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Old 2005-01-20, 02:25   #27
Peter Nelson
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Oct 2004

232 Posts
Post Onboard video slowing RAM access

Nomadicus, thanks for pointing out that using some onboard video solutions could slow RAM access and therefore prime95.

I had in fact considered that this effect might come into play.

You are right that this could happen because of the video controller and the processor both wanting access to the memory they share. (because the video doesn't have its own dedicated memory).

Well I can suggest running your gui desktop in a low resolution and low colour depth which will reduce the amount of memory accessing caused by screen refresh and updating the screen buffer when something on screen changes.

Secondly I think some interleaving effect may happen minimising the effect (if the memory controller and/or processor is clever?)

As prime95 keeps memory read/write within the Level2 cache as much as possible, the main memory should not be contended with the video accesses most of the time.

Both my machines with Intel 865G chipset do share video/main memory this way. I would be interested to know of other chipsets and/or motherboards where the graphics has dedicated video ram and thus avoids this problem.

My prime95 benchmarks with 865G integrated graphics seem to be comparable with other people running on dedicated card with own memory. I therefore do not think this effect is major.

If you have any benchmark results indicating a wide variation between onboard and plug-in graphics please post them or summarise your experiences.

For applications other than prime95 programs may well use memory beyond the L2 cache, so the difference would be more noticeable.
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