Barcelona Architecture: AMD on the Counterattack
by Anand Lal Shimpi on March 1, 2007 12:05 AM EST- Posted in
- CPUs
AMD Virtualization Improvements
The performance-related improvement to Barcelona comes in the way of speeding up virtualized address translation. In a virtualized software stack where you have multiple guest OSes running on a hypervisor there's a new form of memory address translation that must be dealt with: guest OS to hypervisor address translation, as each guest OS has its own independent memory management. According to AMD, currently this new layer of address translation is handled in software through a technique called shadow paging. What Barcelona offers is a hardware accelerated alternative to shadow paging, which AMD is calling Nested Paging.
Supposedly up to 75% of the hypervisor's time can be spent dealing with shadow pages, which AMD eliminates by teaching the hardware about both guest and host page tables. The translated addresses are cached in Barcelona's new larger TLBs to further improve performance. AMD indicates that Barcelona's support for Nested Paging requires very little to implement; simply setting a mode bit should suffice, making the change easy for software vendors to implement.
Power Management
The most recent aspect of Barcelona's design that AMD revealed is how it handles power management. Although all four cores still operate on the same power plane (same voltage), Barcelona's Northbridge now runs on a separate power plane. Barcelona's core and Northbridge voltages can vary between 0.8V - 1.4V independently of one another.
In a conventional platform architecture, the Northbridge and the CPU are already on separate power planes given that the Northbridge is external to the CPU. The benefit of this arrangement is that the two chips can power down independently of one another, so when the memory controller has little to do, it can power down until needed. With AMD's K8, this wasn't true as the Northbridge and CPU core(s) were on the same power plane. In Barcelona, they are separated to improve power efficiency.
The individual cores still share the same reference voltage, but each core has its own PLL so that they can run at different clock speeds depending on load. While voltages of all four cores have to be equal, clock speed and thus current draw can be reduced depending on load - which will amount to power savings under normal usage conditions. The implications on the desktop are particularly interesting since it's rare that most desktop workloads will keep all cores pegged at 100% utilization.
Barcelona supports up to 5 independent p-states for each core, varying only in clock speed. The p-states are completely hardware controlled, so you will not need a driver to enable support for the power management features. AMD also increased the amount of clock gating done on Barcelona compared to K8 at both the block level and logic level. AMD wouldn't give us any more detail than this, but given how long it's been since the K8's introduction we'd expect that there's a lot that can be done.
The performance efficiency enhancements to Barcelona, coupled with updated power management, further clock gating and 65nm process allow AMD's first quad core part to operate within the same thermal envelope as current Opterons.
The performance-related improvement to Barcelona comes in the way of speeding up virtualized address translation. In a virtualized software stack where you have multiple guest OSes running on a hypervisor there's a new form of memory address translation that must be dealt with: guest OS to hypervisor address translation, as each guest OS has its own independent memory management. According to AMD, currently this new layer of address translation is handled in software through a technique called shadow paging. What Barcelona offers is a hardware accelerated alternative to shadow paging, which AMD is calling Nested Paging.
Supposedly up to 75% of the hypervisor's time can be spent dealing with shadow pages, which AMD eliminates by teaching the hardware about both guest and host page tables. The translated addresses are cached in Barcelona's new larger TLBs to further improve performance. AMD indicates that Barcelona's support for Nested Paging requires very little to implement; simply setting a mode bit should suffice, making the change easy for software vendors to implement.
Power Management
The most recent aspect of Barcelona's design that AMD revealed is how it handles power management. Although all four cores still operate on the same power plane (same voltage), Barcelona's Northbridge now runs on a separate power plane. Barcelona's core and Northbridge voltages can vary between 0.8V - 1.4V independently of one another.
In a conventional platform architecture, the Northbridge and the CPU are already on separate power planes given that the Northbridge is external to the CPU. The benefit of this arrangement is that the two chips can power down independently of one another, so when the memory controller has little to do, it can power down until needed. With AMD's K8, this wasn't true as the Northbridge and CPU core(s) were on the same power plane. In Barcelona, they are separated to improve power efficiency.
The individual cores still share the same reference voltage, but each core has its own PLL so that they can run at different clock speeds depending on load. While voltages of all four cores have to be equal, clock speed and thus current draw can be reduced depending on load - which will amount to power savings under normal usage conditions. The implications on the desktop are particularly interesting since it's rare that most desktop workloads will keep all cores pegged at 100% utilization.
Barcelona supports up to 5 independent p-states for each core, varying only in clock speed. The p-states are completely hardware controlled, so you will not need a driver to enable support for the power management features. AMD also increased the amount of clock gating done on Barcelona compared to K8 at both the block level and logic level. AMD wouldn't give us any more detail than this, but given how long it's been since the K8's introduction we'd expect that there's a lot that can be done.
