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This is for a server running Ubuntu Server 10.10.

The server is an old desktop PC. It had 2 sticks of 256 MB of 182-pin DDR 400 MHz RAM in it (total 512 MB of RAM).

I just ordered a 1 GB stick of compatible RAM for the machine (now would have a total of 1.25 GB of RAM).

A friend told me that it might run better if I removed both sticks of 256 MB RAM and used just the 1 GB stick I will be receiving. This seems counterintuitive since then there would only be 1 GB of RAM instead of 1.25; is it possible that it would be better to run with 1 GB or is he totally wrong?

Thanks for the help.

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There's a LOT more to RAM than just the capacity. You would need to supply full information before a yes or no answer could be provided. –  John Gardeniers Jan 17 '11 at 2:02
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5 Answers

up vote 1 down vote accepted

Older machines would only support a certain maximum amount of memory with a given amount of CPU cache; going beyond that maximum would disable the cache for part or all of the memory. You say your system is "old", but without more details we can't be sure if it would exhibit this behavior. In any case, it would not be difficult to test the cached memory speed in both configurations.

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Thanks, good explanation. Any advice on the best way to test the speed of installed memory? –  James Marthenal Jan 17 '11 at 2:07
    
Arf. I haven't had to do anything like that for a while, so I've forgotten what tool to use for that. –  Ignacio Vazquez-Abrams Jan 17 '11 at 2:11
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Depends how the banks are laid out on the motherboard. Sometimes mixing slower memory with faster can cause all the memory to run at the lower speed. See this wikipedia explanation.

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You aren't wrong, but the question is with regards to a single stick versus a mismatched pair, so multi-channel memory doesn't come into this. –  Ignacio Vazquez-Abrams Jan 17 '11 at 1:59
    
Depends on the motherboard. Some can use dual-channel for the overlapping portion of memory. In any event his comment is relevant because the question for him is whether the addition of the extra 256 MB will slow down the 1 GB stick and, if so, whether that effect will negate the benefit of the extra 256 MB. –  Mark Sowul Mar 5 '11 at 1:50
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There are two ways this can hurt you:

  1. If the 1GB stick is faster than the 256MB — say 667Mhz vs 533Mhz — then keeping them paired could force the 1GB stick to also only run at 533mhz. Whether 1GB at at 667Mhz is faster for your machine than 1.25GB at 553Mhz depends on your usage patterns and other components in the machine.
  2. Assuming DDR2, right now you're probably running dual-channel mode which effectively doubles your memory speed. Going to either mismatched RAM or a single stick will no longer allow dual channel mode. Note that this hits you whether you run 1GB or 1.25GB, but it is likely to be a hit. What you really want to do is order another 1GB stick to exactly match your new stick. This way you get both more RAM (a whole 2GB) and dual channel mode.
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The difference in memory speed is going to be completely academic compared to the key question, which is: are you hitting swap or not? Going to disk is abysmal compared to even the worst-case RAM scenario here.

If you are always using all of 1GB but not all of 1.25GB, you will absolutely get better performance with 1.25.

If you are sometimes using over 1GB, you'll get better performance for those times by every bit more you have, including that 0.25GB.

If you are always over 1.25GB, you probably should buy even more RAM, but in the meantime, again, every bit helps.

If you're consistently under 1GB, then you can start worrying about whether the extra bit slows you down. But don't worry for too long, because modern software will catch up to you.

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This may not be the answer for your question but it could help.

Keep in mind what are your requirements, in the hypothetical case the two sticks slow down the CPU it would be much and 256 mb extra for the ram are important, you will win more than lose.

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