[LAU] Isolating/dedicating Interrupts (was: irqbalance)?

Len Ovens len at ovenwerks.net
Sun Nov 22 15:34:23 UTC 2015

On Sun, 22 Nov 2015, Peter P. wrote:

> * Len Ovens <len at ovenwerks.net> [2015-11-21 18:55]:
> [...]
>> I would probably turn HT off anyway and the i5 runs cooler
>> (=quieter BTW with similar cooling setup) If I wanted more speed, I would
>> look at a xeon with no HT or an i7 and turn ht off in BIOS if I could. That
>> is I would add more cores rather than HT.
> Interesting, Len, why do you recommend to disable HyperThreading? What
> problems / disadvantages wrt. audio could it cause?

I found that it did not effect audio so long as I had jack set higher than 
64/2 for latency, but with HT turned off, I can get 16/2 with good 
stability and no xruns over a 24 hour period (assuminig cron is turned off 
:)  )

HT means that the system (not the OS, but HW) can stop the high priority 
audio process on one half of a core, to run whatever on the other half of 
the core. Linux may have learned to handle this better than the last time 
I tested this... I am guessing the only way the OS could fix this is to 
not assign anything to the other half core along with a high priority 
thread... but I really don't know that part.

There just seem to be some things that are bad for low latency/RT work:
  - anything where the cpu is doing something the OS is not aware of:
 	- HT
 	- Temperature monitoring (this seems to be fast enough not to
 	  matter so long as the temp is kept low)
 	- Some older "In the cpu GPU stuff".
 	- Other CPU utilities that run outside of the OS (SMS I think)

  - CPU speed changes. In theory this should not effect things, but I have 
gotten lower latency with my cpu set hard to 800Mhz than with it running 
3.2 Ghz and speed set to OnDemand. The best setting is Performance which 
sets top speed. Turn off things in BIOS like Boost (Intel) and the thing 
in AMD that allows one core to over speed if the rest are idle(ish).

It seems I often get xruns at the point the cpu speed goes down, but it is 
ok on an upward change. (seems is the operative word)

More on speed: Every once in a while it is good to run a cpu hog for each 
core for about 10 minutes to see where the temperature levels off at. If 
the temperture is too high:
  - cleaning required
  - better cooling setup required
  - set cpu speed mode to User and run the cpu at a fixed speed lower than 

The last one may be the only thing possible on a laptop.

* by CPU hog I mean something that pulls the core to 100% use. (compiling 
Ardour seems to do that for me, but there are utilities out there just to 
stress systems like cpuburn or stress)

Len Ovens

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