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| What Nehalem is really about
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Date: August 19th, 2008
Author: Johan De Gelas
As IDF has started, the first benchmarks of Nehalem will probably pop up. It is without a doubt an impressive architecture that gets a much better platform to run on, but this CPU is not about giving you better frames per second in your favorite game than the Penryn family. Let me make that more clear: even when the GPU is not the bottleneck, it is likely that most games will not significantly faster than on Penryn. We, the people behind it.anandtech.com will probably have the most fun with it, more than your favorite review crew at Anandtech.com :-). And no, I have not seen any tests before I type this. Nehalem is about improving HPC, Database and virtualization performance, much less about gaming performance. Maybe this will change once games get some heavy physics threads, but not right away.
Why? Most Games are about fast caches and super integer performance. After all, most of the Floating point action is already happening on the GPU. All Core 2 CPUs were a huge step forward in integer performance (not in the least because of memory disambiguation) compared to the CPUs of that time (P4 and K8). Nehalem is only a small step forward in integer performance. And the gains due to slightly increased integer performance are mostly negated by the new cache system. In a previous post I told you that most games really like the huge L2 of the Core family. With Nehalem they are getting a 32 KB L1 with a 4 cycle latency, next a very small (compared to the older Intel CPUs) 256 KB L2 cache with 12 cycle latency and after that a pretty slow 40 cycle 8 MB L3. When running on Penryn, they used to get a 3 cycle L1 and a 14 cycle 6144 KB L2. That is a 24 times larger L2 than Nehalem!
The percentage of L2 caches misses of most games running on a Penryn CPU is extremely low. Now that is going to change. The integrated memory controller of Nehalem can't help much, as the fact remains that the L3 is slow and the L2 is small.
But that doesn't mean Intel made a bad choice. Intel made a superbly good choice by improving the performance where Core (Merom/Penryn) was mediocre to good. Penryn was already a magnificent gaming CPU, but it could not beat the AMD competitor in HPC benchmarks. And AMD gave good resistance in the database performance benchmarks. That is all going to change.
Most Database code can not use the wide architecture of Penryn very well. The number of instructions per cycle get lower than 0.5 and waiting for the memory is the most probably cause. SMT or Hyperthreading can do wonders here: while one thread waits for a memory stall, the other thread continues working and vice versa.
Secondly, quad (and eight) socket performance is going to improve a lot as four Nehalems only have to keep four L3 in sync, while a similar Tigerton system has to keep 8 L2 caches in sync. That is why the cache system is perfect for server performance, but a little less interesting for gaming performance.
The massive bandwidth that the integrated tri-channel memory controller delivers will do wonders for HPC code. And the new TLB architecture with EPT will make Nehalem shine compared to it's older Core brothers.
No, Nehalem was made to please the IT and HPC people. Bring it to it.anandtech.com, it is not that interesting for you gamers ;-) | 
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发表于 2008-8-19 22:47
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