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Opteron - single dual core vs two single cores

Discussion in 'AMD Overclocking' started by CharlesBlackstone, Aug 5, 2006.

  1. Hi, I'm about to buy a workstation, and wonder what the functional
    difference would be between getting a machine with two single-core
    processors, and a machine with one dual-core processor? Is it
    essentially the same thing, or are the two single-cores more
    independent in terms of function, memory access, etc. that would affect
    overall performance?

    Thanks very much......
    CharlesBlackstone, Aug 5, 2006
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  2. CharlesBlackstone

    Paul Guest

    Two single-cores versus a dual core:

    Benchmarks start on page 10:

    The advantage of two single core processors, is the number
    of chipset chips on the motherboard. The AMD processor is
    a "hub" with Hypertransport busses sprouting out of it.
    With two single cores, you can have two true PCI Express X16
    slots, plus lots of tunnels for peripheral busses. If
    there is only one CPU socket on a motherboard, generally
    the I/O features are a bit less (they don't have to be,
    but companies don't generally use large numbers of daisy
    chained tunnel chips). This is mostly related to motherboard
    marketing, and the proposed uses for single socket versus
    dual socket systems (workstation versus server perhaps).

    Example block diagram on page 2, of a dual socket board:

    Note that if the "H0" socket is not occupied by a processor,
    you lose the use of a PCI Express x16 slot, an ethernet
    interface, and the use of four more DIMM slots. The board
    will still work with just one processor (single or dual core)
    in socket "H1". If you buy a motherboard with a single socket,
    well, you never have to worry about this, but you also have
    fewer facilities of all sorts on the motherboard.

    If your box is a pure compute engine, and the only peripheral
    you have is a USB socket for your Ipod, the single socket
    dual core processor wins. If the box is intended to hold
    a wealth of hardware, then using two sockets might be a
    better answer, even if it is computationally slightly slower.

    With two sockets, you have two memory controllers, and
    you would think, more memory bandwidth. A single socket
    setup has access to fewer total sticks of RAM, which
    might be important consideration if the problems you
    handle need a lot of RAM.

    "Cache-Coherent Nonuniform Memory Access (ccNUMA)" - the theory

    "Non-Uniform Memory Architecture (NUMA): Dual Processor
    AMD Opteron Platform Analysis" - benchmarks
    (Conclusion - set node interleave off ; use "numa aware" OS)

    That last article shows that you can get 12.8GB/sec memory
    bandwidth, as long as each single core processor only accesses
    its own local memory at 6.4GB/sec. While the dual socket
    system has the ability to use any memory by either processor,
    there is a penalty associated with going to the "other" memory.

    If your system has ECC memory, I think features like scrubbing
    and the like, may also affect whether things like node
    interleave can be turned on.

    Dual sockets have their complexities, as do setting up dual core
    processors. For the chunk of cash you'll be paying for either
    system, I hope the person setting it up does a good job.

    Paul, Aug 6, 2006
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  3. For workstations, 1 core per socket is kinda dead. Go for the dual-core.
    The X2's just came down, a lot.

    Besides, I don't believe AMD is selling 940-pin single core Opterons
    anymore, anyway.
    Douglas Bollinger, Aug 6, 2006
  4. CharlesBlackstone

    wizzywiz Guest

    Paul, many, many thanks for your excellent answer. I sure appreciate

    wizzywiz, Aug 8, 2006
  5. CharlesBlackstone

    wizzywiz Guest

    Paul, many, many thanks for your excellent answer. I sure appreciate

    wizzywiz, Aug 8, 2006
  6. CharlesBlackstone

    willbill Guest


    AMD 940 Opteron single core; Newegg prices:

    same speed
    dual core (all
    90nm (all free))
    today(shipping) today 3-31-2006(shipping)

    246/90nm/oem $159.99(free) $162.00(+$3.99)

    246/90nm/retail $185.00(free) $180.00(+$3.99)
    2.0GHz 270/$498.00 $476.00(+$3.99)

    248/120nm/retail $223.00(free) $237.50(+$.99)/120nm
    2.2GHz 275/$699.99 $689.00(+$3.99)

    250/90nm/retail $339.00(free) $339.00(+$.99)
    2.4GHz 280/$869.00 $919.00(+$3.99)

    252/90nm/retail $468.00(free) $461.00(+$3.99)
    2.6GHz 285/$1,065 N/A

    254/90nm/retail $689.00(free) $691.00(+$3.99)

    i also see, at Newegg today, that all 3 Xeon Woodcrest
    CPUs that they offer, are out of stock

    when those Woodcrest CPUs are in stock is when
    AMD may have to adjust pricing on their
    940 Opteron CPUs

    the open question is: how long before
    that happens?

    willbill, Aug 9, 2006
  7. CharlesBlackstone

    Tony Hill Guest

    There are some differences between the two, the biggest being that
    with a single dual-core processor you have all your memory hanging off
    one memory controller while a pair of single-core chips each have
    memory hanging off their own memory controllers. The
    advantage/disadvantage here is that with the one dual-core chip you
    can run with only 2 DIMMs but are limited to a maximum of 4 or 8 DIMMs
    (depending on the board), while with the two single-core chips you
    want a minimum of 4 DIMMs but can expand up to 8 or 16 DIMMs.
    Therefore, if you're application needs lots of memory (8GB+) a pair of
    single-core chips is processors might be better.

