CAS 2 vs. CAS 3 speed differences in RAM with Athlon 64?

Discussion in 'IBM' started by ANTant, Sep 19, 2004.

  1. ANTant

    ANTant Guest

    Hello.

    I am going to upgrade my old gaming box to an Athlon 64 system. I
    noticed the memory CAS speeds. Does CAS 2 (faster) and CAS 3 (slower)
    make any big speed differences for 1 GB of PC3200 RAM on a Socket 754
    ATX motherboard with an AMD Athlon 64 3000+/2.0 Ghz to 3400+ 2.4 Ghz
    (both with 512 KB socket and 754 CPU)? CAS 2 is expensive so... I am
    wondering if getting 3 is really worth the price. I am mainly gaming,
    watching movies, using the Internet, etc. Gaming is the big one to
    note.

    You can see my current Athlon XP 2200+ system at
    http://alpha.zimage.com/~ant/antfarm/about/computers.txt ... I will be
    replacing the motherboard (ASUS K8V SE Deluxe or a MSI K8T Neo F...),
    CPU, RAM, and sound card (getting an Audigy 2).

    Thank you in advance. :)
    --
    "Oh, look what Kyle got me, it's a red Mega... Ants in the pants? Ants
    in the pants?! Ants in the Pants?!! ..." --Eric Cartman in South Park's
    Damien Episode (Season 1; Episode 8)
    /\___/\
    / /\ /\ \ Phillip (Ant) @ http://antfarm.ma.cx & http://aqfl.net
    | |o o| | E-mail: or
    \ _ / Nuke ANT from e-mail address if your e-mail was returned.
    ( )
     
    ANTant, Sep 19, 2004
    #1
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  2. ANTant

    DDStech Guest

    The CAS ratings are latency cycles. What this means is how many clock ticks
    before the memory can read/right again. For CAS 2, this is two mhz, for CAS
    3 this is 3mhz. In theory, this calculates to CAS 3 being 30% slower than
    CAS 2. Now you know why CAS 2 has a premium on it.

    For gaming, if frames per second are your main concern, then you need CAS 2.
     
    DDStech, Sep 20, 2004
    #2
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  3. You were right when you said clock ticks. That is not 2, or 3, 'MHz' however.
    If CAS delay were the only thing to memory access that would be right but,
    in reality, that occurs once per data stream so the one 'difference' is
    spread over multiple reads making it relatively minor in the total picture,
    say a few percent.
     
    David Maynard, Sep 20, 2004
    #3
  4. ANTant

    DDStech Guest

    Can you explain the difference? The clock cycle is in mhz. Perhaps I am
    jumping to oonclusions here.

    However, there is a huge difference between cas2 and cas3 memory settings.
    It is easily noticed by the human eye. Windows snap open alot quicker,
    programs pop up faster. Games run noticibly faster.
     
    DDStech, Sep 20, 2004
    #4
  5. ANTant

    Lachoneus Guest

    However, there is a huge difference between cas2 and cas3 memory settings.
    Are you sure you didn't just have window animation turned off when you
    were running CAS2? Because I could barely tell the difference at all.
    Benchmark improvements were almost in the noise. IMHO, CAS2 is not
    worth the price premium.
     
    Lachoneus, Sep 20, 2004
    #5
  6. ANTant

    ANTant Guest

    Wow, 30% slower? That's a big number! Thanks for the comments! :)


    --
    "Oh, look what Kyle got me, it's a red Mega... Ants in the pants? Ants
    in the pants?! Ants in the Pants?!! ..." --Eric Cartman in South Park's
    Damien Episode (Season 1; Episode 8)
    /\___/\
    / /\ /\ \ Phillip (Ant) @ http://antfarm.ma.cx & http://aqfl.net
    | |o o| | E-mail: or
    \ _ / Nuke ANT from e-mail address if your e-mail was returned.
    ( )
     
    ANTant, Sep 20, 2004
    #6
  7. ANTant

    ANTant Guest

    Hmmm... not 30% speed difference? OK, I really need to know if CAS 2 is
    really worth buying for major speed difference. 30% is a lot. If it is
    5%, I would just get CAS 3 and save my money.
    --
    "Oh, look what Kyle got me, it's a red Mega... Ants in the pants? Ants
    in the pants?! Ants in the Pants?!! ..." --Eric Cartman in South Park's
    Damien Episode (Season 1; Episode 8)
    /\___/\
    / /\ /\ \ Phillip (Ant) @ http://antfarm.ma.cx & http://aqfl.net
    | |o o| | E-mail: or
    \ _ / Nuke ANT from e-mail address if your e-mail was returned.
    ( )
     
    ANTant, Sep 20, 2004
    #7
  8. Well, even if it could be expressed in Mhz, 2 or 3 is way off no matter how
    you look at it. Take a 100Mhz FSB. 1 'clock' represents '100 Mhz'. 2 would
    then be half that speed, or '50 Mhz'. (I'm using SDR PC100 SDRAM here for
    simplicity's sake). The number is obviously larger the faster the FSB is,
    so there's no way we're going to come around to '2 or 3 MHz'.

