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Thread: Asymmetric 3D cache on high end 7000 series

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    Asymmetric 3D cache on high end 7000 series

    Interesting confirmation here:

    https://www.techpowerup.com/303084/a...e-two-chiplets

    If you can afford a 7900X-3D or better, then you only get the stacked cache on one of the CCD die.

    That's potentially cool, as the non stacked die can boost to the same sort of single thread frequency as the non 3D variant. Or it's a nightmare.

    I guess thread scheduling just got more interesting

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    Re: Asymmetric 3D cache on high end 7000 series

    Quote Originally Posted by DanceswithUnix View Post
    Interesting confirmation here:

    https://www.techpowerup.com/303084/a...e-two-chiplets

    If you can afford a 7900X-3D or better, then you only get the stacked cache on one of the CCD die.

    That's potentially cool, as the non stacked die can boost to the same sort of single thread frequency as the non 3D variant. Or it's a nightmare.

    I guess thread scheduling just got more interesting
    The kernel thread schedulers have been living in interesting times for a while now. For x86, first it was Intel and their E cores, not this. Plus the various CCD stuff AMD did. Then for mobiles (with which Linux has to cope due to Android), this has been going for a while.

    The other obvious takeaway: with 5800X3D having great DDR4 and timings was nice but not essential. 5800X3D was basically very easy to get great performance from without expensive RAM or having to configure sub-timings.

    For Zen4 3D this is probably no longer the case. The 3D CCD should be fine, but if anything needs to use the "ordinary" CCD then because of that, you'd want fancy expensive RAM with good timings.

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    Re: Asymmetric 3D cache on high end 7000 series

    One commentator mentioned that most games (so far) haven't really made use of more than eight cores, which is what the vcache is aimed at. It will be interesting to see if any games fail to use the CCD with the vcache, which as you say, will be down to the scheduler.

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    Re: Asymmetric 3D cache on high end 7000 series

    Quote Originally Posted by kompukare View Post
    The kernel thread schedulers have been living in interesting times for a while now. For x86, first it was Intel and their E cores, not this. Plus the various CCD stuff AMD did. Then for mobiles (with which Linux has to cope due to Android), this has been going for a while.

    The other obvious takeaway: with 5800X3D having great DDR4 and timings was nice but not essential. 5800X3D was basically very easy to get great performance from without expensive RAM or having to configure sub-timings.

    For Zen4 3D this is probably no longer the case. The 3D CCD should be fine, but if anything needs to use the "ordinary" CCD then because of that, you'd want fancy expensive RAM with good timings.
    That will again be an interesting point. The cores on the plain CCD should still have access to the 3D L3 cache, just with a bit more latency.

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    Re: Asymmetric 3D cache on high end 7000 series

    This is going to play havoc with direct die heatpipe coolers! Also sounds like AMD trying to save money by only using one die with the stacked cache. It wouldn't surprise me the next generation will have both with the stacked cache.

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    Re: Asymmetric 3D cache on high end 7000 series

    Sounds interesting, can't be any worse for scheduling than intels P and E cores at a guess.

    I am probably skipping this generation and the one after anyway so hopefully it will all be sorted for my next upgrade.

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    Re: Asymmetric 3D cache on high end 7000 series

    Quote Originally Posted by Percy1983 View Post
    Sounds interesting, can't be any worse for scheduling than intels P and E cores at a guess.
    I think it'll be a little bit tougher, since it's pretty obvious when you would schedule between a P and an E core, but now you've/the scheduler has to choose between higher frequency or more cache, both of which are valid performance gains.

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    Re: Asymmetric 3D cache on high end 7000 series

    Quote Originally Posted by Percy1983 View Post
    Sounds interesting, can't be any worse for scheduling than intels P and E cores at a guess.
    Bit OT, but how do the P/E cores seem to effect things in Windows, I've not touched any of the newer Intel stuff for a while, but have just had an AIO with a 12th Gen i7 in it, need to test it to see how much it screws stuff up.

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    Re: Asymmetric 3D cache on high end 7000 series

    Quote Originally Posted by [GSV]Trig View Post
    Bit OT, but how do the P/E cores seem to effect things in Windows, I've not touched any of the newer Intel stuff for a while, but have just had an AIO with a 12th Gen i7 in it, need to test it to see how much it screws stuff up.
    I think you need to be using Windows 11 in order for the scheduler to work correctly with the P & E cores, although I don't have an Intel cpu so can't verify that. You can always turn off the the E cores in the bios, as one way of preventing problems (if a bit extreme).

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    Re: Asymmetric 3D cache on high end 7000 series

    Quote Originally Posted by CAT-THE-FIFTH View Post
    This is going to play havoc with direct die heatpipe coolers! Also sounds like AMD trying to save money by only using one die with the stacked cache. It wouldn't surprise me the next generation will have both with the stacked cache.
    What problem do you see with coolers? In a normal 7950X you can have one of the compute die generating more heat than the other, and the IO die doing its own thing. Just seems more of the same to me.


    Quote Originally Posted by kalniel View Post
    I think it'll be a little bit tougher, since it's pretty obvious when you would schedule between a P and an E core, but now you've/the scheduler has to choose between higher frequency or more cache, both of which are valid performance gains.
    I'm sort of expecting a whitelist tbh.

    Old games will only have a few threads, don't need the 3D cache and will benefit from increased frequency. They will already be fast enough, but using the high freq cores will make benchmarks look better.
    Modern stuff will more likely use more cores, need more cache, and so will lean on the 3D cache cores.
    Productivity programs will just max out all the cores, so are moot.

    Besides, if the scheduler gets it wrong then the fast clocked core will still see the cache on the 3D die like an L4 so should still go faster than a straight 7950X.

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    Re: Asymmetric 3D cache on high end 7000 series

    Quote Originally Posted by DanceswithUnix View Post
    Besides, if the scheduler gets it wrong then the fast clocked core will still see the cache on the 3D die like an L4 so should still go faster than a straight 7950X.
    Is the L3 shared between CCX's? I thought the L3 was "Per CCX" - i.e. the other compute die could not use it.
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    Re: Asymmetric 3D cache on high end 7000 series

    Quote Originally Posted by DanceswithUnix View Post
    What problem do you see with coolers? In a normal 7950X you can have one of the compute die generating more heat than the other, and the IO die doing its own thing. Just seems more of the same to me.
    Because it's an asymmetrical orientation. This is going to cause problems with direct die air coolers for sure.

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    Re: Asymmetric 3D cache on high end 7000 series

    Quote Originally Posted by badass View Post
    Is the L3 shared between CCX's? I thought the L3 was "Per CCX" - i.e. the other compute die could not use it.
    That had me wondering if I made a bad assumption somewhere, and I think you might be right (though I've had a hard time finding definitive references).

    The L3 seems to be wired such that cores can see all the L3 through the Infinity Fabric, and there seem to be plenty of "cross core latency" sort of benchmarks. That makes sense for efficient sharing of data across threads. *But*, it's a victim cache so only the cores local to the L3 write to it.

    I did find an article that said the big cache cores would be scheduled first, with the normal 8 cores kicking in when there are enough threads for things like Blender. That might be rubbish though, or for that matter that's software so it could change

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