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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bobsmith1492 - Monday, February 7, 2005 - link
Granted the T8000 here is an Intel fanboy, but please notice Anand was comparing clock-for-clock.T8000 - Monday, February 7, 2005 - link
There is one big difference between this review and the reviews where the Pentium M did very well: CLOCKSPEED!While others where able to get over 2.8 Ghz with aircooling, Anand got just 2.4 Ghz. This may be a coincidence, but it is the difference between surprisingly good performance and a few % below others.
As most of the benchmarks where based on the stock 2 Ghz, the difference became even greater.
So this review just shows that the stock speed Pentium M performs about 30% less with about 30% less clockspeed than overclocked versions.
A slightly redesigned version with higher voltages is not extremely unlikely to hit at least 3 Ghz. Combining that with a desktop chipset will result in stellar performance, as the benchmark scores in this review (x1.5) indicate.
But since there is no slightly redesigned version and Intel has no good reason to make one, the current Pentium M desktops will only appeal to overclockers and silent computing people.
Also, for some reason, Anand found the 90W TDP of the 2.4 Ghz A64 closer to the 20W of the P-M than to the 110W of the 3.8 Ghz P4.
CSMR - Monday, February 7, 2005 - link
That's a very good option Zebo, thanks for posting it.teutonicknight - Monday, February 7, 2005 - link
One suggestion: Why don't you start using a newer version of Premiere for testing? I personally don't use it, but every that I know who does says before Premiere Pro, the program sucked. I'm sure the render results would be much more realistic and accurate if you used a more up to date version of the programRegs - Monday, February 7, 2005 - link
I was wondering the same thing too Jeff. If you feed it more bandwidth, it would eliminate the pipeline stalls and maybe give it a chance to reach higher clock speeds. Right? Or is it still prohibited by the shorter pipeline to reach higher clock speeds?Longer pipeline = wasted clock cycles. But to me that sounds like the PM should actually scale a lot better with a speed boost. Why exactly does it scale badly compared to a P4? Could it be remedied in anyway with a dual channel memory bus?
ozzimark - Monday, February 7, 2005 - link
there's something wrong with the 3400+ in the spec tests. why is the 3000+ beating it consitantly?Warder45 - Monday, February 7, 2005 - link
Maybe I missed something but I don't see the reason for all the negitivity in the final words. The 2.4Ghz P-M was very close to the A64 2.4Ghz in many of the tests, 3D rendering seemed to slow it down but that looked like it. With better boards and memory the P-M might best the A64 in a clock for clock match up.I do agree the prices are way too high. I think Intel really needs to wake up and smell what they have cooking here. With more support and more aggressive priceing they could easily have a winner in the HTPC and SFF markets.
plewis00 - Monday, February 7, 2005 - link
Surely when someone builds a mainboard with the Sonoma (i915) platform using PCI-E and DDR2-533 then it will change. And I wouldn't have thought that's that far off assuming they don't charge rip-off prices for the technology. It would also be perfect for Shuttle systems where the emphasis is on quietness and coolness rather than so much on performance.Zebo - Monday, February 7, 2005 - link
CSMRSo's this one very soon..
http://www.xtremesystems.org/forums/showthread.php...
...more than excellent performance wise if Dothan is excellent...power differential hopefully for AMD will be nominal.
Sokaku - Monday, February 7, 2005 - link
While it is true that the A64 has way more bandwidth, I doubt that is the reason why it crushed the P-M in the Professional Applications. I think the real cause is to be found in the P-M's abillity to do FP divisions. The P-III had a pipelined FP unit, however div operations were extremly expensive. My guess would be that Intel haven't thrown much effort into improving on this.