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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?"
But seriously, you may want to take a look at this Tom's Hardware article detailing the weaknesses of an integrated chip.
For those looking for the quick answer, I'll do my best to summarize. First off, since integrated graphics tend to be low cost solutions, transistor counts are nowhere near current add-in boards. From the article, Nvidia's FX5200 has 47 million transistors (FX5600=80 million and FX5900=130 million), while their onboard solution (equivalent to GeForce4 MX440) has only 27 million.
Then, there's the question of memory bandwidth. Dual channel DDR 400 has a peak of 6.4GB/s, which is shared, while an equivalent GeForce4 MX440 would have a dedicated 8GB/s.
Now, to your question. Does this consume a ton of bandwidth and affect performance? Well, that would all depend on what you're doing with it.
If you're running 3D games and the like, then both performance and bandwidth will be an issue and limit your framerates. Comparing the previous review and this review of add in boards, shows about a 25% reduction in framerate (at 1024x768) between an add in GeForce4 MX440 and an NForce2 integrated chipset in UT2003, and an almost 40% reduction in 3DMark 2001. Since the machines were not identical, don't take the numbers as gospel, but they were similar enough to make a meaningful comparison IMHO.
That being said, for normal 2D work, bandwidth utilization is negligible and shouldn't seriously impact performance as shown by this SysMark 2002 test. AFAIK, this doesn't take into account extremely intensive RAM->CPU loads, but I wouldn't expect results to vary significantly, since memory requirements for 2D work are relatively low.
Be warned though, that Tom's Hardware did note image quality issues with most of the integrated chips-which they theorized was the result oflow cost manafacturing, not a limit of the technology itself. This theory is bolstered by the fact that their low cost add in card (Radeon 9200) suffered the same problems.