4 ms·

For well-optimised floating point calculations, this is generally true (even taking into account the SMT capabilities of Power8). Intel also has 8-wide (and mor

by berkut 9y ago

For well-optimised floating point calculations, this is generally true (even taking into account the SMT capabilities of Power8). Intel also has 8-wide (and more) SIMD in terms of intrinsics (AVX/AVX512), whilst Power is still limited to 4-wide with AltiVec, although with Power9 VSX should allow wider (I think). Where Power8/9 really shines is memory bandwidth - it's orders of magnitude faster than leading edge Intel stuff currently. But if you're not memory constrained, Intel machines can still have a bit of an edge.

kev009 9y ago
AVX512 will severely clock throttle an intel chip. I only want to see real world tests on a workload when comparing the two.
- berkut 9y ago
  Yes, but even when throttled, it can often be faster than full-clock less wide instructions, if your workload allows for it. So it's sometimes worth it and there is a benefit.
  - flamedoge 9y ago
    It's power throttled by design. I don't think it's heat throttled. I'm sure Intel could push for higher power or optimize power to allow full bandwidth performance, perhaps there isn't a strong demand for sustained, full-width vector execution, yet.
    - nordsieck 9y ago
      > I don't think it's heat throttled. "Processors have two modes of thermal protection, throttling and automatic shutdown. When a core exceeds the set throttle temperature, it will start to reduce power to bring the temperature back below that point." https://www.intel.com/content/www/us/en/support/articles/000005597/processors.html https://www.intel.com/content/www/us/en/support/articles/000...
      - flamedoge 9y ago
        I mean in the context of fully loading AVX512 vector processor.
dragontamer 9y ago
Power systems also have an absurdly sized L3 cache. Case in point: Power9 is advertised to have 120MB of L3 cache. I'm sure there are a lot of problems that fit inside of 120MB.
classichasclass 9y ago
Near as I can determine, VSX in POWER9 is still 128-bit.
- berkut 9y ago
  I'd heard rumours... Nothing concrete though, so I could be wrong.
philipkglass 9y ago
Where Power8/9 really shines is memory bandwidth - it's orders of magnitude faster than leading edge Intel stuff currently. But if you're not memory constrained, Intel machines can still have a bit of an edge. That makes Power sound very appealing. I've never met a real life scientific simulation that wasn't often constrained by memory bandwidth. I'm sure they exist, but they aren't all that plentiful. I'd love to try running the code that matters to me on Power. (For one thing, I don't even know if it would pass basic regression tests; numerical code is touchy. Are gcc/gfortran reliably adequate on Power like they are on x64?) Unfortunately, as everyone else is commenting, entry level x64 is dirt cheap and entry level Power isn't.
- classichasclass 9y ago
  gcc is perfectly cromulent on Power (speaking from experience with my own PowerPC and POWER6 systems, and when I had time on one of Raptor's POWER8 systems). The speed differential between that and IBM xlc is pretty small IME nowadays, too. I'm more hesitant to say the same about clang/llvm but it's been awhile and I'm probably not current.
- gnufx 9y ago
  It's true that raw FP performance isn't the be-all and end-all, but things aren't necessarily memory-bound either. HPC codes at scale are often dominated by communication. I'm surprised if numerical issues are that much of a worry. x86 has now caught up in doing FMA, in particular. I don't know what the build systems actually are, but most of Fedora (and Debian?) is available for POWER, and will typically have at least minimal tests in the package. It's not as if POWER is new for HPC; the Daresbury HPC-X machine was quite high in the top500 15 years or so ago, as an example I was physically close to.
gnufx 9y ago
"Orders of magnitude" is implausible, and certainly not supported by published STREAM results for POWER8, even excluding HBM in Knights Landing, for instance. I'd love to see some actual performance figures for POWER9, though, or even facts about its SIMD, in particular.

by berkut 9y ago

kev009 9y ago

AVX512 will severely clock throttle an intel chip. I only want to see real world tests on a workload when comparing the two.

berkut 9y ago
Yes, but even when throttled, it can often be faster than full-clock less wide instructions, if your workload allows for it. So it's sometimes worth it and there is a benefit.
- flamedoge 9y ago
  It's power throttled by design. I don't think it's heat throttled. I'm sure Intel could push for higher power or optimize power to allow full bandwidth performance, perhaps there isn't a strong demand for sustained, full-width vector execution, yet.
  - nordsieck 9y ago
    > I don't think it's heat throttled. "Processors have two modes of thermal protection, throttling and automatic shutdown. When a core exceeds the set throttle temperature, it will start to reduce power to bring the temperature back below that point." https://www.intel.com/content/www/us/en/support/articles/000005597/processors.html https://www.intel.com/content/www/us/en/support/articles/000...
    - flamedoge 9y ago
      I mean in the context of fully loading AVX512 vector processor.

dragontamer 9y ago

Power systems also have an absurdly sized L3 cache. Case in point: Power9 is advertised to have 120MB of L3 cache. I'm sure there are a lot of problems that fit inside of 120MB.

classichasclass 9y ago

Near as I can determine, VSX in POWER9 is still 128-bit.

berkut 9y ago
I'd heard rumours... Nothing concrete though, so I could be wrong.

philipkglass 9y ago

Where Power8/9 really shines is memory bandwidth - it's orders of magnitude faster than leading edge Intel stuff currently. But if you're not memory constrained, Intel machines can still have a bit of an edge. That makes Power sound very appealing. I've never met a real life scientific simulation that wasn't often constrained by memory bandwidth. I'm sure they exist, but they aren't all that plentiful. I'd love to try running the code that matters to me on Power. (For one thing, I don't even know if it would pass basic regression tests; numerical code is touchy. Are gcc/gfortran reliably adequate on Power like they are on x64?) Unfortunately, as everyone else is commenting, entry level x64 is dirt cheap and entry level Power isn't.

classichasclass 9y ago
gcc is perfectly cromulent on Power (speaking from experience with my own PowerPC and POWER6 systems, and when I had time on one of Raptor's POWER8 systems). The speed differential between that and IBM xlc is pretty small IME nowadays, too. I'm more hesitant to say the same about clang/llvm but it's been awhile and I'm probably not current.
gnufx 9y ago
It's true that raw FP performance isn't the be-all and end-all, but things aren't necessarily memory-bound either. HPC codes at scale are often dominated by communication. I'm surprised if numerical issues are that much of a worry. x86 has now caught up in doing FMA, in particular. I don't know what the build systems actually are, but most of Fedora (and Debian?) is available for POWER, and will typically have at least minimal tests in the package. It's not as if POWER is new for HPC; the Daresbury HPC-X machine was quite high in the top500 15 years or so ago, as an example I was physically close to.

gnufx 9y ago

"Orders of magnitude" is implausible, and certainly not supported by published STREAM results for POWER8, even excluding HBM in Knights Landing, for instance. I'd love to see some actual performance figures for POWER9, though, or even facts about its SIMD, in particular.