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Update performance.md

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@ -164,11 +164,16 @@ Under Windows, we also support the GNU GCC compiler via MSYS2. The performance i
Downclocking
--------------
You should not expect the simdjson library to cause downclocking of your recent Intel CPU cores.
The SIMD instructions that simdjson relies upon (SSE and AVX under x64, NEON under ARM) [are routinely part of runtime libraries](https://golang.org/src/runtime/memmove_amd64.s). What distinguishes the simdjson library is that it is built from the ground up to benefit from these instructions.
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. We do use vectorized multiplications, but only using 128-bit registers. 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. We do use vectorized multiplications, but only using 128-bit registers. 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) by simdjson. Thus even if your CPU supports AVX2, you do not need to use AVX2. You are in control.