Hi
I was exited to see if her is a measurable difference.
So I added a Scoped timer around
{
ScopedTimer t("emit(valueChanged())");
emit(valueChanged(value, pSender));
}
I did 3 test runs QT_NO_GLIB unset and 3 set. Every time I play 10
seconds of the same track:
./mixxx --devloper
Debug [Main]:
Stat("emit(valueChanged())","count=20178,sum=1.0176e+08ns,average=5043.1ns,min=123ns,max=1.20981e+07ns,variance=1.71805e+10ns^2,stddev=131074ns")
Debug [Main]:
Stat("emit(valueChanged())","count=20268,sum=9.97117e+07ns,average=4919.66ns,min=127ns,max=1.2527e+07ns,variance=1.78212e+10ns^2,stddev=133496ns")
Debug [Main]:
Stat("emit(valueChanged())","count=20289,sum=1.01444e+08ns,average=4999.94ns,min=124ns,max=1.23328e+07ns,variance=1.50495e+10ns^2,stddev=122676ns")
QT_NO_GLIB=1 ./mixxx --developer
Debug [Main]:
Stat("emit(valueChanged())","count=20603,sum=9.09682e+07ns,average=4415.29ns,min=128ns,max=1.56305e+07ns,variance=1.53043e+10ns^2,stddev=123711ns")
Debug [Main]:
Stat("emit(valueChanged())","count=20242,sum=1.04024e+08ns,average=5139ns,min=137ns,max=1.51701e+07ns,variance=2.29287e+10ns^2,stddev=151422ns")
Debug [Main]:
Stat("emit(valueChanged())","count=20601,sum=1.01946e+08ns,average=4948.57ns,min=130ns,max=1.25509e+07ns,variance=1.57278e+10ns^2,stddev=125410ns")
There is no measurable difference!
Is the test significant?
Kind regards, Daniel
Am 21.10.2016 um 14:53 schrieb RJ Ryan:
> Gah, the sorts of things we do in the audio callback thread. This
> definitely sounds worth testing, thanks Nils!
>
> I'm not sure we would see any difference in profiling since it would
> likely affect tail latency only. Does anybody have a test environment
> they could run some good A/B tests on? It sounds like a netbook or other
> resource constrained environment would be a good place to start -- set
> the latency to a value it just starts to xrun on and then test with and
> without this flag.
>
> On Fri, Oct 21, 2016 at 8:02 AM, Nils Goroll <[email protected]
> <mailto:[email protected]>> wrote:
>
> Hi,
>
> TL;DR: setting QT_NO_GLIB may make QT event delivery more efficient
> which could
> help reduce latencies.
>
> -
>
> I'm using mixxx master on Debian 8.6. While checking for possible
> underrun
> causes, I noticed that an strace of the portaudio callback thread
> shows many
> writes which appear unnecessary:
>
> poll([{fd=56, events=POLLOUT|POLLERR|POLLNVAL}], 1, 11) = 1 ([{fd=56,
> revents=POLLOUT}])
> ioctl(56, SNDRV_PCM_IOCTL_STATUS, 0x7fd3153d4b20) = 0
> write(3, "\1\0\0\0\0\0\0\0", 8) = 8
> write(3, "\1\0\0\0\0\0\0\0", 8) = 8
> ioctl(56, SNDRV_PCM_IOCTL_STATUS, 0x7fd3153d4b20) = 0
> poll([{fd=56, events=POLLOUT|POLLERR|POLLNVAL}], 1, 11) = 1 ([{fd=56,
> revents=POLLOUT}])
> ioctl(56, SNDRV_PCM_IOCTL_STATUS, 0x7fd3153d4b20) = 0
> write(3, "\1\0\0\0\0\0\0\0", 8) = 8
> write(3, "\1\0\0\0\0\0\0\0", 8) = 8
> ioctl(56, SNDRV_PCM_IOCTL_STATUS, 0x7fd3153d4b20) = 0
> poll([{fd=56, events=POLLOUT|POLLERR|POLLNVAL}], 1, 11) = 1 ([{fd=56,
> revents=POLLOUT}])
> ioctl(56, SNDRV_PCM_IOCTL_STATUS, 0x7fd3153d4b20) = 0
> write(3, "\1\0\0\0\0\0\0\0", 8) = 8
> write(3, "\1\0\0\0\0\0\0\0", 8) = 8
> write(3, "\1\0\0\0\0\0\0\0", 8) = 8
> write(3, "\1\0\0\0\0\0\0\0", 8) = 8
> futex(0x2d9f4dc, FUTEX_CMP_REQUEUE_PRIVATE, 1, 2147483647,
> 0x2d9f4b0, 2230) = 1
> futex(0x2d9f4b0, FUTEX_WAKE_PRIVATE, 1) = 1
> ioctl(56, SNDRV_PCM_IOCTL_STATUS, 0x7fd3153d4b20) = 0
>
> turns out that these are caused by glib g_wakeup_signal() writes to
> the glib
> eventfd caused by postEvent()s because QT defaults to
> QEventDispatcherGlib. See
> bottom of the email for some exemplary stack traces.
