Unlock AMD X3 455?

Discussion in 'Gigabyte' started by red floyd, Mar 8, 2012.

  1. red floyd

    red floyd Guest

    I've got a GA-880GA-UD3H, and an Athlon II X3 455.

    I'm using a stock heatsink, and a 500W power supply. Unlocking the
    CPU reveals a fourth core. Given the PS and the heatsink, should I
    have any issues if the CPU is unlocked/
     
    red floyd, Mar 8, 2012
    #1
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  2. red floyd

    Paul Guest

    The CPU support chart for the motherboard, handles up to 140W processors,
    so that's not going to be a problem.

    http://www.gigabyte.com/support-downloads/cpu-support-popup.aspx?pid=3789

    The disabled core, was probably wasting power just sitting there.
    When they're disabled, it doesn't mean they're not still connected
    to VCore.

    There was a proposal at one time, sort of a "what-if", to turn off
    power on individual cores. But to do that, might require separate
    race tracks (power rail rings) on the silicon die. And I don't
    really think they like that idea too much. There is a limit to how
    many race tracks you can have around the edge of the die. So I would
    assume the locked core was still receiving power.

    They could remove the clock entirely from that core, or use clock gating,
    as other means of reducing power. But on modern silicon, that doesn't
    necessarily reduce the power on that core to zero either.

    My expectation would be, your idle power won't change, while
    your 100% load power will go up slightly. I wouldn't expect
    the change to be major.

    *******

    You need to test the "before" and "after" cases.

    Ideally, you'd want test software, which tests execution of all
    possible instructions. Intel and AMD will have these in-house
    (after the FDIV bug, such applications are crucial, and should be
    quite detailed in what they check). I'm not aware of anything like that,
    available for end users. Only the factory can thoroughly certify the core
    (with test vectors for 100% of chips, or software tests as a
    functional test of correctness of design).

    The next best thing, is to use an app that looks for execution
    errors. As an old timer, I'd use Prime95 (stress test option),
    as it does some math, and knows what the answer should be.

    Prime95 makes the processor hot, so it also tests the adequacy of
    the cooling system.

    http://www.mersenne.org/freesoft (download page)

    First, you'd run Prime95 on the three cores, without unlocking
    the fourth. This establishes the baseline conditions for the
    processor. You check CPU temp with Speedfan (almico.com), while
    Prime95 is running. Prime95 starts a test thread per core, so
    there will be three test threads for the baseline run.

    You shouldn't get any errors, in a four hour run of Prime95, with
    your three cores. You don't want any of the test threads to stop
    on an error. If the test is clean after four hours, you can
    stop it (stop, and exit).

    If your processor isn't stable and error free with the three
    cores, then stop right there. You're not ready to unlock.
    Things would only get worse if you unlocked. You need to fix
    the reason for the errors first, before you go any further.

    During the test run, you note the temps, and whether the cooling
    in your system is good enough. Say we aim for 60C or 65C as
    an upper bound for the Tcase perhaps. (Tcase and Tdie can be
    different, and you have to be careful not to freak out, by
    measuring the wrong one and jumping to conclusions. The die
    gets hotter than the case, and some measurement methods
    are measuring the silicon die, which can be 25C hotter than
    the casing.) On modern systems, the temp measurement
    is more likely to be the silicon die kind.

    So if the three-core run is error free, and there seems
    to be some cooling margin, then you're ready to unlock.

    Before unlocking, you back up your Windows C: drive. That
    protects you in case the fourth core causes damage to
    the Registry or the like, on the Windows partition. (Doing
    the Prime95 tests with a Linux LiveCD, is a way to avoid this,
    but then the temperature measurement step is more complicated.)

    So with Windows backed up, you enable the fourth core, and do
    another Prime95 run. Did one of the test threads stop ?
    Did it stop quickly ? Perhaps the fourth core really stinks.
    Or, if the four hour run passes with just as much ease and
    the baseline run, you're home free. (Prime95 only covers a
    fraction of all the possible instructions on the processor,
    so someone at the factory would laugh at this test as an
    "acceptance test". But what can we do ?)

    I wouldn't expect the temp rise, on a Prime95 run with the
    fourth core enabled, to go that much higher. As long as you
    have a bit of headroom when three cores were running, it'll
    probably be OK.

    As for a 500W supply, that would only be an issue, if you
    had a high end video card drawing over 200W. Then you
    might be concerned. If you have a low end video card,
    again, I don't expect a problem. If you really wanted
    to know, you'd calculate an estimate of the system
    power, to be sure. If your video card has no PCI Express
    connectors on the end of it, for example, you've got
    nothing to worry about. Such cards are 50W or less.

    Paul
     
    Paul, Mar 8, 2012
    #2
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  3. red floyd

    red floyd Guest

    On 3/7/2012 8:43 PM, Paul wrote:
    [too long to quote]
    Thank you, Paul, for an informative, thorough, and detailed
    reply. It was very useful.

    -- red floyd
     
    red floyd, Mar 9, 2012
    #3
  4. red floyd

    red floyd Guest

    I suspect it will be OK. I accidentally ran for a couple of weeks on an
    unlocked CPU without realizing it... I only noticed after a reboot that
    Linux was reporting 4 cores.

    The only issue I had was that the USB keyboard sometimes wasn't being
    properly reset by the BIOS, and I had to reboot multiple times to get
    into setup.

    Thanks again,

    red floyd
     
    red floyd, Mar 9, 2012
    #4
  5. red floyd

    Paul Guest

    As long as you run a decent test to prove the fourth core works,
    it should work great.

    AMD sometimes takes perfectly good CPUs, and uses them to fill
    that SKU (make three core processors). And in other cases, the
    CPU really does have a defective core. So there's no way to know
    what you're really dealing with. Looking at the reviews on Newegg,
    sometimes gives a hint as to which type they're shipping at the
    moment.

    Three core CPUs were invented, as a way to increase yield from
    the four core production line. But if the four core yield is
    good, they can't provide enough defective ones, to make the
    three core product. And that's when they ship some of the
    good ones to fill demand.

    Since the defective core, is connected to the same power source
    as the good cores, at least you know it isn't so defective, as
    to render the entire chip dead. But it could still have an
    "at-speed fault", and then you need software which tests all
    ~1000 x86 instruction combinations, to know for sure you have a
    good one. And I'm not aware of any applications (for free) that do that.
    While running Prime95 is fun, it would only cover a small
    portion of all the instruction set. You could execute a SQRT()
    some day, and the answer could come back wrong, from the bad core :)

    Even on "good" CPUs, they occasionally find an instruction sequence
    which causes "noise problems" on the silicon die. And the computation
    ends up wrong. But when these are discovered, they're found by
    doing a test at the factory, which involves instruction sequences
    which don't happen in real programs. So even what we consider to
    be "good working CPUs", actually have hidden weaknesses. It's the
    fact that compilers only use 20% of the instruction set, and
    seldom achieve max code density for FPU sequences, that prevents
    those bugs from causing end users a problem.

    Paul
     
    Paul, Mar 9, 2012
    #5
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