    However, 'MHz' refers to a repetitive, cyclic, thing and the CAS '2 clock
    ticks' doesn't repeat every '2 clock ticks' (which would be needed to claim
    it's '50 MHz'). It occurs at the beginning of a burst read and then again
    before the next burst. The repetition rate, which one might say is a 'MHz'
    number, is, then, more related to the number of bytes in the data stream
    than it is to whether it's CAS 2 or 3. To make matters even more
    complicated, there are other delays besides simply CAS (why you see SDRAM
    specs with those three numbers. e.g. 3-2-2 [for CAS 3])

    Let's take a simplified example for a read without precharge. The delay
    before read is CAS plus Trcd, 3+2, 5 clocks. Then a 4 clock data read.
    That's 9 clocks. At 100MHz FSB that's 90ns which translates to 11.1 Mhz, if
    it repeated over and over.

    However, the repetition depends on there being constant reads, that the
    selected bank doesn't change, in which case the extra 2 (or 3) clock
    precharge time must be added, and we're ignoring things like bank
    interleaving and command overlap.

    In other words, it's gets rather complicated to do simple calculations like
    "2 is 33% faster than 3" and have much meaning. Note that in our simplified
    example, going from CAS 3 to CAS 2 memory lowers the clocks needed from 9
    to 8; roughly 11% better, not 33%. But, as noted, that doesn't take into
    account other memory cycles, such as the 2 clock precharge.

    And then, the main memory access is being L2 cached, so that it's from the
    L2 that the processor really reads, and that tends to mask 'improvements'
    in the main memory access times since that is, after all, the purpose of L2
    cache to begin with: to minimize the effect of main RAM delays.
    I seriously doubt that whatever difference you think you're seeing is
    because of CAS 2 vs 3. Odds are something else changed at the same time.
     
    David Maynard, Sep 20, 2004
    #8
  9. ANTant

    Yousuf Khan Guest

    No chance. CAS stands for Column Address Strobe, and its latency is the
    number of cycles it takes to activate it. If you picture RAM as a grid, then
    you will have the memory arranged in rows and columns. Similarly the RAS is
    the Row Address Strobe, with its own latency timing.

    Due to the mainly linear nature of memory accesses, you will only need to
    activate the CAS only once in awhile and keep it open, while you cycle
    through the memory using the RAS. This is known as a burst transfer. Really,
    RAS is the more important measure of the speed of RAM, because it's done so
    much more often. But RAS timings don't really change from module to module,
    it's always stuck at about 3 cycles in DDR-RAM. So the only timing that
    might be slightly better or worse is the CAS timing, so that's what they
    advertise. Really, CAS timing is sort of a measurement of what kind of a
    delay you can expect for a burst of data to begin coming to you after you've
    told it to start, since CAS is one of the first activities done.

    Now a DDR-200 (aka PC1600) runs at 100Mhz, DDR-266 (PC2100) runs at 133Mhz,
    DDR-333 (PC2700) runs at 166Mhz, and DDR-400 (PC3200) runs at 200Mhz. CL3
    would mean 3 cycles for CAS, and CL2 would mean 2 cycles. So a DDR-200 CL2
    module would have a CAS latency time of 2/(100) = 50ns; while a DDR-400 CL3
    would have a CAS latency of 3/(200) = 15ns. So a DDR-400 even with a CL3 has
    considerably lower latency than a CL2 DDR-200.

    So yes, CAS latency does make a small difference, but we're only talking
    about nanoseconds at the beginning of a burst.

    Yousuf Khan
     
    Yousuf Khan, Sep 20, 2004
    #9
  10. ANTant

    Yousuf Khan Guest

    Did you make the observations of the RAM on the same machine, or is this an
    observation based on comparing two separate machines (i.e. one running CAS2
    and the CAS3, obviously)?

    Yousuf Khan
     
    Yousuf Khan, Sep 20, 2004
    #10
  11. ANTant

    Yousuf Khan Guest


    You'd be lucky if it made even 5% of a difference. The best way to look at
    CAS latency is as a measurement of how quickly a RAM module reacts to a
    command. It's actual sustained speed is represented by its PC rating
    (PC2100, PC2700, etc.), or alternatively its DDR rating (DDR-266, DDR-333,
    etc.), same thing. The sustained speed is the same no matter whether you are
    getting an expensive CAS2 or a cheaper CAS3. The CAS rating is just a
    measurement of its initial reaction time.

    The above is assuming that that we're comparing two modules with the exact
    same sustained speed ratings, but different CAS ratings. If we're talking
    about PC2100 CL2 vs. PC2700 CL3, then by all means get the PC2700, as it
    would be much faster all around.

