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
Low Latency L2 Cache
We mentioned at the start of this review that the Pentium M featured a very large, yet low latency L2 cache - at 2MB for the current 90nm Dothan based Pentium M. But how much low latency are we talking? To find out, we turned to ScienceMark 2.0 and Cachemem.First, let's have a look at the latency of the Pentium M's 64KB L1 cache:
| Cachemem L1 Latency | ScienceMark L1 Latency | |
| AMD Athlon 64 | 3 cycles | 3 cycles |
| Intel Pentium 4 (Northwood) | 1 cycle | 2 cycles |
| Intel Pentium 4E (Prescott) | 4 cycles | 4 cycles |
| Intel Pentium M | 3 cycles | 3 cycles |
With such a large L1 cache, it is difficult to get much lower than 3 cycles, as we see that the Pentium M has a similar L1 access latency as the Athlon 64. What is also important to note, however, is that the Pentium M does have a lower L1 access latency than the Pentium 4E.
But what we came here to look at was L2 cache latency, which matters much more in real world application performance where not everything fits into L1:
| Cachemem L2 Latency | ScienceMark L2 Latency | |
| AMD Athlon 64 | 17 cycles | 18 cycles |
| Intel Pentium 4 (Northwood) | 16 cycles | 16 cycles |
| Intel Pentium 4E (Prescott) | 23 cycles | 23 cycles |
| Intel Pentium M | 10 cycles | 10 cycles |
Here's where things get very interesting - the Pentium M has the lowest L2 cache access time of any of the modern day desktop microprocessors. With a 10 cycle L2 latency, any application that fits within the Pentium M's 2MB cache will most definitely perform very well on the CPU. It is the 10 cycle L2 that allows the Pentium M to be competitive with much higher clocked CPUs in most mobile applications as they are normally office application tasks that are generally very cache-friendly. Keep this in mind as we look at the actual performance numbers of the Pentium M.
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fitten - Tuesday, February 8, 2005 - link
Also, it's interesting that there are many benchmarks chosen which are known to stress the weaknesses of the Pentium-M... not that it isn't interesting information. For example, there seems to be a whole lot of FPU intensive benchmarks (around 15 or so, all of which the Pentium-M should lose handily - known before they are even run) so kind of just hammering the point home I guess.Anyway, the Dothans held up pretty well from what I can see... Most of the time (except for the notable FPU intensive and memory bandwidth intensive benchmarks), the Dothan compares quite well with Athlon64s of the same clock speed that have the advantage of dual channel memory.
fitten - Tuesday, February 8, 2005 - link
The other interesting thing about the Athlon64 vs. Dothan comparison is that even with dual channel memory bandwidth on the Athlon64's side, the single channel memory bandwidth of the Dothan still keeps it very close in many of the benchmarks and can even beat the dual channel Athlon64s at 400MHz higher clock in some.Anyway, the Pentium-M family is a good start. Some tweaking here and there (improved FPU with better FPU performance and maybe another FPU execution unit, improved memory subsystem to make good use of dual channel) and it will be at least as good as the Athlon64s across the board.
I own three Athlon64 desktops, two AthlonXP desktops, and two Pentium-M laptops and the laptops are by no means "slow" at doing work.
KristopherKubicki - Tuesday, February 8, 2005 - link
teutonicknight: We purposely don't change our test platform too often. Even though we are using a slightly older version of Premiere, it is the same version we have used in our other processor analyses.Hope that helps,
Kristopher
kmmatney - Tuesday, February 8, 2005 - link
There's also a Celeron version that would have been intersting to review. The small L2 cache should hurt the performance, though. I think the celeron version using something like 7 Watts. It would make no sense to put a celeron-M in such an expensive motherboard, though.Slaimus - Tuesday, February 8, 2005 - link
I think this indirectly shows how AMD needs to update its caching architecture on the K8. They basically carried over the K7 caches, which is just too slow when paired with its memory controller. Instead of being as large as possible (as evidenced by the exclusive caches) at the expense of latency, the K8 needs faster caches. The memory bandwith of L2 vs system memory is only about 2 to 1 on the K8, which is to say the L2 cache is not helping the system memory much.sandorski - Monday, February 7, 2005 - link
I think the Pentium M mythos can now be laid to rest.mjz5 - Monday, February 7, 2005 - link
to #29:your 2800 is the 754 pin.
the 3000+ reviewed is the 939 pin which is 1.8. the 3000+ for the 754 is 2.0 ghz
kristof007 - Monday, February 7, 2005 - link
I don't know if anyone else noticed but the charts are a bit off. My A64 2800+ is running at a stock 1.8 ghz .. while in the review the A64 3000+ is running at 1.8 ... weird!knitecrow - Monday, February 7, 2005 - link
#251) Intel and AMD measure TDP differently... and TDP is not the same as actual power dissipation. The actual dissipation of 90nm A64 is pretty darn good.
2) A microprocessor is not made of Lego... you can't rearrange/tweak parts to make it faster. It takes a lot of time, energy and talent to make changes -- even then it may not work for the best. Prescott anyone?
Frankly I’ve been waiting for a good review of P-M's actual performance. I really don't trust those "other" sites.
k00kie - Monday, February 7, 2005 - link