MacOS - vncviewer: High CPU load due to writing the image to the screen #372

fredowski opened this Issue Oct 25, 2016 · 2 comments


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fredowski commented Oct 25, 2016 edited


i looked at XCode Instruments to see where the CPU cycles are consumed in the vncviewer process. I wanted to check if I can see a difference in the jpeg vs. jpeg-turbo build. I see roughly 20% of the load going to jpeg decoding, while 80% is spend more or less in writing the image to the screen. So on Mac the jpeg decoding seems to be a minor problem compared to image writing. I run this on a MacBook Pro with HighResolution Display - so maybe something is converted there...

Here is a screenshot showing the Main Thread vs. one of the worker threads with jpeg decoding:

Here is the first part of the main thread:


and here is the remaining part.


60% of the total CPU power for vncviewer is spend in the OS routine: argb32_image_mark_RGB24.

Googling seems to show more problems with this:


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@CendioOssman CendioOssman added the bug label Oct 28, 2016

Yes, I'm afraid this is a somewhat known issue. Unfortunately none of the suggested fixes we've found so far have worked.


It also happens on all Macs, not just high resolution displays.

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