What about Hyper-Threading and IMC?

Core's impressive execution resources and massive shared cache seem to make it the ideal CPU design for SMT. However, there is no Simultaneous Multi Threading anywhere in the Core architecture. The reason is not that SMT can't give good results (See our elaborate discussion here), but that the engineers were given the task to develop a CPU with a great performance ratio that could be used for the Server, Desktop and Mobile markets. So the designers in Israel decided against using SMT (Hyper-Threading). While SMT can offer up to a 40% performance boost, these performance benefits will only be seen in server applications. SMT also makes the hotspots even hotter, so SMT didn't fit very well in Core's "One Micro-Architecture to Rule them All" design philosophy.

As far as including an Integrated Memory Controller (IMC), we were also told that the transistors which could have been spent in the IMC were better spent in the 4 MB shared cache. This is of course highly debatable, but it is a fact cache consumes less power. The standard party line from Intel is that keeping the memory controller on the chipset allows them to support additional memory types without having to re-spin the CPU core. That is certainly true, and with the desktop/mobile sectors using standard DDR2 while servers are set to move to FB-DIMM designs, the added flexibility isn't terrible. Techniques such as memory disambiguation and improved prefetch logic can also help to eliminate any advantage an IMC might offer. Would an IMC improve Core's performance? Almost certainly, but Intel will for the time being pursue other options.

Conclusion 1 : AMD K8 versus Intel P8

The Intel Core architecture is clearly the heir and descendant of the hugely successful P6 architecture. However, it has state of the art technology on board such as micro-op/macro-op fusion, memory disambiguation and massive SIMD/FP power.

Compared to the excellent AMD K8/Hammer architecture, the Core CPU is simply a wider, more efficient and more out of order CPU. When I suggested to Jack Doweck that the massive execution resources may not be fully used until SMT is applied, he disagreed completely. Memory disambiguation should push the current limits of ILP in integer loads a lot higher, and the massive bandwidth that the L1 and L2 can deliver should help Core to come close to the execution utilization percentages of the current P-M. 33% more execution potential could thus come very close to 33% more performance, clock-for-clock.

So is it game over for AMD? Well, if you read the previous pages, it is pretty clear that there are some obvious improvements that should happen in AMD's next generation. However, there is no reason at all to assume that the current K8 architecture is at the end of its life. One obvious upgrade possibility is to enhance the SSE/SIMD power by increasing the wideness of each unit or by simply implementing more of them in the out of order FP pipeline.

To sustain the extra (SIMD) FP power, AMD should definitely improve the bandwidth of the two caches further. The K7 had a pretty slow L2-cache, and the K8 doubled the amount of bandwidth that the L2 could deliver for example. It's not unreasonable to think a 256-bit wide cache bus could be added to a near-future AMD design.

Finally, there is also a lot of headroom for increasing integer performance. The fact that Loads can hardly be reordered has been a known weak point since the early K7 days. In fact, we know that engineers at AMD were well aware of it then, and it is surprising that AMD didn't really fix this in the K8 architecture. Allowing a much more flexible reordering of Loads - even without memory disambiguation - would give a very healthy boost to IPC (5% and more). It is one of the main reasons why the P-M can beat the Athlon 64 clock-for-clock in certain applications.

Those are just a few examples that are well known. It is very likely that there are numerous other possible improvements that could take the K8 architecture much further.

Looking at the server version of Core ("Woodcrest") and considering that it is very hard to find a lot of ILP in server applications, the only weakness of Core is that there is no multi-threading in each Core. This small disadvantage is a logical result of the design goal of Core, an architecture which is an all-around compromise for the server, desktop and mobile markets. The lack of Hyper-Threading in Xeon Core products might give Sun and IBM a window of opportunity in the heavy thread server application benchmarks, but since Tigerton (65 nm, two Woodcrests in one package, 4 cores) will come quickly, the disadvantage of not being able to extract more TLP might never be seen. Our astute readers will have understood by now that it is pretty hard to find a weakness in the new Core architecture.

Conclusion 2 : The free lunch is back!

It is ironic that just a year ago, Intel and others were downplaying the importance of increasing IPC and extracting more ILP. Multi-core was the future, single thread performance was a minor consideration. The result was that the reputed Dr. Dobbs journal headlined : "the free lunch is over" [1] claiming that only larger caches would increase IPC a little bit and that the days that developers could count on the ever increasing clockspeeds and IPC efficiency of newer CPU to run code faster were numbered. Some analysts went even further and felt that CPU packages with many relatively simple, small in-order CPUs were the future.

