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What is the performance difference of the Intel Mac mini using matched or unmatched RAM?
As noted elsewhere in the Intel Mac mini Q&A, Apple states that "for optimum performance, it is recommended to use two SO-DIMMs (one in each slot) of the same DIMM size (e.g. two 512 MB SO-DIMMs)" with the "pre-Early 2009" Intel-based Mac mini systems (all of which have Intel GMA 950 graphics).
For the "Early 2009" Mac mini models, which have NVIDIA GeForce 9400M graphics, Apple stopped recommending that one install RAM in pairs and did not install RAM in pairs by default either on the low-end configuration (MB463LL/A). For the "Late 2009" Mac mini models, Apple likewise makes no recommendation as to whether or not one should install memory in pairs.
BareFeats posted a great "shootout" comparing the performance of a Mac mini "Core Duo" 1.66 with a matched pair of 1 GB RAM SO-DIMM modules to a Mac mini "Core Duo" 1.66 with an unmatched pair of RAM (with a 256 MB and 1 GB module), as well as a Mac mini G4/1.42 and iMac "Core Duo" 1.83 17".
BareFeats has done a great job as always, and the article should be read in its entirety. Nevertheless, for example, regarding the "real-world" performance difference between the matched and unmatched systems, The site reported that it took 47 seconds to "Render Soft Focus" in iMovie HD with the matched system and 48 seconds with the unmatched system.
However, in a "1024x768 Maximum" test of Quake 3, the matched system churned out 86 FPS, whereas the unmatched system only 57 FPS, a whopping 29 FPS difference.
As one would expect given those results, the author concludes in a "performance summary" that:
The Intel [Mac] mini ["Core Duo" 1.66] seemed only slightly affected by unmatched memory pairs until we ran Quake 3. In that test, matching memory was 51% faster. We recommend matching memory of either two 512 MB modules or two 1 GB modules. Just booting up the mini and doing nothing else consumes 270 MB, so [the pre-installed] two 256 MB modules just doesn't cut it.
Site sponsor Other World Computing also published extensive benchmarks comparing Intel-based Mac mini models with matched and unmatched memory and for "pre-Early 2009" systems results also show a larger amount of mismatched memory to be modestly faster than or about the same as a smaller amount of matched memory, except in graphic intensive tasks, where the mismatched memory results in reduced performance.
Interestingly, for the "Early 2009" models, OWC's benchmarks reflect steadily increasing performance for non-graphics related tasks with more memory installed, whether or not it "matched", but non-matching RAM continues to reflect a performance hit in some graphics-related tasks, despite Apple no longer deeming it to be worth mentioning.
Ultimately, as all Intel-based Mac mini systems have "integrated" video -- even the "Early 2009" and "Late 2009" models -- where the video memory is "shared" with the system, it should not be entirely unexpected that mismatched memory reduces the performance of graphics-related tasks. If access to the system RAM is faster, video-related tasks will be faster as well.