3 ms·

Most benchmarks show the 3900X with single thread performance competitive with Intel's offerings, including for software whose developers spend plenty of time o

by zrm 6y ago

Most benchmarks show the 3900X with single thread performance competitive with Intel's offerings, including for software whose developers spend plenty of time optimizing it. Can you think of a reason they haven't realized this performance advantage in practice?

jdsully 6y ago
Zen is great if you have moderate utilization code around the ~2-3 instructions per cycle mark. This is common for most compiled C and C++ where it hasn’t been specifically tuned for ILP. In these scenarios even if Intel has an extra ALU it won’t be used anyways. It took 3 days of tuning to get to a point where Intel started pulling away from my Zen2 chip. No matter what I did the Zen2 would not get faster because it was already at max utilization. Lemire had a blog [1] talking about this on here a few months ago as well. Most people were surprised because frankly Zen2 is very competitive with Intel for normal compiled code. https://lemire.me/blog/2019/12/05/instructions-per-cycle-amd-versus-intel/ https://lemire.me/blog/2019/12/05/instructions-per-cycle-amd...
- zrm 6y ago
  Are you measuring actual performance though or just IPC? We know Intel has that problem with AVX-512. You can get a lot of throughput per cycle with those instructions but the cost is they cause the processor to run hot and have to downclock. It's possible (and really expected) that the same thing happens to some extent at unusually high IPC. Getting 15% higher IPC doesn't really buy you anything if the processor has to lower its clock speed by 15% to execute that type of code.
  - jdsully 6y ago
    Actual wallclock time. The code is 100% scalar - LZF is very byte oriented and I haven't found a good way to vectorize it. IPC counts were used to help guide tuning but not a determination of success. I don't really understand why people think this is controversial. Somehow an Intel cpu on a generation behind process is still competitive with a 7nm CPU with more cache. Intel must have one hell of an architecture to make that possible. AMD did a good job for the most common workloads but they have work to do with Zen3 if they want to maintain that lead when Intel finally figures out its manufacturing.
    - zrm 6y ago
      Have you looked at what 7zip is doing? That should be a similar type of code and it runs very well on Zen2.
      - jdsully 6y ago
        7zip doesn't use LZF. LZF is mandated by the file format I'm working with.

by zrm 6y ago

jdsully 6y ago

Zen is great if you have moderate utilization code around the ~2-3 instructions per cycle mark. This is common for most compiled C and C++ where it hasn’t been specifically tuned for ILP. In these scenarios even if Intel has an extra ALU it won’t be used anyways. It took 3 days of tuning to get to a point where Intel started pulling away from my Zen2 chip. No matter what I did the Zen2 would not get faster because it was already at max utilization. Lemire had a blog [1] talking about this on here a few months ago as well. Most people were surprised because frankly Zen2 is very competitive with Intel for normal compiled code. https://lemire.me/blog/2019/12/05/instructions-per-cycle-amd-versus-intel/ https://lemire.me/blog/2019/12/05/instructions-per-cycle-amd...

zrm 6y ago
Are you measuring actual performance though or just IPC? We know Intel has that problem with AVX-512. You can get a lot of throughput per cycle with those instructions but the cost is they cause the processor to run hot and have to downclock. It's possible (and really expected) that the same thing happens to some extent at unusually high IPC. Getting 15% higher IPC doesn't really buy you anything if the processor has to lower its clock speed by 15% to execute that type of code.
- jdsully 6y ago
  Actual wallclock time. The code is 100% scalar - LZF is very byte oriented and I haven't found a good way to vectorize it. IPC counts were used to help guide tuning but not a determination of success. I don't really understand why people think this is controversial. Somehow an Intel cpu on a generation behind process is still competitive with a 7nm CPU with more cache. Intel must have one hell of an architecture to make that possible. AMD did a good job for the most common workloads but they have work to do with Zen3 if they want to maintain that lead when Intel finally figures out its manufacturing.
  - zrm 6y ago
    Have you looked at what 7zip is doing? That should be a similar type of code and it runs very well on Zen2.
    - jdsully 6y ago
      7zip doesn't use LZF. LZF is mandated by the file format I'm working with.