There are a few possible reasons to this. Remove the Quartz Compositor component from the pipeline and CPU usage drops dramatically, especially because Flash and Silverlight have hardware-accelerated video rendering and video decoding. Long story short: Quartz Compositor + very frequent screen updates (30 to 60 times per second with video) = high CPU usage. Quartz Extreme is the modern incarnation of the Quartz compositor, which does everything I described above on the GPU itself (although these operations aren't "free" just because they're offloaded to the GPU the GPU still consumes power performing these compositing steps.)įor a developer perspective of exactly how a program (such as Flash, or Silverlight) can take full screen control and temporarily disable the Quartz Compositor, see this Apple developer doc. The compositing window manager used on Mac OS X is called Quartz Compositor and it has been a part of OS X for a long time. But then, when the compositing window manager is disabled (at the request of privileged programs such as Flash and Silverlight), suddenly that extra layer of "overhead" is gone. This is partly to enable "effects", and partly for security and stability reasons, because programs are unable to directly corrupt the desktop (either maliciously or by bad coding) the compositing window manager won't let it. The browser, the video player, and every other component on the system has to go through the compositing window manager, like a gatekeeper or spokesperson, which sits between the hardware and the user space. The compositing window manager has to keep asking for the video content to be transferred back to the CPU because, under normal circumstances, the only process on the system that is allowed to write directly to the video framebuffer is the compositing window manager. The fans spin up and more power is eaten when the video is windowed precisely because the compositing window manager is always saying, "OK, what does the video look like right now?" and it has to be read back into the CPU (and probably buffered in RAM) before the compositing window manager then decides to write it right back out to the graphics card, along with all the other composited data (the browser, the taskbar, etc.)
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