_RCU_TRACE=m


I also got high latencies without CONFIG_RCU_TRACE set at all. setting
CONFIG_RCU_TRACE to y or m causes also high latency though in my test
runs. Hence I doubt the rcu Tracer is really the problem.

As I mentioned in my last mail. Only thing I could reproduce reliably is
that the measurement results depend heavily on kind of the non-rt
Workload. For with hackbench or cache calibrator I couldn't produce
abnormal high latencies. Nor could I produce the hight latencies on a
system booted via flash. Hence my suspects stays on the fec irq thread
and filesystem access routines.

With CONFIG_NO_HZ=y or CONFIG_PPC_BESTCOMM_GEN_BD=y the latency
increases by approx. 100..150us, each.


Since I was producing high latencies independently from the rcu
settings. I didn't spend a lot of time playing around with the GEN_BD
and dynamic clock. All I can say is that disabling them also cause high
latency. ;-) The average results don't differ significantly in my test
runs.

In the meantime I have measured the impact more carefully and posted the
results to the list:

- CONFIG_RCU_TRACE or CONFIG_PREEMPT_RCU_BOOST not set causes high
latencies up to 500..600us.

- CONFIG_NO_HZ=y increases the latency by 90 us, at least.

- CONFIG_PPC_BESTCOMM_GEN_BD=y seems not to harm.

Do you still get high latencies with:

CONFIG_PREEMPT_RCU_BOOST=y
CONFIG_RCU_TRACE=y
CONFIG_NO_HZ is not set

With this setting I have not yet realized latencies > 150us. Could you
please give it a try? If I change one of the parameters above, latency
increases in short time.

Thanks.

Wolfgang.


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/