    Another difference is that the single-core chips usually have one
    clock speed above the fastest dual-core chips. For example, right now
    the fastest dual-core Opteron is the 285, which runs at 2.6GHz. The
    fastest single-core Opteron is the 254 which runs at 2.8GHz.

    In terms of performance, there are a handful of pluses and minuses on
    either side. Two processors, each with their own memory controller,
    give you more theoretical bandwidth. The downside is that you now
    have a NUMA setup with half your memory being remote and therefore
    suffering a latency penalty. Cache access also can suffer a bit of a
    latency penalty as accessing the cache on a second core in a dual-core
    chip is much faster then accessing cache on a physically different
    chip in a dual-processor setup.

    In the end though, the performance differences mostly get lost in the
    noise. A single dual-core chip will usually come out on top, but not
    by any big margin. The *REAL* difference in all this is cost. A
    one-socket motherboard and a single dual-core chip will be a fair bit
    cheaper than a 2P motherboard with a pair of single-core chips. This
    is especially true if you opt for a dual-core Athlon64 X2 vs. a pair
    of single-core Opteron chips. Even though the X2 chips are marketed
    as desktop chips they could make for quite a good workstation setup
    with the right motherboard. Only possible shortfall here is that
    unbuffered ECC memory is a bit rare, so if error correction is
    required for your workstation you might need to do a bit of digging.
    Tony Hill, Aug 9, 2006
  8. CharlesBlackstone

    Scott Alfter Guest

    Hash: SHA1

    OTOH, the two cores in a dual-core processor have equal access to memory.
    In a dual-processor system with memory attached to one processor, processor
    #2 will have to go through processor #1 to access memory. The dual-Opteron
    at which I'm typing this message is set up that way, with a couple of 512-MB
    DIMMs hanging off one processor. Prime95 running on processor #2 takes
    noticeably longer per iteration than Prime95 running on processor #1. (I'd
    provide numbers, but they're currently doing different types of work (one is
    factoring and the other is LL-testing).)

    Putting memory on both processors would fix this, but that would be an
    additional expense you would want to keep in mind.

    / v \ Scott Alfter (remove the obvious to send mail)
    (IIGS( http://alfter.us/ Top-posting!
    \_^_/ rm -rf /bin/laden >What's the most annoying thing on Usenet?

    Version: GnuPG v1.4.5 (GNU/Linux)

    -----END PGP SIGNATURE-----
    Scott Alfter, Aug 9, 2006
  9. I didn't have any trouble getting unbuffered ECC DDR a few weeks ago and
    yes a system like that makes a nice workstation or Intranet Web/ASP server
    -- my purpose -- for not a lot of money. Funny, Crucial doesn't list the
    2GB kit I got in their DDR listings but looking up the mbrd, an Asus A8N-E,
    still shows it as available; for DDR2, they show it in the PC-5300 stuff
    *but* at 3x the price of the DDR!!... which makes s939 still an attractive
    bet, to me anyway. Of course you won't find it in the "boutique" lines
    which cater to the OC/gamer market and which many on-line vendors also seem
    to concentrate on.

    Oh and BTW I can find nothing on ECC reporting so who knows if it really
    works... whats new??:)
    George Macdonald, Aug 10, 2006
  10. CharlesBlackstone

    DRS Guest

    You also need to factor in commercial OS licencing. MS views dual-core as a
    single CPU for licencing purposes. You pay more to licence Windows on 2
    DRS, Aug 10, 2006
  11. * DRS:
    Nope. Windowsxp Pro works fine with up to two cpus no matter how much
    cores it has...

    Benjamin Gawert, Aug 10, 2006
  12. CharlesBlackstone

    DRS Guest

    My bad, multiprocessor licencing is currently restricted to servers.
    DRS, Aug 10, 2006
  13. CharlesBlackstone

    AD. Guest

    Not for OS itself unless you outgrow the low end Windows Server version
    - at 4 sockets I think? But you are right about other licenses eg the
    per CPU SQL Server licenses.
    AD., Aug 11, 2006
  14. Scott, how do you tell one processor to do one job, and the other
    processor to do the other job?

    CharlesBlackstone, Aug 16, 2006
  15. CharlesBlackstone

    jukka Guest

    Scott, how do you tell one processor to do one job, and the other
    Scott could write a multi-threaded application which uses multiple
    threads. Each thread can potentially run on a discrete physical or
    logical processing unit.
    jukka, Aug 16, 2006
  16. CharlesBlackstone

    Keith Guest

    In WinXP: Task Manager => Processes => Set Affinity (I think, I'm
    on a UP Laptop, right now).
    Keith, Aug 16, 2006
  17. CharlesBlackstone

    krw Guest

    Small correction: TaskManager => Processes => <RMB> on process =>
    Set Affinity => select processors process is allowed to run on
    krw, Aug 19, 2006
  18. CharlesBlackstone

    Scott Alfter Guest

    Hash: SHA1

    With Prime95, there's a menu option (under Advanced) that sets affinity.
    Since I have it set up to run as a service, this is more convenient than
    having to set affinity in the Task Manager on every reboot.

    / v \ Scott Alfter (remove the obvious to send mail)
    (IIGS( http://alfter.us/ Top-posting!
    \_^_/ rm -rf /bin/laden >What's the most annoying thing on Usenet?

    Version: GnuPG v1.4.5 (GNU/Linux)

    -----END PGP SIGNATURE-----
    Scott Alfter, Aug 19, 2006
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