jwzumwalt wrote:Memory split is a "suggestion" to the OS on how to use memory. How much should be dedicated to video and how much should be available to caching etc. When you split (allocate) memory, the OS will disregard the designated amounts of memory if it gets in trouble. But it uses your selection as a guideline - as long as everything goes well.
I use 256mb video and 256mb OS because I do a lot of graphics intensive programming (graphics instrumentation). I have seen the results of others benchmarks and they have found it very difficult to get the OS to use more than about 150mb. However, if the CPU would ever need more than 256mb, it will happily prioritize and start stealing from the video memory.
That is NOT how memory split currently works on Pi - the memory split is a static setting at boot time, and can't be altered at runtime.
There is/was some work on dynamic memory split (CMA) to vary the GPU split at runtime - but its not in normal use (beta) as hasn't found to be reliable yet.
How much to set it at ?
If you have a 512MB Pi (modern model B), set it at 128MB and forget about it.
That's enough for every use of the GPU and leave "more than enough" to the CPU.
On a 256MB Pi (Model A / or older 256MB model B) - then you may find yourself changing it (smaller GPU RAM means more available for CPU and so often faster opeation in X, particulally web browsing).
16MB leaves 240MB for the OS, but then you can't view accelerated video e.g. youtube/ or mpeg video clips
64MB allows accelerated video
128MB is needed for heavy graphics use (XBMC) or the Pi Camera
As said, things are less cramped on a modern 512MB model B - 128Mb leaves 384MB for the OS, which is plenty.