Comment # 81
on bug 97879
from Marek Olšák
I merged a different patch that just improves GL_MAP_READ_BIT and returns a mapping with CPU caches enabled, with the downside that it does a copy from the uncached buffer into a cached temporary buffer, maps that, and if GL_MAP_WRITE_BIT is set, the temporary buffer is copied into the original buffer on unmap. The reason GPU drivers prefer uncached allocations is that the GPU access to the buffer is faster while the CPU access is slower (except for sequential writes). If we enabled caching, the GPU access would be slower, but the CPU access would be faster. Or we could have 2 buffers and do copying back and forth like for GL_MAP_READ_BIT, but again the copy also isn't free and it's worse if you have to do 2 copies - one in Map and the other in Unmap. The malloc + memcpy solution on the app side is probably the best one.
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