4 ms·

The workloads don't support that many threads. 8-core A53 chips exist, but they don't perform anywhere near what a 2-core A57 does for typical mobile workloads.

DannyBee 12y ago
Yes, it doesn't make a whole heck of a lot of sense to add cores when they aren't cpu bound at all. The idea that "the slow cores can just handle the background stuff" makes little to no sense in practice. The most power effective thing to do is not add more cores (and all that entails), but get done the work something wants done as fast as humanly possible so you can go back to sleep. Having slow cores handle this for you sounds great in theory. In practice, it's been a huge bust (The one exception being dedicated-function coprocessors, etc) This is one reason that, for example, higher speed wifi/etc chipsets often become less power intensive than the old chipsets (past the initial spike from it being new hardware) - they can sleep more
- jacquesm 12y ago
  For low end servers it would make sense though.
- CHY872 12y ago
  Do you have any numbers re "little to no sense in practice"? What I've heard suggests that the tradeoffs behind high performance ARM lead to significantly worse power efficiency to the benefit of peak performance; which means that a core like the A53 ends being a very attractive proposition for power saving. Certainly I don't know why ARM would recommend their big.LITTLE architecture (or at least why people would use it) unless it did. Furthermore, Amdahl's law means that even in theory it should be advantageous for performance to include a few extra low power cores to boost peak performance (above adding a larger high power core).
  - DannyBee 12y ago
    "Certainly I don't know why ARM would recommend their big.LITTLE architecture (or at least why people would use it) unless it did. " Because they want to compete with others who keep upping the core numbers. "Furthermore, Amdahl's law means that even in theory it should be advantageous for performance to include a few extra low power cores to boost peak performance (above adding a larger high power core)." Again, in practice, exactly zero of the chips that have done this and been put in phones (nvidia's, etc) have actually saved power by doing so. Or did things get miraculously better when nobody was looking?
- pcwalton 12y ago
  > The most power effective thing to do is not add more cores (and all that entails), but get done the work something wants done as fast as humanly possible so you can go back to sleep. And what if you can actually scale your CPU-bound tasks linearly? Then you can use the extra cores to get the work done faster so you can go back to sleep faster. I'm not speculating here: I've seen power measurements demonstrating this in real world applications on mobile (in my case, the browser engine). If you can use all your cores to get the work done faster, you can decrease overall power consumption. You're assuming we aren't going to parallelize our applications. I don't make that assumption.
  - DannyBee 12y ago
    "You're assuming we aren't going to parallelize our applications. I don't make that assumption. " I'm assuming parallelizing doesn't help enough :) Which it hasn't so far. So let me rephrase "so far, it makes no sense". and it's not like they haven't existed for quite a while, so it's not "well, it's a chicken and egg problem".
    - pcwalton 12y ago
      > I'm assuming parallelizing doesn't help enough :) Which it hasn't so far. So let me rephrase "so far, it makes no sense". Not according to my numbers. And yes, we have measured the impact of parallel rendering specifically on power consumption on mobile. On real-world Web sites, not random microbenchmarks.

DannyBee 12y ago

Yes, it doesn't make a whole heck of a lot of sense to add cores when they aren't cpu bound at all. The idea that "the slow cores can just handle the background stuff" makes little to no sense in practice. The most power effective thing to do is not add more cores (and all that entails), but get done the work something wants done as fast as humanly possible so you can go back to sleep. Having slow cores handle this for you sounds great in theory. In practice, it's been a huge bust (The one exception being dedicated-function coprocessors, etc) This is one reason that, for example, higher speed wifi/etc chipsets often become less power intensive than the old chipsets (past the initial spike from it being new hardware) - they can sleep more

jacquesm 12y ago
For low end servers it would make sense though.
CHY872 12y ago
Do you have any numbers re "little to no sense in practice"? What I've heard suggests that the tradeoffs behind high performance ARM lead to significantly worse power efficiency to the benefit of peak performance; which means that a core like the A53 ends being a very attractive proposition for power saving. Certainly I don't know why ARM would recommend their big.LITTLE architecture (or at least why people would use it) unless it did. Furthermore, Amdahl's law means that even in theory it should be advantageous for performance to include a few extra low power cores to boost peak performance (above adding a larger high power core).
- DannyBee 12y ago
  "Certainly I don't know why ARM would recommend their big.LITTLE architecture (or at least why people would use it) unless it did. " Because they want to compete with others who keep upping the core numbers. "Furthermore, Amdahl's law means that even in theory it should be advantageous for performance to include a few extra low power cores to boost peak performance (above adding a larger high power core)." Again, in practice, exactly zero of the chips that have done this and been put in phones (nvidia's, etc) have actually saved power by doing so. Or did things get miraculously better when nobody was looking?
pcwalton 12y ago
> The most power effective thing to do is not add more cores (and all that entails), but get done the work something wants done as fast as humanly possible so you can go back to sleep. And what if you can actually scale your CPU-bound tasks linearly? Then you can use the extra cores to get the work done faster so you can go back to sleep faster. I'm not speculating here: I've seen power measurements demonstrating this in real world applications on mobile (in my case, the browser engine). If you can use all your cores to get the work done faster, you can decrease overall power consumption. You're assuming we aren't going to parallelize our applications. I don't make that assumption.
- DannyBee 12y ago
  "You're assuming we aren't going to parallelize our applications. I don't make that assumption. " I'm assuming parallelizing doesn't help enough :) Which it hasn't so far. So let me rephrase "so far, it makes no sense". and it's not like they haven't existed for quite a while, so it's not "well, it's a chicken and egg problem".
  - pcwalton 12y ago
    > I'm assuming parallelizing doesn't help enough :) Which it hasn't so far. So let me rephrase "so far, it makes no sense". Not according to my numbers. And yes, we have measured the impact of parallel rendering specifically on power consumption on mobile. On real-world Web sites, not random microbenchmarks.