Shared Video Memory and Memory Bandiwidth Issues?

klystron2 asks: "Does shared video memory consume a huge amount of memory bandwidth? We all seem to know that a notebook computer with shared video/main memory will have performance drawbacks.... But what exactly are they? It's easy to see that the amount of main memory decreases a little bit, but that shouldn't make a big difference if you have 1GB of RAM. Does the video card trace through memory every time the screen is refreshed? Therefore consuming a ton of memory bandwidth? If this is the case then the higher the resolution and the higher the refresh rate, the lower the performance of the system, right? I have searched the Internet for an explanation on shared memory and have come up empty. Can anyone explain this?"

Pro/Con

[Tune]

Besides the obvious penalty caused by the DAC, there should also be a minor improvement in applications where the CPU needs to have direct access to (physical) screen memory (older 2D games) or the GPU needs data stored in main memory (3D bitmaps). In the "standard setup" this would require data transfer from main memory to video memory (and vice versa) including the overhead of PCI/AGP synchronization.
Wrt. performance the benefits of a separated frame buffer outweigh those of shared memory, in my experience. I'm not sure if this is true as well wrt. the performance/power consuption ratio (use suitable definition), however. Especially when the (Dvi LCD/TFT) screen already has a frame buffer and VSync is only 20-40Hz. (Ditch GPU altogether?)

Anyone with ideas, data?

--
As far as we know, our computer has never had an undetected error -- Weisert

I'd assume so, too

[Scorchio]

I can't say for current day laptops, but early ARM based Acorn computers had shared video and system memory. It did indeed suck bandwidth, which seems was a problem when it came to IO. If you set the resolution high enough, the screen would turn black while loading from disk, presumably so any incoming data from the drive doesn't get lost while the bus is in use by the video output. Fortunately, at the time, the OS and software in general didn't constantly suck at the disk, as is common today.

I presume today's bus speeds, processor caches and other buffers are sufficiently fast and large enough to share the memory without too much of a noticable effect...