memory latency

Discussion in 'Buying Tips and Advice' started by 2ms, Sep 30, 2007.

  1. 2ms macrumors 6502

    Nov 22, 2002
    I notice that some SODIMMS have half the latency of others. For example, most "value" types are CAS latency 5 whereas some higher-end SODIMMS have CAS Latency 2.5.

    I know the clock speed rating of different memory chips is irrelevant since the memory/FSB clockrate cannot be changed (overclocked) on my iMac even if I wanted to (which I don't).

    However, it does seem like lower latency memory would have to provide better performance than higher latency. Intuitively, to me it's like the way a faster hard drive will make for a faster computer than a slower hard drive.

    Am I wrong? Is there any reason, other than the greater expense, to not buy lower latency memory? By the way, my imac is a 2.16GHz C2D, in case that has any bearing on things.
  2. stainlessliquid macrumors 68000

    Sep 22, 2006
    You most likely wouldnt be able to tell a difference.
  3. 2ms thread starter macrumors 6502

    Nov 22, 2002
    Do you say this about computers and memory latency in general, or is there something specific about low latency memory in standard-clocked C2Ds in particular that would make it less noticable than in other computers?
  4. gnasher729 macrumors P6


    Nov 25, 2005
    You need to check both CAS latency and clock speed. This latency is measured in multiples of the clock speed. CAS = 5 at 666 MHz is exactly the same latency as CAS = 2.5 at 333 MHz.

    There are also significant amounts of latency in other parts of the system. It takes quite a while from the time that a processor instruction tries to access some memory, to the time that L1 cache and L2 cache have responded that they don't have that memory available, plus the time that the request is actually sent to the memory, plus the time the data makes it back through the processor to the instruction trying to read the memory.
  5. phungy macrumors 68020


    Dec 5, 2006
    I'm looking at some G.Skill RAM and both are 667Mhz PC2-5300, one has latency of 5 and the other has 4. The one with 4 is "better"?

    Also, I read up on timing but am still confused (e.g. 5-5-5-12). What do those mean and lower numbers mean better timing right?
  6. Freyqq macrumors 601

    Dec 13, 2004
    generally a better CL is a better dimm for overclocking..but apples can't really overclock so it isn't that big an issue..
  7. CanadaRAM macrumors G5


    Oct 11, 2004
    On the Left Coast - Victoria BC Canada
    The original question has a flaw in its premise, and should be disregarded

    DDR DIMMs and SODIMMs are completely different from DDR2 DIMMs and SODIMMs

    You cannot compare the latency of DDR and DDR2 directly. A DDR DIMM at CL 2.5 is not "more advanced" than a DDR2 DIMM at CL5

    DDR PC2700 modules commonly have a CAS latency of 2.5 at 333 MHz

    DDR2-667 (PC2-5300) modules commonly have a CAS latency of 5 at 667 MHz. There are some 'overclocker' DDR2 DIMMs that run CL4, there are almost no SODIMMs that run CL4 at 667.

    (Note: There is one company that blatantly advertises CL3 and CL4 for their DDR2 SODIMMs --- that is true, in a limited sense. If you take a 667 MHz module at CL5 and reduce the speed enough, to 533 MHz or to 400 MHz, you can advance the Latency timings. However the performance will be worse, and the RAM cannot be run in a Mac at those speeds. I consider that company to be dishonest and misleading)

    And, there has never been any evidence from Apple that Macs will take advantage of lower latency if the RAM is capable -- there is no BIOS setup where you can manually adjust latency timings.

    There are a number of measurements of latency, the most important is Column Address Strobe or CAS latency (CL), which is always the first number in the series, and is the measure that is used when only one number is referred to. The other numbers (5-5-5-12) are measurements of different forms of latency, the 2nd through 4th numbers are of minor importance only, and only significant in overclocking situations.

    Given two identical DIMMs, a lower latency value is better -- if the machine is capable of exploiting it.

    Another caution - PC overclocking memory very often can only achieve lower latency or higher clock speeds if the memory buss voltage is increased from the standard 1.8 Volts to 2.0 to 2.2 Volts. Again, this is not possible on a Mac (or a Dell, for that matter).
  8. phungy macrumors 68020


    Dec 5, 2006

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