>
> Also, the receiving side (main thread) needs to read all these off
> the eventfd
> (ultimately happening in g_wakeup_acknowledge()).
>
> I _think_ that glib could do better and avoid notifications
> altogether unless
> the receiving thread is actually polling and reducing the eventfd
> notifications
> to just one, but nevertheless there seems to be an easy way out:
> Instruct QT not
> to use the glib event loop by setting
>
> QT_NO_GLIB=whatever
>
> This instructs QT to fall back to QEventDispatcherUNIX which should
> be less
> efficient for a large number of file descriptors, but appears to be
> significantly more efficient in handling QT events: Only one event
> write is
> generated to wake up poll as can be seen in the strace of the
> portaudio callback
> thread:
>
> poll([{fd=73, events=POLLOUT|POLLERR|POLLNVAL}], 1, 11) = 1 ([{fd=73,
> revents=POLLOUT}])
> ioctl(73, SNDRV_PCM_IOCTL_STATUS, 0x7f72abd81b30) = 0
> ioctl(73, SNDRV_PCM_IOCTL_STATUS, 0x7f72abd81b30) = 0
> poll([{fd=73, events=POLLOUT|POLLERR|POLLNVAL}], 1, 11) = 1 ([{fd=73,
> revents=POLLOUT}])
> ioctl(73, SNDRV_PCM_IOCTL_STATUS, 0x7f72abd81b30) = 0
> write(4, "\0", 1) = 1
> ioctl(73, SNDRV_PCM_IOCTL_STATUS, 0x7f72abd81b30) = 0
> poll([{fd=73, events=POLLOUT|POLLERR|POLLNVAL}], 1, 11) = 1 ([{fd=73,
> revents=POLLOUT}])
> ioctl(73, SNDRV_PCM_IOCTL_STATUS, 0x7f72abd81b30) = 0
> futex(0x19315ec, FUTEX_CMP_REQUEUE_PRIVATE, 1, 2147483647,
> 0x19315c0, 730) = 1
> futex(0x19315c0, FUTEX_WAKE_PRIVATE, 1) = 1
> ioctl(73, SNDRV_PCM_IOCTL_STATUS, 0x7f72abd81b30) = 0
>
>
> This won't make that much of a difference, but reducing the number
> of syscalls
> is always a good idea, so I wonder if setting QT_NO_GLIB should be
> recommended?
>
> Cheers, Nils
>
> -
>
>
>
>
> (gdb) break write
> Breakpoint 1 at 0x7ffb7a767bf0: write. (2 locations)
> (gdb) cont
> Continuing.
>
> Breakpoint 1, write () at ../sysdeps/unix/syscall-template.S:81
> 81 ../sysdeps/unix/syscall-template.S: No such file or directory.
> (gdb) bt
> #0 write () at ../sysdeps/unix/syscall-template.S:81
> #1 0x00007ffb7b2e5ef2 in ?? () from
> /lib/x86_64-linux-gnu/libglib-2.0.so.0
> #2 0x00007ffb7ef0b81e in QCoreApplication::postEvent(QObject*,
> QEvent*, int) ()
> from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
> #3 0x00007ffb7ef1ca44 in QMetaObject::activate(QObject*,
> QMetaObject const*,
> int, void**) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
> #4 0x00000000004fb547 in ControlDoublePrivate::valueChanged (
> this=this@entry=0x31e38e0, _t1=_t1@entry=3, _t2=_t2@entry=0x39ce2d0)
> at lin64_build/control/moc_control.cc:101
> #5 0x00000000004ea565 in setInner (pSender=0x39ce2d0, value=3,
> this=0x31e38e0)
> at src/control/control.cpp:203
> #6 ControlDoublePrivate::set (this=0x31e38e0, value=3,
> pSender=0x39ce2d0)
> at src/control/control.cpp:190
> #7 0x0000000000b07be2 in set (v=<optimized out>, this=<optimized out>)
> at src/control/controlproxy.h:78
> #8 SoundDevicePortAudio::callbackProcessClkRef (this=0x39ce190,
> framesPerBuffer=1024,
> out=0x3c3952f0, in=0x0, timeInfo=<optimized out>,
> statusFlags=<optimized out>)
> at src/soundio/sounddeviceportaudio.cpp:916
> #9 0x00007ffb82094f04 in NonAdaptingProcess (bp=bp@entry=0x3bba98e8,
> streamCallbackResult=streamCallbackResult@entry=0x7ffb0f971d08,
> hostInputChannels=0x0, hostOutputChannels=0x2bf24b80,
> framesToProcess=<optimized out>) at src/common/pa_process.c:873
> #10 0x00007ffb82096ec0 in PaUtil_EndBufferProcessing
> (bp=bp@entry=0x3bba98e8,
> streamCallbackResult=streamCallbackResult@entry=0x7ffb0f971d08)
> at src/common/pa_process.c:1585
> #11 0x00007ffb8209e37e in CallbackThreadFunc (userData=0x3bba9880)
> at src/hostapi/alsa/pa_linux_alsa.c:4346
> #12 0x00007ffb7cb2b0a4 in start_thread (arg=0x7ffb0f972700) at
> pthread_create.c:309
> #13 0x00007ffb7a77462d in clone () at
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:111
> (gdb) cont
> Continuing.
