Interview with AMD's Fred Weber - The Future of AMD Microprocessorsby Anand Lal Shimpi on March 31, 2005 12:00 AM EST
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Along with Scott Wasson of The Tech Report and Kyle Bennett of HardOCP, we recently had some time to sit down and talk with AMD's CTO Fred Weber about his vision of the future of microprocessors. We took the opportunity to compare and contrast his vision with our discussions from this year's Spring IDF that we had written about.
Be sure to read our IDF Spring 2005 - Predicting Future CPU Architecture Trends article before continuing as it provides a lot of background information necessary for this piece.
The ILP/TLP Debate in AMD's ShoesWhen we talked to Intel at IDF, we had the distinct impression that the focus on improving microprocessor performance as a whole had shifted pretty significantly from ILP to TLP. To put it plain and simple, making individual cores faster was no longer top priority; rather, getting multiple cores to work together was the new focus.
Weber's stance on ILP vs. TLP tended to agree with what we had heard from Intel; TLP is the future and using ILP to increase performance is at a point of extremely diminished returns. That being said, we asked Fred where he thought the improvements in ILP would be going forward and he responded with the following four areas:
Fred's number one increase for single core, single thread performance was clock frequency, so we will inevitably see that clock speed will go up as time goes on. It is quite possible that combined with a reduction in branch prediction latency, future versions of the Athlon 64 will use a lengthened pipeline to reach higher operating frequencies. If paired with Prescott-caliber branch predictors, a somewhat deeper pipelined K8 would provide additional frequency headroom without too much worry.
- Reducing Memory Latency
- Instruction Combining
- Branch Prediction Latency
Behind clock frequency, Weber saw reducing memory latency as the other major way of increasing single core performance. Reducing memory latency in this sense basically means two things:
More than once during our conversations with Weber, it became clear that future multi-core AMD processors will continue to have their L1 and L2 caches separate, but a shared L3 cache will eventually be introduced to help reduce memory latency and keep those cores fed.
- higher levels of cache hierarchy, and
- better prefetching.
To Weber's second point, the use of helper threads (compiler or application generated threads that go out and work on prefetching useful data into cache before it's requested) will also improve single core performance. Intel has been talking about using helper threads since before Hyper Threading, but there is no idea of when we can expect real world implementation of helper threads at this point.
The topic of instruction combining was also interesting because it is something that we have only seen used in the Pentium M (Micro-Ops Fusion). Weber couldn't elaborate on an AMD implementation of some form of instruction combining, but we did get the distinct impression that it's something that's in the cards going forward. It looks as if elements from both AMD's and Intel's present day architectures will shape tomorrow's designs.
In the end, Fred left us with the following: if you see single core performance improving at a rate of 40% per 12 - 18 months, it will now improve at about half that rate for the foreseeable future.