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Old 05-17-2022, 07:36 AM   #1
MikeLacey
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Default /dev/cpu_dma_latency settings

Hi,

Running REAPER from the command line I noticed that it was reporting permission denied on /dev/cpu_dma_latency

I changed the permission on that (device) file and now REAPER is reporting
/dev/cpu_dma_latency: set to 0

Some interesing information at:

https://blog.csdn.net/msdnchina/arti...tails/98659435

and at:

https://access.redhat.com/articles/65410

Might be worth bearing in mind when trouble shooting random latency spikes. ymmv

Edit:

I've been running with cpu_dma_latency writable so that reaper can set it as needed for a while and I haven't seen problems.

Here's how you set things up to that it is writable each time your system boots.

* Create a group called audio - if it doesn't already exist
* Add the user that runs reaper to that group
* reboot, run the groups command as that user to check it's in the audio group.
* as root, create /etc/udev/rules.d/99-cpu-dma-latency.rules containing the single line below

DEVPATH=="/devices/virtual/misc/cpu_dma_latency", OWNER="root", GROUP="audio", MODE="0660"

* save the file
* In reaper, in Options/Preferences/Audio/Device check the option "Disable power management if supported"
* click Ok
* reboot

Check that the permissions are as they should be.

ls -l /dev/cpu_dma_latency

It should look something like this:

crw-rw---- 1 root audio 10, 123 Jun 21 21:18 /dev/cpu_dma_latency
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Last edited by MikeLacey; 06-21-2022 at 12:48 PM.
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Old 05-17-2022, 10:39 AM   #2
s wave
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I have often used secondary PSUs to avoid the power spikes that cause latency issues and more. (Kind of having modular - style power)

I use dedicated power to areas that may cause this. I am SURE power usage compromises PC performance. tc good stuff
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Old 05-17-2022, 11:58 AM   #3
MikeLacey
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It's very hard to pin down.

If a latency spike is due to c-state transitions it will probably be because of multiple CPUs waking at about the same time and... that's going to be difficult to replicate.

REAPER sets the value to zero, the number of milliseconds latency it wants (seriously), so I'm going to allow REAPER to do as it intends for a while and see how things go.
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Old 06-21-2022, 12:48 PM   #4
MikeLacey
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Added info to first post in thread.
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Old 06-21-2022, 01:33 PM   #5
Xasman
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Interesting thread, MikeLacey. Thanks for posting!
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