>
> Breakpoint 1, write () at ../sysdeps/unix/syscall-template.S:81
> 81 in ../sysdeps/unix/syscall-template.S
> (gdb) bt
> #0 write () at ../sysdeps/unix/syscall-template.S:81
> #1 0x00007ffb7b2e5ef2 in ?? () from
> /lib/x86_64-linux-gnu/libglib-2.0.so.0
> #2 0x00007ffb7ef0b81e in QCoreApplication::postEvent(QObject*,
> QEvent*, int) ()
> from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
> #3 0x00007ffb7ef1ca44 in QMetaObject::activate(QObject*,
> QMetaObject const*,
> int, void**) () from /usr/lib/x86_64-linux-gnu/libQtCore.so.4
> #4 0x00000000004fb547 in ControlDoublePrivate::valueChanged (
> this=this@entry=0x3228840, _t1=_t1@entry=0.82023751735687256,
> _t2=_t2@entry=0x3228740) at lin64_build/control/moc_control.cc:101
> #5 0x00000000004ea565 in setInner (pSender=0x3228740,
> value=0.82023751735687256,
> this=0x3228840) at src/control/control.cpp:203
> #6 ControlDoublePrivate::set (this=0x3228840,
> value=value@entry=0.82023751735687256,
> pSender=0x3228740) at src/control/control.cpp:190
> #7 0x00000000007f6b09 in set (value=0.82023751735687256,
> this=<optimized out>)
> at src/control/controlobject.h:94
> #8 EngineVuMeter::process (this=0x3224a90, pIn=<optimized out>,
> iBufferSize=2048)
> at src/engine/enginevumeter.cpp:86
> #9 0x00000000007f0a03 in EngineMaster::process (this=0x325ecb0,
> iBufferSize=iBufferSize@entry=2048) at
> src/engine/enginemaster.cpp:523
> #10 0x0000000000b0a2eb in SoundManager::onDeviceOutputCallback
> (this=<optimized
> out>,
> iFramesPerBuffer=iFramesPerBuffer@entry=1024) at
> src/soundio/soundmanager.cpp:547
> #11 0x0000000000b07c76 in SoundDevicePortAudio::callbackProcessClkRef
> (this=0x39ce190,
> framesPerBuffer=1024, out=0x3c3952f0, in=<optimized out>,
> timeInfo=<optimized out>,
> statusFlags=<optimized out>) at
> src/soundio/sounddeviceportaudio.cpp:947
> #12 0x00007ffb82094f04 in NonAdaptingProcess (bp=bp@entry=0x3bba98e8,
> streamCallbackResult=streamCallbackResult@entry=0x7ffb0f971d08,
> hostInputChannels=0x0, hostOutputChannels=0x2bf24b80,
> framesToProcess=<optimized out>) at src/common/pa_process.c:873
> #13 0x00007ffb82096ec0 in PaUtil_EndBufferProcessing
> (bp=bp@entry=0x3bba98e8,
> streamCallbackResult=streamCallbackResult@entry=0x7ffb0f971d08)
> at src/common/pa_process.c:1585
> #14 0x00007ffb8209e37e in CallbackThreadFunc (userData=0x3bba9880)
> at src/hostapi/alsa/pa_linux_alsa.c:4346
> #15 0x00007ffb7cb2b0a4 in start_thread (arg=0x7ffb0f972700) at
> pthread_create.c:309
> #16 0x00007ffb7a77462d in clone () at
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:111
>
>
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>
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>
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