At AnandTech, we were pretty skeptical about the "threading is our only savior" future, as Tim Sweeney, the leading developer behind the Unreal 3 engine, explained the challenges of multi-threaded development of the next generation of games. The fat, wide OoO core running at high clockspeeds was buried a little too soon. Yes, Intel's Core does not use the aggressive domino and LVS circuit-design strategy that NetBurst designs used to achieve stunning clockspeeds. At the same time, it is a fat, massive reordering CPU which gives free lunch to developers who don't want to spend too much time on debugging heavily threaded applications. Multi-core is here to stay, but getting better performance is once again the shared responsibility of both the developer and the CPU designer. Yes, dual-core is nice, but single threaded performance is still important!

I would like to express my thanks to the following people who helped to make this article possible:
Jack Doweck, "Foo", "Redpriest", Jarred and Anand

References

[1] The Free Lunch Is Over: A Fundamental Turn Toward Concurrency in Software By Herb Sutter


Intel's Next Generation Microarchitecture Unveiled, by David Kanter, Real World Technologies
Faster Load Times
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  • Spoonbender - Tuesday, May 02, 2006 - link

    How do you know? As the article said, the prefetching *might* in some cases decrease performance, even if it'll usually be an advantage. But I don't really think you have enough information to make a valid comparison. My point was simply that generally speaking, a 64KB, 2-way associative cache will have better hit rates than a 32KB 8-way associative. Of course, having fancy prefetching is always a good thing, but its effect *is* limited. If it was a huge improvement, people would have done that 8 years ago, instead of just messing with cache size and associativity. Reply
  • Betwon - Tuesday, May 02, 2006 - link

    Your information is too old and should be updated now.
    Prefetcher give much improvement in reducing the miss-rate.
    About 30-90% miss rate reduced.
    The good prefetcher tech is one of the most important performance factors.

    http://www.hpcaconf.org/hpca11/slides/hpca_inst_sl...">http://www.hpcaconf.org/hpca11/slides/hpca_inst_sl...
    Reply
  • Betwon - Tuesday, May 02, 2006 - link

    Who is James E. Smith? I think that you should know him.

    Data Cache Prefetching Using a Global History Buffer -- the prefetcher bring the great performance improvement! From 20-110%
    abstract
    http://ieeexplore.ieee.org/search/freesrchabstract...">http://ieeexplore.ieee.org/search/frees...+buffer%...
    Of course, you can download the full-text pdf file, if you have a IEEE member account. I can download and view it, but can not release it.
    slides ppt
    http://www.ece.gatech.edu/~leehs/ECE7102/slides/ka...">http://www.ece.gatech.edu/~leehs/ECE7102/slides/ka...
    Reply
  • Sunrise089 - Monday, May 01, 2006 - link

    flak flak flak

    Seriously - props to the author on a good article, but if I had one comment it would be that there are length issues to trying to provide the ammount of background needed for this sort of article. I think it's best to either just draw the comparisons between the two chips, or do a full-length many thousands of words write-up on the technical importance of the various topics. I read the article, and while writen well and informative in it's conclusions, I cannot say all the background was enough to make me really understand the concepts better. For example I already knew what out-of-order execution was, but only being able to read a few hundred words more on it didn't allow me to really learn enough to understand all of the reasons why the K8 had a disadvantage in that area, and if all you wanted was for me to understand that it did indeed have the disadvanatge, you could have just said so.
    Reply
  • JohanAnandtech - Monday, May 01, 2006 - link

    It is indeed an issue I struggled with. Writing full length articles on these subjects doesn't sound like a good idea for me: I personally do not like lengthy articles either. So I tried to keep a balance between being technical and keeping it understandable.

    Anyway, Just ask about the points where you were lost. Especially on the OoO matters: it is much more interesting than "AMD has a disadvantage". Basically, reordering happens between the decoding and the execute phase.

    Pushing loads forward helps in two ways:
    1.Whenever a load fails to get it's data from the L1-cache, the CPU has to find other instructions to execute. As loads are very common, it is easier to fill the gaps than when you can not move loads before other loads.

    2. If a load gets pushed forward and a L1-cache miss for that load occurs, it isn't that bad. This is very simplified, but assume the load has been pushed 5 cycles forward, and your L2-cache latency is 10, you only have to wait 5 cycles instead of 10.
    Reply
  • Furen - Monday, May 01, 2006 - link

    I'll be the grammar nazi today, lol.

