[Interest] Poor QMethod::invoke performance across threads on i.MX53

[Thiago Macieira]

On Thursday 14 May 2015 18:18:52 Robert Daniels wrote:
> I'm working on a project that uses a QThread to process communication over a
> TCP connection. In trying to improve performance I've tracked down an odd
> slow-down when we use QMethod::invoke to force a method to be called on the
> correct thread.

You don't need threads to get acceptable performance of TCP. The networking is 
usually much slower than the processing itself.

> This is an embedded project running on a Freescale i.MX53.  On this board,
> I've measured the time to make this call and it is on average around 400000
> ns. I've measured this on our i.MX6 board as well (with only one core
> enabled) and it takes only 60000 ns. This doesn't look right to me. I would
> expect the difference to be at most 2x, not 6x.

Can you benchmark this in any way? What is consuming CPU time here? Is it in 
kernel mode? Is it waiting? Is it the QSemaphore inside the blocking queued 
connection>

> Is there something I'm missing here in my expectations or is there something
> fundamentally wrong causing this slow-down?

There are problems, but not related to the issue you're reporting.

> class WorkerThread : public QThread
> {
>     Q_OBJECT
> 
> public:
>     WorkerThread()
>     {
>         moveToThread(this);

This is wrong. Never do "moveToThread(this)", since it's very difficult to then 
destroy the QThread object. This is unrelated to the problem and it's probably 
only for simplicity of your testcase.

> private slots:
>     void onTimer()
>     {
>         qDebug() << "Begin test.";
>         QElapsedTimer timer;
>         qint64 total = 0;
>         const int cnt = 100000;
> 
>         for (int i = 0; i < cnt; i++) {
>             timer.start();
>             worker.myMethod(i);
>             total += timer.nsecsElapsed();

nsecsElapsed may not have enough resolution on your board. The problem might 
not be slowness, but simply that the clock isn't good enough.

Move the timer.start() and timer.elapsed() calls outside of the loop.

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-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center