    Yousuf Khan
     
    Yousuf Khan, Sep 20, 2004
    #11
  12. ANTant

    ANTant Guest


    This was assuming PC3200 like Kingston brand. I guess I will go with CAS
    3 then to save money and since the speed difference isn't that big. I
    have until Wednesday night to decide. ;)
    --
    "Oh, look what Kyle got me, it's a red Mega... Ants in the pants? Ants
    in the pants?! Ants in the Pants?!! ..." --Eric Cartman in South Park's
    Damien Episode (Season 1; Episode 8)
    /\___/\
    / /\ /\ \ Phillip (Ant) @ http://antfarm.ma.cx & http://aqfl.net
    | |o o| | E-mail: or
    \ _ / Nuke ANT from e-mail address if your e-mail was returned.
    ( )
     
    ANTant, Sep 20, 2004
    #12
  13. ANTant

    Tony Hill Guest

    Generally speaking you're looking at about a 0-3% difference in
    performance, probably about 1.5% on average. It makes sense if you're
    going for a high-end system, where the extra $50 for CAS2 memory makes
    about as much difference as an extra $200 worth of CPU speed.
    However, given that you're looking at a 3000+ to 3400+, the saved on
    CAS3 memory would be better spent elsewhere.

    If you were to compare a system with an Athlon64 3000+ and CAS2 memory
    to one with an Athlon64 3200+ with CAS3 memory, the two systems would
    cost about the same amount but the latter system would usually be
    faster.


    Note that the difference is definitely *NOT* 33% as another poster
    mentioned. From the test bellow, an Athlon64 3200+ (2.0GHz) has a
    total latency for an uncached memory access of 83ns, or 166 clock
    cycles for the processor (this is a slight over-simplification, but it
    will suffice for the example).

    http://www.aceshardware.com/read.jsp?id=60000258

    This test used CAS2 memory. Going to CAS3 memory would add a single
    200MHz clock cycle on the memory side of things, or 10 clock cycles as
    far as the processor is concerned. The result would be an uncached
    memory access time of 88ns. So we get a performance improvement of
    about 6% when going from CAS3 memory to CAS2.

    Of course, uncached memory access is only part of the story. In
    reality, well over 90% of all memory access is handled by the L1 and
    L2 cache, though even at that the MUCH slower main memory access takes
    a disproportionate amount of time (for comparison to that 166 clock
    cycles for memory, L1 cache normally takes 2 clock cycles and L2 cache
    takes usually 7-20 clock cycles).

    In the end, main memory access makes up at most about 50% of
    performance. So, 6% faster memory gets you, at most, a 3% speed
    boost. Typically the benefit is actually smaller than that, or about
    the 1.5% improvement I mentioned above.
     
    Tony Hill, Sep 20, 2004
    #13
  14. ANTant

    Jon Danniken Guest

    Of all the explanations I have read concerning memory timings, that is far and away
    the most concise and practical one I have ever had the pleasure to read. Thanks,
    Yousuf!

    Jon
     
    Jon Danniken, Sep 20, 2004
    #14
  15. ANTant

    AD. Guest

    Ignoring the mega/milli prefixes for a second, aren't you confusing
    'somethings' with 'somethings per second'?

    Or did I misread your question?

    Cheers
    Anton
     
    AD., Sep 20, 2004
    #15
  16. ANTant

    AD. Guest

    Great post, but shouldn't that be 20ns?

    Cheers
    Anton
     
    AD., Sep 20, 2004
    #16
  17. ANTant

    Lachoneus Guest

    You'd be lucky if it made even 5% of a difference. The best way to look at

    ... snip ..
    ....

    Okay, I don't want to throw a monkey wrench in things here, but there's
    also CAS2.5, which is what I got for my new Athlon 64. It was the same
    price as the CAS3 when I bought it, so I thought it wouldn't hurt.

    I don't know exactly what it means, but I'd imagine it's somewhere
    between CL2 and CL3 in performance, meaning you'd be lucky if the
    performance difference in any real-world situation wasn't lost in the
    noise. :)
     
    Lachoneus, Sep 20, 2004
    #17
  18. ANTant

    DDStech Guest

    After reading Toms hardware article on memory timings I stand corrected. It
    appears to gain about 6% over CAS 3. However, 6 % performance gain is worth
    a few extra bucks in my opinion. Also, most of the time, your only quoted
    the CAS latency. What you find when you get the chip is that its not
    3-2-2-5, but, 3-4-4-9. The ram I'm using is 2-2-2-5.

    I ran into this problem with my last system. I bought one stick of ram, that
    I was able to run at 2-2-3. When I added ram, I bought a similar type, but
    it would only run at 3-3-4. This made a noticible difference in performance.
    Windows didn't snap open as fast, and games ran slower.
     
    DDStech, Sep 20, 2004
    #18
  19. ANTant

    chrisv Guest

    Clueless.
     
    chrisv, Sep 20, 2004
    #19
  20. ANTant

    Flow Guest

    I ran cas 2,5 memory but when it went bust i upgraded to a cas 2 stick.
    I find that it overclocks better and runs just that little smoother opposed
    to the cas 2,5 memory.
    Dunno about percentages though.Im a gamer and will never go back to slower
    ram.
    I have all mem settings in bios at most agressive possible and it runs
    better then with slower ram.
    This is ofcourse my opinion,can't back it up with figures.
    If you like to save some money then go for the cheaper ram at a small cost
    of performance,if you want the best then get the more expensive memory.
     
    Flow, Sep 20, 2004
    #20
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