Performance Improvements

Performance improvements

Here we enumerate all of the areas where Mixxx can be optimized, roughly in order of importance. The more things we can complete on this list, the more machines Mixxx can run on, and the larger our user base.

Thread scheduling

The most important thread is the audio callback thread. If this misses its deadline (the latency period,) pops and clicks can be heard in the output. As a result, it needs real-time priority, preferably with a hard deadline matching the latency period. (And we need to make sure it runs as succinctly and efficiently as possible so it can complete its work within the latency period (which can ideally be as low as 1ms.) Taking any longer would be shooting itself in the foot.)

TO DO: Ensure the audio callback thread gets real-time priority when run as a regular user. - RJ discovered that Mixxx's requests for real-time priority on this thread are having no effect. Running as a regular user on Linux shows that the priority range is from 0 to 0, and as root from 1 to 99, but it is set to 1 (the lowest) by default. However, calling setPriority(QThread::TimeCriticalPriority) (while running as root) does result in priority escalation. (xwax has been able to solve this problem on Linux. See the rig_realtime function in rig.c.)

Thread consolidation

As Mixxx's functionality grows and it's extended to work with arbitrary numbers of resources, we need to consolidate the work to handle each resource into a single thread. This currently needs to be done for:

• MIDI Script Engine - currently one thread per attached controller, need one thread with n ScriptEngines in it
• (Caching)Reader - currently one thread per deck, need one thread with n Readers in it

CPU

(Anything found that wastes CPU time should be listed here)

• Line 881 in enginebuffer.cpp process() fires continuously while Mixxx is idle: fixed by rryan

// Stopped. Wheel, jog and scratch controller all scrub through audio.
rate=(wheel->get()*40.+m_pControlScratch->get()+m_jogfilter->filter(m_pJog->get()))*baserate; //*10.;
m_pJog->set(0.);

• Line 43 in controlobjectthreadmain.cpp eventFilter() also fires incessantly while idle:

emit(valueChanged(ce->value()));

• Line 96 in controlpotmeter.cpp setValueFromThread() also fires very often while idle:

emit(valueChanged(m_dValue));

• Line 107 in controlpotmeter.cpp setValueFromEngine() also fires very often while idle:

emit(valueChangedFromEngine(m_dValue));

• EQ code takes up 50% of our current CPU usage on a Pentium 4 (with static EQs! HQ eqs are even slower...)

Time-critical code

(Please list all time-critical code sections/functions here to be considered for assembly language reimplementation.)

• EQ code - SSE-enhanced implementation is in progress in the features_hydra branch (unfortunately it wasn't given its own.)
• EngineBufferScale: EngineBufferScaleLinear and EngineBufferScaleST - also see features_hydra
• SoundDevicePortAudio - also ripe for SSE reimplementation

For reference:

• Intel64 & IA-32 manuals
• AMD64 manuals

Memory

(Anything that wastes memory by inefficient storage (where unnecessary) or leaks goes here)

• https://bugs.launchpad.net/mixxx/1.7/+bug/410841
• Currently loading four two copies of MP3 files into memory. (r2509 in 1.7 removes the second copy as soon as the Analyser is done with it, plugging a big leak.)
• Vinyl control leaks part of the lookup table, or something like that when it gets deleted/recreated
• SoundSource's should use memory mapped IO. Making that cross platform is tricky.