[Ardour-Users] Ardour's DSP load rising and falling whilst idle [WAS: Denormals]

[Q]

THE STORY SO FAR:
-- With the session I'm currently working on, Ardour's DSP load keeps 
rising from ~50% to near/actually at 100%. This is making it impossible 
to mix as I cannot run playback for long before it grinds to a halt

-- Sometimes the session will run fine for a little while and the rise 
in DSP load starts when I stop the transport, although given the next 
point I wonder whether this is just a coincidence

-- Ardour's DSP load for this session will rise to 100% when the 
transport has been stopped for some time, then drop (sometimes as low as 
30ish percent, but usually about 50%). It will keep on rising and 
falling periodically, although I've not ascertained any frequency to it

-- This rising/falling of DSP load occurs whether other programs are 
running or not. Looking at the Task Manager, no other processes are 
using anything more than 1% CPU (and there's only one or two processes 
doing even that) when Ardour's DSP load goes up to 100%

-- All tracks and busses in the session have "Protect against denormals" 
and "Use DC bias" selected and I have "Stop plugins with transport" 
selected (although apparently this feature might not actually do 
anything). The processor denormal handling features are disabled (but 
not when using A3 -- the same load issues are seen in A3 as well, 
although the load is not as high and it only rarely goes over 90%)

-- I'm running 2.8.12 built from source under Ubuntu Studio 11.10. The 
machine is a Phenom X4 9950 with 4 GB RAM and it is running at the full 
2.60 GHz all the time (the cpufreq panel applet says it's running On 
Demand, but I think it has no choice but to run at 2.60 GHz)

-- Jack is running in RT mode, 2048 f/p and 2 p/b, 44.1 kHz, duplex 
(can't change that: when it's set to playback only, it'll only allow the 
default device, which isn't my actual soundcard), Unlock Memory selected

-- Disabling plugins and re-introducing them has not turned up any which 
are obviously causing the spikes in DSP load. I'm using EQ10Q (many 
instances), Invada Compressor (several instances), IR (two), TAP Tube 
Warmth, Invada Stereo Phaser, MS Spatialiser (a couple), Invada HPF, C* 
AMP VTS, C* Cabinet II, Richard Furse Simple Delay Line (a couple), 
Invada Delay Munge (a few), Tape Delay Simulation


THE NEW BIT:
I have just de-activated all plugins in the session and have watched the 
DSP load for five or ten minutes. With Ardour idling and no plugins 
activated, the DSP load is generally 5.0 to 5.5%, but periodically 
doubles to 10% and over. As above, there are no other processes using 
much CPU when this happens.

Yeah, 10% is nothing, but it seems that something is causing these 
periodic rises in Ardour's load, which is greatly amplified when there 
are a load of plugins activated.

Further investigations with re-introducing plugins seems to indicate 
that the three instances of Invada Delay are the biggest problem. They 
cause a considerable increase in DSP load, which is pushing these spikes 
to near 100%. Without them, when the load spikes it's only going from 
around 25% to around 50%, which doesn't cause any xruns. A single Invada 
Delay pushes the load up to 75%. Without the spikes it wouldn't be an 
issue, but for the time being I'll have to find an alternative to the 
Invada Delay so that when the load spikes inexplicably it's not grinding 
the machine to a halt.

So when Ardour is ostensibly doing nothing, what could be causing this 
periodic rise/fall in the DSP load?

As ever, any hints, helpful suggestions or advice gratefully received.

Thanks in advance,

Q

PS If this too general or not Ardour-specific let me know and I'll take 
it over to LAU.