Intel's Pentium M on the Desktop - A Viable Alternative?
by Anand Lal Shimpi on February 7, 2005 4:00 PM EST- Posted in
- CPUs
Memory Latency and Bandwidth
Currently, there are only two manufacturers of desktop Pentium M motherboards who are selling into the channel - AOpen and DFI. Both AOpen and DFI's motherboards came about not because of widespread consumer demand, but because they each had one customer that needed a Pentium M motherboard for a specific application. Once the boards were designed and built, they were later repackaged and made available to the public as an afterthought.The major issue with both of these motherboards is that they are based on the 855GME chipset. The 855GME only features AGP 4X support, but the killer is in its single-channel DDR333 memory controller. Without DDR400 support, the 855GME starves the Pentium M for bandwidth, as it is only capable of delivering 2.7GB/s of bandwidth to main memory while the Pentium M at 2.0GHz needs 3.2GB/s of bandwidth to remain most efficient. Overclocking the memory bus is somewhat of an option, but not exactly the most desirable one for reasons that we will get to later.
One solution is Intel's recently released mobile 915 chipset, which features a dual-channel DDR1/DDR2 memory controller. The dual channel controller is more than capable of supplying the appropriate memory bandwidth to the Pentium M, if not a bit overboard, but right now mobile 915 isn't an option on the desktop.
With an unsatisfactory amount of memory bandwidth, the Pentium M will undoubtedly be held back in performance in applications where memory bandwidth is most important. As we all know, memory bandwidth and latency are interdependent, so let's see how the latency to main memory compares.
For our memory latency tests, we once again turn to ScienceMark 2.0:
| CPU | Memory Latency (in ns) |
| AMD Athlon 64 3200+ (2.0GHz) | 50ns |
| Intel Pentium 4E 560 (3.6GHz) | 80ns |
| Intel Pentium M 755 (2.0GHz) | 80ns |
With an on-die memory controller, the Athlon 64 obviously offers the lowest latency memory access out of the group. The reason why we used a 2.0GHz Athlon 64 for this comparison was to show the memory latency seen by a CPU clocked identically to the Pentium M. As strong as the Pentium M's branch predictor may be, the trip to main memory will always be longer than the Athlon 64 - increasing the penalty from having a longer pipeline.
When you compare the Pentium M to the Pentium 4, you see the real harm in only having a single channel DDR333 memory controller - the time for the Pentium M to get to main memory is very similar to that of the Pentium 4, even when the latter is using higher latency DDR2 memory. High memory latency will send the performance of the Pentium M tumbling as soon as it leaves the sanctity of its low latency L2 cache.
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bob661 - Tuesday, February 8, 2005 - link
The only problem with this chip is that the marketing is oriented towards the mobile market and therefore not a direct competitor to the A64. It would be nice if it was. It might bring some cats out of the bag on the AMD side. Competition in the marketplace is good for us all.jvrobert - Tuesday, February 8, 2005 - link
Really, AMDroids, get a grip. You're all excited because the AMD chips beat a mobile processor pretty handily, and because you are making some silly assumption that the Pentium-M in its current form is Intel's "last chance".First, Intel doesn't need a last chance. They make enough money to make AMD look like a Mexico City taco stand. So enough of those delusions of grandeur.
But on a technical front, if Intel ramps the clockspeed up to the 2.8 range (easy), and releases a desktop class chipset for the Pentium M it would match or exceed any current chip. And these are _basic_ steps. What if they made more improvements?
jvrobert - Tuesday, February 8, 2005 - link
bob661 - Tuesday, February 8, 2005 - link
#45You are a rock. The point of the article was to compare the P-M to desktop CPU's because most of us here wanted to know it will perform. And you know what? It performed very nicely.
classy - Tuesday, February 8, 2005 - link
I just can't help but to laugh at some folks. Its a nice chip but clearly not in the A64 ballpark. Its that simple. As far as the 2.8 oc, that was only accomplished in one reveiw. All the reviews show the same thing you have oc so it can it compete. What's interesting though is most of these Intel fanboys don't want to see a comparison of an oc'ed A64 vs a Dothan. Smoke city :)FrostAWOL - Tuesday, February 8, 2005 - link
IF the Pentium-M and P4 are electrically incompatible then someone please explain this:HP Blade system Pentium-M with Serverworks GC-SL chipset
http://h18000.www1.hp.com/products/servers/prolian...
FrostAWOL
jae63 - Tuesday, February 8, 2005 - link
Great review & of interest to those of us with HTPCs. Too bad the price point is so steep.One minor correction on page 11:
"The Pentium M does a bit better in the document creation tests, as they are mostly using applications that will fit within the CPU's cache. However, the introduction of a voice recognition program into the test stresses the Pentium M's floating point performance, which does hamper its abilities here."
Actually NaturallySpeaking uses almost no floating point but is very memory intensive. The performance hit that you are seeing is because it uses a lot of memory bandwidth and its dataset doesn't fit in the L2 cache.
Here's some support for my statement, by the main architect of NaturallySpeaking, Joel Gould:
http://tinyurl.com/6s4mh
segagenesis - Tuesday, February 8, 2005 - link
#43 - I think you have the right idea here. This processor is not meant to be performance busting but rather a low energy alternative to current heat factories present inside every P4 machine. I would love to have this in a HTPC machine myself but the cost is still too damn high. Hopefully higher production will bring the cost down.Aileur - Tuesday, February 8, 2005 - link
I guess the pentium M isnt ready (yet) for a full featured gaming machine, but with that kind of power, passively cooled, it would make for one hell of an htpc.PrinceGaz - Tuesday, February 8, 2005 - link
#45- It was not an unfair review, on the contrary it seemed very well done. The reason the P-M was compared with fast P4 and A64's is because they cost about the same.Maybe someone else buys your computers for you, but most of us here have to spend our own money on them so cost is the best way to decide what to compare it with.