Re: PM related performance degradation on OMAP3

[Tero Kristo <t-kristo@ti.com>]

On Tue, 2012-04-24 at 14:50 +0200, Jean Pihet wrote:
> Hi Tero,
> 
> On Tue, Apr 24, 2012 at 2:21 PM, Tero Kristo <t-kristo@ti.com> wrote:
> > On Tue, 2012-04-24 at 16:08 +0530, Santosh Shilimkar wrote:
> >> + Tero
> >>
> >> On Tuesday 24 April 2012 03:20 PM, Jean Pihet wrote:
> >> > Hi Grazvydas, Kevin,
> >> >
> >> > I did some gather some performance measurements and statistics using
> >> > custom tracepoints in __omap3_enter_idle.
> >> > All the details are at
> >> > http://www.omappedia.org/wiki/Power_Management_Device_Latencies_Measurement#C1_performance_problem:_analysis
> >> > .
> >> >
> >> Nice data.
> >>
> >> > The setup is:
> >> > - Beagleboard (OMAP3530) at 500MHz,
> >> > - l-o master kernel + functional power states + per-device PM QoS. It
> >> > has been checked that the changes from l-o master do not have an
> >> > impact on the performance.
> >> > - The data transfer is performed using dd from a file in JFFS2 to
> >> > /dev/null: 'dd if=/tmp/mnt/a of=/dev/null bs=1M count=32'.
> >
> > Question: what is used for gathering the latency values?
> I used ftrace tracepoints which are supposed to be low overhead. I
> checked that the overhead cannot be measured on the measurement
> interval (>400us), given the fact that the time base is 31us (32 KHz
> clock).

If you want to get accurate measurements, you could use ARM performance
counters, namely the cycle counter. I have a couple of patches for that
purpose I've used if you are interested.

-Tero