The performance efficiency enhancements to Barcelona, coupled with updated power management, further clock gating and 65nm process allow AMD's first quad core part to operate within the same thermal envelope as current Opterons.
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JustKidding - Friday, March 2, 2007 - link
So what you are saying is that it's not the size of your cache that matters as much as how well you use it.VooDooAddict - Thursday, March 1, 2007 - link
With Cache size differences usually having small impact on performance for Athlon64s, the slight trade off for better yields and margins seems the better choice for AMD here.
Regs - Thursday, March 1, 2007 - link
Where was this article 8 months ago? ;)I agree with Anands closing article that AMD now needs it's own "snowball effect" for the next couple of years. 4-5 years with a sitting target against a giant like Intel prooved to be costly in terms of competivness.
We all saw it coming when Intel developed the first Pentium M. It looks like AMD got the message as well and started the Barcelona project. Maybe AMD learned their lesson.
iwodo - Thursday, March 1, 2007 - link
So bascially all intel 's C2D improvement are made into Barcelona. And apart from Virtualization improvement there are nothing new from AMD that Intel doesn't have?On performance note Barcelona doesn't seem to offer better clock scaling. I.e even if it is 30% faster then its current K8 it will only have slight advantage against C2D clock per clock. Not to mention it is up against Penryn. Although Penryn is nothing much then a few minor tweaks and more cache. It does allow intel to scale higher in clock speed.
And given AMD slow roll out rate, and AMD limited production capacity Barcelona never seem like much of a threat.
The article does not mention anything about FP improvement. Are AMD keeping them secret for now or is that all we are going to see?
Spoelie - Thursday, March 1, 2007 - link
The FP improvement is the SSE improvement, and according to the theory it's more powerful than what core2 duo is offering.There are improvements mentioned that are not in core2 (+ other way around, like instruction fusing), and improvements that are inspired on the same principle but implemented differently. The architectures themselves differ widely (see earlier article that compares K8 with Core2 - reservation station etc.) so different implementations of principally the same optimizations on a different architecture will have vastly different effects. Even after these improvements, the capabilities (how much can you decode, etc) of each read nothing alike. And if it were all the same, AMD has the platform advantage, so it would still end up faster by virtue of nothing else but that. Some guesstimates made by varying sites would put Barcelona ahead in FP code and at the same level or slightly behind in INT code. But those are just guesstimates.
What I'm trying to say here is that barcelona is still very different from core2, and that we just don't know yet in which direction the pendulum will swing ;)
Shintai - Thursday, March 1, 2007 - link
No....precisely in theory is where Barcelona lacks. Core 2 Duo could in theory do 6 64bit or 3 128bit SSE instructions per cycle. Barcelona can do 4 64bit or 2 128bit. AMD provided this information aswell.Griswold - Thursday, March 1, 2007 - link
Wishful thinking.Spoelie - Thursday, March 1, 2007 - link
Hmmm, in the earlier article, there was explicit emphasis on the fact that 2 of the 3 units are symmetric in core2, but I'm not too sure what it means. It does imply however that those 3 units of core2 can only be used fully in certain combinations, and are not 3 independent units. On 128-bit performance, what was said is this: "so the Core architecture has essentially at least 2 times the processing power here [compared to K8]". Not 3 times, but "at least" 2 times, so again the 3 times will probably only be in certain situations.The next paragraph said this:
"With 64-bit FP, Core can do 4 Double Precision FP calculations per cycle, while the *Athlon64* can do 3."
So K8 was not at such a big disadvantage when it came to 64-bit SSE, if Barcelona doubles everything SSE, it should come ahead in this area.
So to me it looks like for 128-bit, core2 will be faster in some situations, on par in others, and for 64-bit, Barcelona would be ahead.
If this is wrong, I do not know where some of the articles I read over time came from, implying Barcelona would be better overall in SSE.
Shintai - Friday, March 2, 2007 - link
Core 2 got 3 individual SSE ports:http://www.realworldtech.com/page.cfm?ArticleID=RW...">http://www.realworldtech.com/page.cfm?ArticleID=RW...
AMD says 4 double:
http://www.anandtech.com/showdoc.aspx?i=2768&p...">http://www.anandtech.com/showdoc.aspx?i=2768&p...
And 64 or 128bit doesnt matter. I dont know how you think that way.
Barcelona got 2 SSE ports. They are able to do 2 128bit or 4 64bit. Most 128bit actually contains 2 64bit or 4 32bit.
Core 2 got 3 SSE ports. They are able to do 3 128bit or 6 64bit.
flyck - Friday, March 2, 2007 - link
core duo has 3 SSE units but they are not symmetric, meaning that not every unit can execute all commands. Core duo can do at best 4DP flops/ cycle. the same as barcelona.