[LAD] cpu spikes

Fokke de Jong fokkedejong at gmail.com
Sun Jan 24 14:03:08 UTC 2016

Hi all,

This is my first post here. I’m note new to audio programming or linux, but I haven’t done much in terms of combining the two. Most of my audio programming has been on os x.

Currently working on some realtime convolution with lots of channels and low latency requirements, but I am running into some unexpected cpu-spikes and hope some of you might have an idea of possible causes.

I’m processing 32 sample-blocks at 48KHz but roughly every 0,6 seconds I get a large spike in cpu usage. This cannot possibly be explained by my algorithm, because the load should be pretty stable. 

I am measuring cpu load by getting the time with clock_gettime(CLOCK_MONOTONIC_RAW, timespec*) at the beginning and end of each callback. When converted to a percentage my cpu load hovers somewhere between 40 an 50% most of the time, but more or less every 900 callbacks (0.8 seconds there is a spike of more than 100%.

I am not doing any IO, mallocing or anything else that could block. My threads are SCHED_FIFO with max priority (I have 4 threads on 4 cores).
The only explanation I can come up with is that my threads are somehow pre-empted even though there are realtime threads. Is that even possible?  And is there a way to check this? Besides pre-emption maybe my caches are severely thrashed but i find that unlikely as it seems to happen on all 4 cores simultaneously. 

I’m running (more or less default install, no additional services run-in) Linux Mint 17.3 with a 3.19.0-42-lowlatency kernel on a core i7-6700 with hyperthreading/turbo disabled.

I remember reading somewhere that realtime threads cannot run more than .95s every second. That would be very bad if it actually meant my threads are blocked  run for a period of 50ms straight…

Anyone have any thoughts on possible causes?


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