Latency Tuning Questions

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15 May 2019 00:45 #133797 by Project_Hopeless
I'm tuning a Dell Inspirion Pentium DualCore E2180 to run a yet to be purchased 7i96e Mesa card.

I've been trying some of the tips found in this older wikki'

http://wiki.linuxcnc.org/cgi-bin/wiki.pl?RealTime

I'm currently trying;
isolcpus=1
intel_idle.max_cstate=0
processor.max_cstate=0
idle=poll"

GRUB_CMDLINE_LINUX_DEFAULT="isolcpus=1 intel_idle.max_cstate=0 processor.max_cstate=0 idle=poll"

The wiki says not to permanently run all these combined settings as the CPU can over heat.

Are these still valid settings for Preempt_RT and Stretch? Are there different settings for Stretch?

The changes helped somewhat. When I run latency test by itself I get the following;
Servo 2K ns
Base 7K ns

Which is much better. When I open the Browser run a utube video they jump to 90-130K which is only slightly better then without the changes. If I bang around, like writing this post they climb to 195K, 140K ns respectively.

Any thoughts?

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15 May 2019 02:41 #133804 by Todd Zuercher
The little I've played with a Peempt-rt machine, this is what I found. Turn off all power management and Core2Duos states in the Bios, have at least 2gb of memory, and try isolcpus. Isolcpus made a pretty big difference on the i5 cpu machine I was messing with. I don't think the cpu hog and idle poll techniques are applicable to Preemt-RT (or were even a good idea when they were. )

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15 May 2019 11:24 #133828 by pl7i92
Replied by pl7i92 on topic Latency Tuning Questions
at 7000 latency most here woudt be in heven stepper systems run perfect on < 150k

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15 May 2019 13:19 #133835 by Todd Zuercher
Problem is he isn't seeing 7k, not even 150k he's getting almost 200k. 7k for a period of time when the machine is idle doesn't count. It needs to be consistent ALL the time regardless of machine state or usage. A large outlier at the wrong time while machining could have devastating results.

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15 May 2019 13:23 #133836 by Todd Zuercher

Todd Zuercher wrote: The little I've played with a Peempt-rt machine, this is what I found. Turn off all power management and Core2Duos states in the Bios, have at least 2gb of memory, and try isolcpus. Isolcpus made a pretty big difference on the i5 cpu machine I was messing with. I don't think the cpu hog and idle poll techniques are applicable to Preemt-RT (or were even a good idea when they were. )


PS I forgot to say, don't forget to turn off Hyperthreading in the BIOS, it is utterly useless for real time.

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15 May 2019 14:58 #133845 by pl7i92
Replied by pl7i92 on topic Latency Tuning Questions
Sorry For missreading the Text
then he needs to find the reason maybe SMI is taking effect
wiki.linuxcnc.org/cgi-bin/wiki.pl?FixingSMIIssues

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15 May 2019 15:15 - 15 May 2019 15:19 #133847 by PCW
Replied by PCW on topic Latency Tuning Questions
Another thing that helps noticeably with Preempt-RT is CPU speed and cache size.
If you replace the E2180 with a E8500 you get about double the performance and
3 times the cache size (for about 6 bucks on Ebay)...

Edit: Note if you intend to use a Ethernet card you should not have a base thread enabled
in your test as it will give inaccurate results.

Heres a E8500 Core Duo on a HP DC7800, with a current kernel
(3 day test doing normal browsing + GLXGears)


Attachments:
Last edit: 15 May 2019 15:19 by PCW.

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15 May 2019 16:49 #133856 by pl7i92
Replied by pl7i92 on topic Latency Tuning Questions
PCW at your knolige it is very easy to do this
while only month into Linux and CNc it is very hard
and on a NEWby it is a nightmare

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15 May 2019 17:30 #133862 by PCW
Replied by PCW on topic Latency Tuning Questions
Changing a CPU should not be a big deal...

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15 May 2019 18:23 #133874 by InMyDarkestHour
Mounting root with the noatime option can give a little reduction when opening files. I have noticed that on some systems opening a file can cause a jump in latency. Getting your hands on an SSD can help as well.

More about fstab and file mounting options.
wiki.debian.org/fstab

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