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This page describes the RAM specifications for many of the MacBook laptops. In the section for my laptop (13" Pro Late 2011), it says:
Additional Notes: For best performance, fill both memory slots, installing an equal memory module in each slot.
This isn't the first time I've heard of this phenomenon, and it's definitely not specific to Apple.
Why is this the case? Other than the obvious decrease in available memory, why would it be worse to run with 6GB (1x4GB + 1x2GB) than with 8GB (2x4GB)?
First...Its not a phenomenon it has to do with the memory channels. Second there would be no performance increase/decrease between your examples. – Ramhound – 2014-10-07T17:24:47.240
@Ramhound if there is no performance decrease, why is there that warning on the Apple website? – LanceLafontaine – 2014-10-07T17:26:38.580
I said in your example if you were not running in dual-channel mode there would be. There is a difference between theoretical performance loss and actual performance loss. – Ramhound – 2014-10-07T17:33:15.667
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Related: How much of a performance hit using asymmetrical dual channel RAM?, If the brand, size, MHz speed and latency settings of two sets of RAM are the same, will they still work correctly in dual channel mode?
– Ƭᴇcʜιᴇ007 – 2014-10-07T18:15:16.260