Hi Tim, On 26/12/16 00:58, termtech wrote: > Hi list. Hope Rui gets this... > I have a puzzling technical question, involving QTractor. > > I have been looking very closely at how Qt's class QThread works. > > Actually, it doesn't. On Linux. As a normal user. > > A long investigation into its source code and ultimately some test programs, > shows that on Linux it ignores what priority you attempt to start it with, > if running with normal user privileges.
Yes, this is expected. See below. > > In Qt's qtbase/src/corelib/thread/qthread_unix.cpp: > > void QThread::start(Priority priority) > { > ... > pthread_attr_t attr; > pthread_attr_init(&attr); > ... > } > > This code in Qt shows it initializes with default attributes which have > scheduling 'other' and thus the range of priority levels is ZERO and thus > QThread is lying about being at QThread::TimeCriticalPriority for example - > it is NOT, it stored your requested level but did not actually set it. > > I just finished examining the following programs using 'htop' plus a > custom program to ask what each's scheduling policy is, > using pthread_attr_getschedpolicy(): > > (With Jack running realtime etc. etc...) > > a) MusE : Two threads running FIFO policy (policy = '1'). Good, expected. > MusE directly uses pthreads, and set policies and levels. > I assume since I'm in the audio group with good rt limits, > some help is going on, 'cause I ain't runnin' as root. > b) QTractor : NO threads running FIFO (all policies = '0'). Bad? Unexpected? > QTractor uses a few different QThreads with various priority settings > including one requesting TimeCriticalPriority. Seem's it's not happening? > c) Qt5Creator test program using QThread set to TimeCriticalPriority: Bad - > thread has policy = '0'. > > The Mixxx project found the exact same thing: > https://www.mixxx.org/wiki/doku.php/performance_improvements > > "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. " > > A further look at the ticket shows they had to go with pthreads directly. > > What are your thoughts on this? Am I doing something wrong? > Is QTractor really running realtime or not? > > After seeing the source and fooling with this, I'm disliking QThread. > > Before creating a QThread, I tried to elevate using seteuid, setegid > and so on, and tried elevating to group 'audio'. > Permissions denied, of course. > This is not a Qt problem - it is a user permission problem. If sched_setscheduler() returns -1, check if errno is set to EPERM. In this case the user trying to perform this operation does not have CAP_SYS_NICE[1] capability, which is *required*. [1] http://man7.org/linux/man-pages/man7/capabilities.7.html If you want this type of feature, set CAP_SYS_NICE to the group audio that you are referring. -- Felipe
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