[Spice-devel] Processing of rendering operations in separate thread

[Frediano Ziglio]

> Hello Yuri

> El vie, 25-11-2016 a las 01:08 +0200, Yuri Benditovich escribió:

> > I'm porting to [qxl-wddm-dod] set of flexvdi changes
> 
> > related to execution of ' present display only' events
> 
> > in separate thread. There are 2 questions below I'd like to ask and k now
> > your opinion.
> 

> > I see there 2 aspects:
> 
> > - reliability
> 
> > - performance
> 

> > Reliability:
> 
> > I see in flexvdi mailing list existing report of
> 
> > BSOD upon system shutdown. Possible cause is lack of
> 
> > synchronization between system flows, hardware availability and worker
> > thread
> > state (last patch in flexvdi 'Terminate working thread on exit' introduces
> > termination procedure but nobody calls it, as I can see)
> 
> > The lack of synchronization may cause also races in
> 
> > power management flows and (possible) on changing
> 
> > operating mode.
> 

> > Question 1:
> 
> > Do you have some additional recommendation which
> 
> > flows shall be specially checked for races with
> 
> > rendering thread?
> 

> Unfortunately, the truth is, we have not thoroughly tested our code to remove
> these races yet. The clients this driver was intended for are still stuck
> using Windows XP/7, and our development is stalled. So, I cannot think of
> any situation you should check that you do not know about yet.

> > Performance:
> 
> > It looks like the change should not affect total CPU consumption for
> 
> > the rendering, it splits more or less the same operations over
> 
> > 2 different threads. It is still possible that the change can improve
> 
> > common user experience due to faster indication of operation completion to
> > the OS.
> 

> We were not trying to reduce total CPU consumption. After all, the driver
> just copies rects from main memory to VRAM and passes them to the spice
> server; there is little to reduce there. Rather, we tried to increase the
> throughput of graphic operations, by not locking the DirectX subsystem while
> we wait for the spice server to accept new drawables. That is, we do not
> mind using more CPU if that results in painting faster.
> On the other hand, I was thinking that maybe we could get the DirectX
> subsystem to provide the rects already in VRAM if we described it as a
> linear memory segment on driver initialization. In that way, the copying
> operation could also be removed. However, I am not sure if this actually
> works or even how to do it, it is just an idea.

> > Question 2:
> 
> > Do you have some ideas how to make quantitive
> 
> > evaluation of this possible improvement of user experience?
> 

> > I think about:
> 
> > - finding scenarios when we receive rendering calls (PresentDisplayOnly)
> > when
> > the worker thread is still processing previous operation. If they exist
> > this
> > can mean that some bottleneck solved in GDI.
> 
> > - writing or getting tool that loads the graphics
> 
> > adapter by heavy operations (like continuos moving of window / scrolling
> > etc)
> > with CPU consumption measurement
> 

> We used a simple tool to measure the performance: it creates a window and
> continuously issues WM_PAINT events where the full background is filled with
> color, then measures the number of processed events per second (not CPU). It
> is quite naive, but it provides a good starting reference, since the tool,
> with the XDDM QXL driver in Windows 7, outputs almost twice as much paint
> events as executing it in Windows 8 with the WDDM QXL driver. There are
> other measurements you can try to obtain, like how much time does it take
> until a paint event gets to the spice server queue, ready to be sent to the
> client (although I'm not sure how to measure it). This delay affects the
> user perception of performance.

> > Please share your thoughts.
> 

> > Thanks,
> 
> > Yuri
> 

If you don't call StopPresentThread to stop the thread and you free the class the thread will continue 
to run accessing freed data.Possibly even the code thread is executing is unloaded from memory too 
so the crash is quite expected (didn't look at the code before). It seems to me that data is correctly 
synchronized, mainly the usage of the ring is shared. 

Personally I would avoid the usage of m_TerminateThread and push a NULL as drawables so sign 
that worker thread should close. This will also avoid leaks if there are still pending drawables in the 
ring. 
m_PresentLock looks useless. 

Frediano 
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