Re: Regression on rt kernel while using POSIX timers

On Mon, Feb 13, 2017 at 8:48 PM, Patel, Vedang [off-list ref] wrote:

On Fri, 2017-02-10 at 20:07 +0100, Sebastian Andrzej Siewior wrote:

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On 2017-02-08 18:41:25 [+0000], Patel, Vedang wrote:

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Hi,

Hi,

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The results for the POSIX timers were not as I expected. The
latency
for real-time kernel (v4.9.4-rt2) was worse compared to the
mainline
kernel (v4.9.4). In almost all the cases, the latency is almost
doubled
 with the max value reaching about 10 times when performing the
tests
under load.

Is it also the case if you boost the priority of ktimersoftd/X
threads?
For clock_nanosleep, the wake-ups happen directly from hard-timer
interrupt. For the posix-timer we have to delay those to the
ktimersoftd
thread which runs usually RT prio 1.

I am getting very similar results even if I change the priority of
ktimersoftd to 99. Are there any recent rt patches which might have
changed the behaviour of POSIX timers?

Also, are POSIX timers really suited for "real-time" applications?I
believe a similar question was raised by Ran Shalit a few days back:
http://www.spinics.net/lists/linux-rt-users/msg16249.html

Right.
I did  testing of recent kernels 4.1.15,  with Atom , and the
non-posix timers jitter was obviously better compared to non-posix
(42usec vs 180us),
On trying 4.4.x kernel I seen very strange behavior for both timers
(maybe I applied the patch wrongly or needed to disable some features
in config?) ,

So I decided eventually to use the 4.1.15 with non-posix timers only
with our Atom board. I hope that's a good decision.

Thanks,
Vedang

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Thanks,
Vedang Patel
Software Engineer
Intel Corporation

Sebastian