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Old 05-15-07, 03:50 PM   #1
LubosD
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Default Driver's bandwidth/speed limitations?

While playing 2 movies at once, or doing simple 2D rendering while playing just 1 movie, I've noticed a strange performance slowdown.

PCIe x16 has a speed limit of 4000 MB/s. When I play two 720p movies on two screens (TwinView), I can notice a certain framedrop. But the CPU load is not the problem, both cores are at 50%.

Let's do some math:
One 720p frame (1280x720) with 4 bytes per pixel gives us about 3,6 MB. When running at 30fps, we have about 110 MB/s. Two movies at once -> 220 MB/s.

Considering that the framedrop is not given by the PCIe's limits and the GPU is not doing any scaling at all (the first screen has 1280x1024 and the second 1280x720), now where the heck is the problem? Why can't I get a completely stable picture?

And it is even worse to draw some 2D graphics while playing a movie. This is an even bigger performance hit than playing another HD movie. Simple 2D stuff like switching a tab in a Kopete chat window is practically able to pause a movie for 0.5 seconds or more.

Should this be considered a driver bug/ugly limitation or am I really hitting some hidden hardware limits?
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Old 05-15-07, 07:04 PM   #2
Lithorus
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Default Re: Driver's bandwidth/speed limitations?

How do you measure the 50% on both cores? top? or htop?

Some system monitors refers to the percentage of total cpu usage.

When I playback 720p h264 stuff at a decent bitrate I'll saturate one of the cores. (core duo 2Ghz).

Sometimes it'll even skip frames.
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Old 05-15-07, 08:55 PM   #3
AaronP
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Default Re: Driver's bandwidth/speed limitations?

XV is not a direct transfer of video data from your app into the GPU. It all has to go through the X server, which is single-threaded and can only process a single frame at a time. The Xorg process is probably hitting 100% of one core's worth of utilization.
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Old 05-16-07, 12:19 AM   #4
LubosD
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Default Re: Driver's bandwidth/speed limitations?

No, the CPU is really at 50% per core, as displayed by gkrellm and htop too.
But single-threaded X server doesn't sound nice at all, I didn't know that.
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