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Tweaking.

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Daniel Lemire 2020-06-17 13:27:17 -04:00
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doc/performance.md
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@ -160,9 +160,11 @@ Recent versions of Microsoft Visual Studio on Windows provides support for the L
Downclocking
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You should not expect the simdjson library to cause downclocking of your recent Intel CPU cores.
On some Intel processors, using SIMD instructions in a sustained manner on the same CPU core may result in a phenomenon called downclocking whereas the processor initially runs these instructions at a slow speed before reducing the frequency of the core for a short time (milliseconds). Intel refers to these states as licenses. On some current Intel processors, it occurs under two scenarios:
- [Whenever 512-bit AVX-512 instructions are used](https://lemire.me/blog/2018/09/07/avx-512-when-and-how-to-use-these-new-instructions/).
- Whenever heavy 256-bit or wider instructions are used. Heavy instructions are those involving floating point operations or integer multiplications (since these execute on the floating point unit).
The simdjson library does not currently support AVX-512 instructions and it does not make use of heavy 256-bit instructions. Thus there should be no downclocking due to simdjson on recent processors. You may still be worried about which SIMD instruction set is used by simdjson. Thankfully, [you can always determine and change which architecture-specific implementation is used](implementation-selection.md). Thus even if your CPU supports AVX2, you do not need to use AVX2. You are in control.
The simdjson library does not currently support AVX-512 instructions and it does not make use of heavy 256-bit instructions. Thus there should be no downclocking due to simdjson on recent processors. You may still be worried about which SIMD instruction set is used by simdjson. Thankfully, [you can always determine and change which architecture-specific implementation is used](implementation-selection.md). Thus even if your CPU supports AVX2, you do not need to use AVX2. You are in control.