Ask the Experts: ARM's Cortex A53 Lead Architect, Peter Greenhalgh

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• Tchamber - Wednesday, December 11, 2013 - link

  Peter,
  Thank you for taking the time to answer our questions!
  As mobile devices become more and more useful/powerful, they have encroached on territory historically dominated by Intel AMD...and they feel the pressure. Does ARM feel pressure from from those two companies? As ARM progresses, will you actively target the desktop space as room for growth?

• OreoCookie - Wednesday, December 11, 2013 - link

  Here's my question: Implementations of previous ARM cores by licensees, most notably the A15, feature much higher clocks than what ARM recommends. How has that influenced the design of the A53? Do you expect ARM's clock frequency design targets to be closer to the clocks in actual implementations?

• Peter Greenhalgh - Thursday, December 12, 2013 - link

  Hi OreoCookie,
  
  ARM processor pipelines allow the processor to be built to achieve certain frequencies, but we don't recommend or advise what they should be. After all, there are still ARM1136 processors being implemented today on 40nm, yet we designed the processor on 180nm!
  
  We and our partners like the freedom to chose whether to push the frequency as far as it will go or to back off a bit and save a bit of area/power. This freedom allows differentiation, optimisation around the rest of the platform and time-to-market (higher frequency = more effort = more time).
  
  Naturally our pipelines have a range of sweet-spot frequencies on a given process node and there is a lot of discussion with lead partners about a new micro-architecture, but we aren't changing the pipelines based on the frequencies we're seeing in current mobile implementations.

• msm595 - Wednesday, December 11, 2013 - link

  As someone starting their Computer Engineering degree and really interested in computer architecture, how can I give myself a head start?

• Peter Greenhalgh - Thursday, December 12, 2013 - link

  Hi Msm,
  
  Most good EE/CE degrees will have a reasonable amount of micro-architecture/architecture courses, but it doesn't hurt to understand what makes all the popular micro-architectures tick. For that matter, a lot of the designs in the 90's were impressive too - check out the Dec Alpha EV8 which never got to market, but was a really interesting processor.

• tabascosauz - Thursday, December 12, 2013 - link

  Hi all (and hopefully this gets a response from our guest Mr. Greenhalgh),
  
  I'm not exactly too well informed in the CPU department, so I won't pretend that I am. I'm just curious as to how A53 will fare against the likes of Krait 450 and Cyclone in terms of DMIPS (as obsolete as some people may think it is, i'd just like to get a sense of it performance-wise) and pipeline-depth.
  
  We're all assuming that Apple has gone ahead and used a ARMv8a instruction set and, as per their own usual routine, swelled up the cores to many times that of their competitors and marketed it as a custom architecture. Since A53 is also based off ARMv8, I'm wondering how this will translate into speed. I think someone's mentioned before that A53 is the logical successor to Cortex-A7, but my mind is telling me that there's more to the number in the name than just a random number that is a few integers below 57.
  
  If this is essentially a quad-core part and succeeds the A7, then are we looking at placement in the Snapdragon 400 segment of the market? It would certainly satisfy the conditions of "mid-to-high end" but I'm a little disappointed in Cortex-A at the moment considering that the A7 was introduced as a sort of energy-efficient, slightly lower performing A9. I mean, the A12 is seen as the A9's successor but it's still ARMv7a and it won't be hitting the market anytime soon, so would it be possible that we could see A53, with its ARMv8 set, on par with the Cortex-A12 in terms of rough performance estimates?
  
  Can't wait until A57; it's bound to be a great performer!

• Peter Greenhalgh - Thursday, December 12, 2013 - link

  Hi Tabascosauz,
  
  Speaking broadly about Dhrystone, the pipeline length is not relevant to the benchmark as perfect branch prediction is possible which means issue width to multiple execution units and fetch bandwidth largely dictates the performance. This is the reason Dhrystone isn't great as a benchmark as it puts no pressure on the data or instruction side memory systems (beyond the L1 cache interfaces), TLBs and little pressure on the branch predictors.
  
  Cortex-A12 is a decent performance uplift from Cortex-A53 in performance so we're not worried about overlap and while the Smartphone market is moving in the direction of 64-bit, there are still a lot of sockets for Cortex-A12. In addition there are many other markets where Cortex-A9 has been successful (Set Top Box, Digital TV, etc) where 64-bit isn't a near-term requirement and Cortex-A12 will be a great follow-on.

• ThanosPAS - Thursday, December 12, 2013 - link

  Question: What is the competitive advantage of ARM powered devices over other manufacturers' products and what your company will do in the future to preserve and enhance it?

• hlovatt - Thursday, December 12, 2013 - link

  Can you explain what you mean by a 'weak' memory model and how this differs from other architectures and how it translates into memory models in common languages like Java?

• Peter Greenhalgh - Sunday, December 15, 2013 - link

  Hi hlovatt,
  
  A weakly ordered memory model essentially allows reads (loads) and writes (stores) to overtake each other and observed by other CPUs/GPUs/etc in the system at different times or different order.
  
  A weakly ordered memory model allows for the highest performance system to be built, but requires the program writer to enforce order where necessary through barriers (sometimes termed fences). There are many types of barrier in the ARM architecture from instruction only (ISB) to full-system barriers (DSB) and memory barriers (DMB) with various variants that, for example, only enforce ordering on writes rather than reads.
  
  The Alpha architecture is the most weakly ordered of all the processor architectures I'm aware of, though ARM runs it close. x86 is an example of a strongly ordered memory model.
  
  Recent programming standards such as C++11 assume weakly ordered and may need ordering directives even on strongly ordered processors to prevent the compiler from optimising the order.