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dual channel and quad channel DDR2

Discussion in 'Hardware' started by deuce, Sep 20, 2007.

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  1. deuce

    deuce Thread Starter

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    Hi, can anyone tell me if there could be a benefit buying 4 x 1 GB DDR2-800 DIMMs, as opposed to 2 x 2 GB DDR2-800 DIMMs? I know the difference between single channel and dual channel is pretty huge, but I'm not sure if it would matter going to quad from dual. I haven't found anything that really helps on the internet.

    The reason I am asking is because 4 x 1GB DIMMS are about 25% cheaper than 2 x 2GB DIMMS, however for future upgrade purposes I will have to get rid of DIMMS with the 1 GB sticks, where with the 2 x 2GB I wouldn't. I think the differences between saving now and saving later when upgrading is negligible...so if there is a noticeable performance difference that would help choose.

    Thanks!
     
  2. Gulo Luseus

    Gulo Luseus

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    Bit of a moot point, this one. 4x 1Gb would give you dual channel twice, which could be beneficial, but in the other hand with 2x2gb, you would still get the dual channel. This would also have a greater bandwidth per stick. Bandwidth is set, so the overall speed will be determined by number of sticks ( sorry, not explaining very well, hope you get the general jist!). if bandwidth were 100, with 2 sticks each gets 50, with 4 sticks each gets 25. Personally i go with the 2x2gb, as the transfer rate is higher, and as there is 2 gig per stick, less latency lag than if u use 2 x1gb sticks.
    Also if you want to upgrade later, its probably better to have 2 slightly more expensive sticks now that you can reuse than 4 you cant. Long term gain vs short term spend... damn, dont you hate the dilemmas?
     
  3. deuce

    deuce Thread Starter

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    Exactly...being a 25% difference--when I actually do hit the next upgrade I think the savings/extra money spent won't really matter. I'm looking more for what will perform best as it is probably about equal in the end, but 2 x 2 has a bonus of being re-usable and puts out less heat in the tight quarters of the DIMMS.


    So--are you saying quad channel isn't really quad channel--just two sets of dual channel?

    Quad channel would double the bandwidth of 1 set of dual channel, but drop the transfer speeds?

    Thanks
     
  4. Nightfirecat

    Nightfirecat

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    I think that's what he's saying.
     
  5. Gulo Luseus

    Gulo Luseus

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    Sorry to hold you so long, and sorry th eprevious post wasnt as clear as it should be, it was late here LOL

    The main point is that as far as the memory goes, the bandwidth to the computer is a fixed number, so regardless of how many sticks you install, the transfer rate between th ecomputer and the whole of the memory is limited to that value.
    Probably the easiest way to put it is that you have, for the sake of arguement, a bandwidth of 100 on memory. Assuming your memory is reasonable in terms od speed, and if it is DDR2 then it is fair to do so, it will use as much bandwidth as it is given, so thememory isnt a bottleneck. Now, when you use dual channel mode, the processor writes to one stick, then immediately to the next stick. Effectively, it puts the first bit of information in the first stick, second in the second, 3rd in th e1st, etc. Because when using one stick, it writes, then has to wait for the memory to write it,so it writes every other time, whereas with dual channel, it uses the time it is waiting for the memory to write info, to write to the other stick. This is th emain reason you see a decent speed boost. Of course, this is offset by other factors, so it doesnt equate to a doubling in speed.
    With quad channel, it writes to stick 1, then stick 3, then stick 2, then stick 4- effectively, quad is the same as double dual, although the architecture is slightly different. If your mobo supports FULL quad channel, it works slightly diffeently, in that it writes to sticks 1 and 3 on cylce 1, and sticks 2 and 4 on cycle 2. However, you have to remember that only one thing can be done at a time, but effectively it doubles up. If this were the main factor, then quad could run 4 times as fast as single, but its not.

    The bandwidth allocated to the memory determines how much information can be written at one time. Effectively, it determines the maximun number of write operations that can be done in 1 second. So, if you have a bandwidth that allows 100 writes/ second, then that is the most it can handle, full stop. In single channel mode, because it is a write/wait/write/wait cycle, you may find you get (say) 40 writes a second. In dual channel mode, because the wait cycle is omitted, you will double this, so you get 80 writes/second. Now, if you have full quad support, you would double this again to get the max write/read speed, which would be 160 ops/sec. But as the bandwidth limits you to 100 ops/sec, then it will top out at that level, meaning the advantages offered are cancelled because the information going to/from the memory is limited by other areas of the computer.
    The other point to bear in mind is that a 2 gig stick can hold more info than a 1 gig, ( I know, obvious!), but this also means that indexing comes into play. When you need to read memory, the CPU has to get the info from the memory. With 4 sticks, it could have to ask 4 sticks if they have the required info, as opposed to just asking 2 sticks, from which to gain info. As such, 2 sticks can relay info back quicker in some cases, due to the interrogation methods.
    I hope this explains it a bit better, I have simplified because there is no point going too far into the subject, and i think the post would just be too long. The real question of which is quicker will depend on the timings of the memory, the read/write speed it can theoretically attain, the bandwidth available to the memory, and whether you are multi tasking- this can slow things down due to having various apps using memory, which is why I said before that there is a moot point. The other factor to consider is that with mem ops being so quick, it is other areas of the system that cause bottlenecks, north and southbridges being the prime culprits- there is a limit to what they can cope with in a given time.
    Essentially, the choice is up to you, as either way it should be an improvement over single channel. I suggested 2gig sticks because for an upgrade, they will at least have some value, whereas 1 gig sticks may not- more and more mobos are coming oput with just the 2 mem slots. Also there is the imminent acceptance of DDR3 memory ( if/when it happens), which would blow the whole thing away.
     
  6. deuce

    deuce Thread Starter

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    I was using dual channel DDR, so it won't really be a difference. I went with 2 x 2 GB because for one, I realized that I wasn't sure if the motherboard that will be used even supports full quad, which if course it doesn't, so that made that decision easy.

    The bandwidth question was actually my intent with the thread, as I wasn't sure if there was any bandwidth left to show a difference with quad, so thank you for the detailed response on that.
     
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