    Last page, paragraph 5: "[...] increasing the <b>wideness</b> of each unit [...]"
    Width, perhaps? "Wideness" refers to either quality or state (neither of which is discrete) while "width" also also applies to measurable fact (128-bits wide, for example). You can talk about the wideness of the units, for example, but you cannot talk about increasing their wideness...

    Great article, by the way, it's been long since I've read such an enjoyable article.
    Reply
  • emboss - Monday, May 01, 2006 - link

    Just a quick note ... on page 4 you have the table with the execution unit details. There's a couple things incorrect (IMO) in the numbers.

    First, you list the number of double precision FLOPs per cycle. Double precision can be done with SSE, so in the K8 you can do 2 DP ADDs and 2 DP MULs every two cycles (due to the 64-bit wide datapaths), a total of 2 DP FLOPs per cycle.

    Core can do two SSE operations per cycle (the two symmetric units), giving it a total of 4 DP FLOPs per cycle. The third SSE unit does not handle FP ops, but instead handles shuffles and the like.

    Obviously, double both of these numbers if you want a "peak" single precision FLOPs per cycle.

    If instead you were meaning about extended precision (64 bit precision, 80 bit floats) x87 operations, it's exactly the same concept as above since Core has apparently has combined SSE/x87 units (and a fully pipelined FMUL, unlike the P4). This gives both the K8 and Core 2 EP FLOPs per cycle.

    Finally, you have the number of SSE units for the K7 wrong. The K7, like the K8, has two SSE units (FADD and FMUL), and the same 64 bit datapath as the K8. Of course, the K7 cannot handle SSE2, so must use x87 instructions for double precision (ie: two DP FLOPs per cycle).


    Apart from that, very nice article! I've been trying to optimise SSE code for the Core processor and have had to do things by trial and error thanks to the complete void of any decent documentation from Intel. One thing in particular was that I was finding "odd" performance properties with SSE that pointed towards it having two FMUL units. Being symmetric units explains a lot!
    Reply
  • JarredWalton - Monday, May 01, 2006 - link

    See above note regarding Core Duo versus Core "Conroe". (Nice naming scheme, Intel. *grumble*) I will let Johan take care of the rest of your comment as appropriate. (His knowledge of the low level details of all of the microarchitectures discussed here definitely surpasses mine!)

    Unfortunately, it's not particularly surprising to find out that optimal code for Core Duo may need to be slightly tweaked in order to extract the most performance from Conroe. Still, they ought to be similar enough that you own by optimizing for Core Duo. The flipside is the optimal code for Conroe could very likely run worse on Core Duo and other processors. Such is the price of progress, I guess.
    Reply
  • prx99 - Monday, May 01, 2006 - link

    Core is not the first x86 having 4 decoders. That was AMDs K5.

    I remember a statement from AMD that in some design they considered adding one more decoder. It turned out to actually slow down the design because the amount of clock speed lost was not compensated for by the smaller amount of performance gained.

    In my interpretation the fusion is done past the initial decoding, so there is not way more that 4 x86 ops can be decoded in a clock cycle (I'm referring to the "4+1" figure). The profit from fusion is not in the decoding stage but in the out of order engine.

    At AMDs, the "1 branch per cycle" rule is limited to branches seen by the predictor. A branch which is generally not taken is invisible to the prediction engine and therefore free.

    The original P4 indeed had a L1 latency of 2. The major P4 redesign in Prescott however increased it to 3.

    Load/store reordering is already done by the P4, but the penalty from a misprediction is fairly high. This is the drawback of any kind of prediction, whether branches or memory access: It speeds up things when being correct, but slows them down quite a bit more when not. This was the general picture seen in the P4: many applications were sped up by some amount, but some suffered greatly because they systematically fooled the P4's engines.

    Gruss, Andreas
    Reply
  • Betwon - Wednesday, May 03, 2006 - link

    Without branch prediction, K8 will become very very poor. Too terrible!

    The prediction is much better than the forever penalty.

    The penalty of disprediction is just the penalty of doing nothing.(don't predict)

    The penalty is fairly high. If you are against the prediction, you will find that the penalty will happen in K8 every 3 instructions averagely. K8@1.8G(without branch predictor ) will fail to win the old Pentium3@1G(with branch predictor ).

    This is the drawback of lack of prediction, whether branches or memory access: It can not speeds up anything, but often slows down.
    Without branch prediction, K8 will be down!
    Reply

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