Intel Core Duo (Yonah) Performance Preview - Part II

What about Clock Speeds?

Whereas the Pentium 4's extremely deep pipeline made clock-for-clock comparisons to the Athlon 64 virtually meaningless, the Pentium M and Yonah processors feature far shorter pipelines akin to AMD's architecture.

The Athlon 64 features a 12-stage integer pipeline, and while Intel has never specifically disclosed the length of Yonah's pipeline, they have made two important statements: it is longer than the Pentium III's 10-stage integer pipeline, and shorter than Conroe/Merom's 14-stage pipeline.  Given the relatively tight range, Yonah's pipeline can pretty much be considered to be very similar to AMD's Athlon 64, give or take a stage of the pipeline. 

The net result is that we can draw some valid conclusions based on comparisons of Yonah to the Athlon 64 X2 at similar clock speeds.

But our Yonah sample ran at 2.0GHz, which ends up being the speed of the slowest Athlon 64 X2 that is currently available: the 3800+.  The highest end Athlon 64 X2s currently run at 2.4GHz, with high speeds just around the corner.  So the question isn't just how competitive Yonah is at 2.0GHz, but rather, how high can Yonah go?  Unfortunately, our test platform wouldn't allow us to overclock our chip very far, but thankfully, we have access to a decent amount of Intel's future roadmaps, so we can at least see what's going to happen to Yonah over the next year. 

While Yonah will make its debut at a maximum speed of 2.16GHz, it will actually only receive a single speed bump before Merom's release at the end of the year.  That means that we'll see a 2.33GHz Yonah after the middle of the year, but we'll have to turn to Merom to get any higher clock speeds. 

Looking back to our initial articles on the Pentium M's architecture, you'll remember that one of the important aspects of its design is that all critical paths in the chip were slowed down to meet a maximum clock target.  This means that Intel set a clock target for the CPU and made sure that the chip ran at that speed or below, and did not optimize any paths that would have allowed the CPU to run higher.  Instead, the Pentium M team depended on the manufacturing folks to give them additional clock speed headroom by providing smaller manufacturing processes every 2 years.  In other words, the Pentium M was never designed for high clock speeds, which is why it debuted at 1.5GHz and still has not even reached 2.33GHz today. 

Intel's next-generation microarchitecture hopes to change that approach ever so slightly by introducing a longer pipeline into the equation, but on a much more conservative basis than the Pentium 4 did just 5 years ago.  Conroe (desktop), Merom (mobile) and Woodcrest (server) will feature a 14-stage integer pipeline, which will allow for higher clock speeds than what Yonah could pull through.  We would expect a debut at a minimum of 2.4GHz and probably at least one speed grade higher.  Learning from their mistakes with the Pentium 4, Intel will balance the reduction in efficiency of a deeper pipeline with a wider 4-issue core (vs. the current 3-issue core used in Yonah).

So it looks like Intel's plan for 65nm is to rely on their deeper pipelined processors (Conroe/Merom/Woodcrest) for higher clock speed, with Yonah falling below the 2.5GHz mark.  And based on what we've seen in the first article, a 2.33GHz Yonah would be competitive with an Athlon 64 X2 4600+, but definitely not outpacing it.  This does bode well for Intel's next-generation processors, especially on the desktop with Conroe. 

If the move to a 4-issue core is able to balance out the negative impact of a deeper pipeline (which admittedly it may or may not do in all cases), a higher clock speed desktop part should be very good competition for AMD's offerings.  Although based on what we've seen thus far, we would be surprised if Conroe vs. Athlon 64 was a blow-out in favor of either manufacturer; more and more, it is looking like Conroe will simply bring Intel up to par with AMD, ahead in some areas, behind in others, and with the lower power advantage as long as AMD is still at 90nm. 


Why the X2 and why not Turion?

One of the other questions that we were asked a lot after the first article was why we insisted on comparing a mobile Yonah processor to a desktop Athlon 64 X2, and not an AMD Turion 64.  Our reasoning was obvious to some, but we felt it made sense to present it more clearly here:

1. As much as Yonah is a mobile processor, it is a great indicator of the performance of Intel's future desktop processors based on the Conroe core.  AMD has already stated that beyond moving to Socket-M2 and some minor updates, there will be no significant architectural changes to the Athlon 64 line next year.  In other words, we know for the most part how AMD's going to be performing next year, but we have no clue how Intel will towards the end of 2006; Yonah helps us fill in the blanks. 
2. AMD will have a dual core Turion based mobile processor out sometime in 2006. However, it will be based on AMD's Socket-M2 platform, meaning that it will include DDR2 support.  Given that we don't know exactly how DDR2 is going to impact the Athlon 64's performance, we couldn't accurately simulate the performance of AMD's upcoming dual core Turion.  Comparing a dual-core Yonah to AMD's single-core Turion also wouldn't be too valid a comparison either.