Phenom II X2 vs. Athlon II X2 - A Question of Cache

A Question of Cache

AMD's dual-core Phenom II X2 555 'Black Edition' has kept us occupied for a while, what with its ability to be unlocked by the proper motherboard/BIOS to a full Phenom II X4 processor. Although we did not succeed in our , we subsequently managed to do so using an MSI 790FX-GD70 motherboard. It was simply a matter of changing a BIOS setting to get it unlocked, which CPU-Z then identified as a Phenom II X4 B55. This experience was documented in the latest April issue of HWM, but that's not all we had in mind with the Phenom II X2 555 BE. 

AMD also has another dual-core series, the Athlon II X2, which is based on a different core and is a true dual-core. This means that unlike the Phenom II X2 555 BE, which has four cores at its heart, there are no extra cores to enable on the Athlon II X2. AMD has released a newer model in the Athlon II series, the X2 255 along with the Phenom II X2 555 BE, but what intrigued us is the similarity between the two. Both are almost at the same frequency, the Athlon II X2 255 runs at 3.1GHz and the Phenom II X2 555 at 3.2GHz. The main difference lies in the amount and hierarchy of its cache.

As most techies know, CPU cache is basically the amount of on-die memory on the processor. It's the closest and fastest source of memory for the CPU and there's drastically less penalty in accessing the cache than the slower system memory (RAM). On modern x86 processors, there's usually a cache hierarchy, where L1 cache has lower latency than L2 and so on.

The Phenom II X2 has a substantial amount of  L3 cache (a total of 6MB) while the Athlon II X2 has no L3 cache. It does have 2MB of L2 cache to compensate, compared to 1MB on the Phenom II X2. Both have the same amount of L1. This difference is what we're looking at today, since besides the cache, the architecture for both processors is similar. Exacly how beneficial is the extra cache and should you opt for an Athlon II or Phenom II series? Before we proceed to our test performance results to find out, here's how the two AMD dual-core processor series stack up:

Test Setup

To level the clock speeds between the Athlon II X2 and Phenom II X2, we tested an Athlon II X2 250 (3.0GHz) with an Phenom II X2 555 BE (underclocked to 3.0GHz from 3.2GHz). These two processors are then tested in various CPU and gaming benchmarks. We have also included results from the quad-core Athlon II X4 630 and 620 as reference points for those who are more inclined to go quad-core due to their usage.

AMD System Test Configuration
 

• CPU - Phenom II X2 555 BE @3.0GHz, Athlon II X2 250, Athlon II X4 630, Athlon II X4 620
• MSI 790FX-GD70 (AMD 790FX + SB750)
• 2 x 1GB Kingston HyperX DDR3-1333 (7-7-7-20)
• Seagate Barracuda 7200.10 200GB SATA hard drive (one single NTFS partition)
• Zotac GeForce GTX 260 (ForceWare 178.24)
• Microsoft Windows XP Professional with Service Pack 2

Benchmarks

The following benchmarks were used in this review:

• BAPCo SYSmark 2007 Preview (ver 1.05)
• SPECCPU 2000 v1.3
• Futuremark PCMark 2005 Pro
• Lightwave 3D 7.5
• 3ds Max8 (SP2)
• Cinebench 10
• Futuremark 3DMark06 v1.1
• AquaMark3
• World in Conflict v1.05
• Crysis v1.1
• Far Cry 2

Results - SYSmark 2007 Preview

Based on the rather significant differences in SYSmark 2007 scores between the Phenom II and the Athlon II, it seems that cache plays quite a major role in a system test suite. The 6MB L3 cache helped the Phenom II get the headstart for Productivity and E-Learning for the most part. In other segments like 3D and video creation, the gap was much more narrow and of course, the quad-core processors are the ones to get.

Results - SPECCPU 2000 v1.3

The floating point performance between the two AMD series was mostly unchanged. Integer performance however is quite dependent on the cache size and the availability of the L3 cache on the Phenom II makes for quite the difference. The slower clocked quad-core CPUs predictably performed well for multi-threaded workloads, though with lower peak scores since the latter is primarily a single-core test where clock speeds take precedence. Please take note that for the rate tests below, we've only shown the maximum throughput obtainable as we couldn't obtain the full deck of results for some reason. Nevertheless, the below results are adequate enough to portray our point.

Results - Futuremark PCMark05 Pro

For the CPU score, the quad-core processors again did well as one would expect. There was however little to separate the Phenom II with the Athlon II in terms of CPU and memory performance. 

Results - Lightwave 3D 7.5

In Lightwave 3D, there were minute differences between the two processors, but like some of the previous benchmarks, at the same frequency, the Phenom II and Athlon II are essentially the same processor.

Results - 3ds Max 8 (SP2) and Cinebench 10

In 3D rendering benchmarks, the difference was almost negligible. The big winners here were mostly the quad-core processors and regardless of their cache designs, both the Phenom II X2 and the Athlon II X2 were outmatched.

Results - Futuremark 3DMark06 & AquaMark3

Next we come to something that should be of interest to gamers. In the two synthetic gaming benchmarks, the Phenom II showed a distinct advantage over the Athlon II. As we have experienced in testing other processors, the presence of the extra cache is crucial when it comes to applications like games. This was particularly exaggerated in the older AquaMark3 benchmark.

Results - Gaming Benchmarks

How about their performance in actual games? Well, we put them through Crysis, Far Cry 2 and World in Conflict. In Crysis, we tried both the CPU and the GPU test and it was clear that while the former mostly had the Phenom II and Athlon II equal, the GPU test was the one that approximates the real-world situation, as the cache advantage was obvious. This was further strengthened by our results for the other two games, where the Phenom II was the faster processor compared to the Athlon II.

Power Consumption

Is the extra cache worth the extra power consumption however? Despite their similarities in so many ways, the fact remains that the Phenom II is a different core from the Athlon II and even with disabled cores, its native TDP is higher. That explains why even at an identical 3.0GHz clock rate, the Phenom II guzzled more power than the Athlon II X2 250.

Cache Matters, Sometimes

One could say it's the same old story, but repackaged for a newer generation. Yes, cache matters for your applications, but it is highly dependent on what you are running. Between the two, the Phenom II X2 and the Athlon II X2, one would choose the Phenom II without question, provided that everything else is equal. It is however not equal - the Phenom II X2 consumes more power while undoubtedly providing its extra performance. This boost appears most significant for applications like games. Applications that one associate with a processor's raw power, like 3D rendering and video encoding benefit from having more cores however.

As for weighing the cost of the two, there's no straight comparison for most cases due to clock speed differences. The only direct match is the Athlon II X2 255 with the Phenom II X2 550, both of which are at 3.1GHz. The price difference between the two however is only around US$13. For that amount, it's likely that gamers would simply pony up the cost for the Phenom II X2, especially when it can be unlocked without too much trouble if they are so inclined.

Of course, the Phenom II X2 will have the higher power consumption, but if that's your main concern, then AMD's own line of low-power processors should be the answer. With dual-cores, quad-cores and now six-cores in the market, there's plenty of choices from both AMD and Intel. It should be easy to find one that fits your budget and after this article, you would hopefully be more prepared to know what to look out for before buying a processor.

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