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Max 50% usage on dual-core CPU

I'm working with ffmpeg which uses Pthreads-w32 in Windows XP Pro SP2. I've used both Pthreads-w32 2.8.0 and 2.9.0 (which as far as I can tell doesn't exist, but is bundled with the Celtic Druid ffmpeg binaries nonetheless).

When using a single-core system (Celeron 2.5Ghz), everything is fine and ffmpeg/Pthreads-w32 eagerly uses 97-99% of my CPU. But when I move the same code to a dual-core system (P4 3Ghz), it will never exceed exactly 50% of the CPU. By default it uses both cores at about 20/30. If I change the affinity and restrict it to one core, then it uses the whole thing but again won't exceed exactly 50% of the total CPU. I get the same results reading from and writing to a flash drive, so I don't think it's an I/O bottleneck.

I'm posting this to the ffmpeg list as well, but it seems like this could be something related to Pthreads-w32. Does anyone have any thoughts about what might be going on?



Chris Stanley

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