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Clarify why we disable SSE for our kernel

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Philipp Oppermann
2019-02-07 12:05:55 +01:00
parent 131be53b1a
commit 1f27ca5352
2 changed files with 2 additions and 2 deletions
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blog/content/second-edition/extra/disable-simd/index.md
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@@ -22,7 +22,7 @@ By using such SIMD standards, programs can often speed up significantly. Good co
[auto-vectorization]: https://en.wikipedia.org/wiki/Automatic_vectorization
However, the large SIMD registers lead to problems in OS kernels. The reason is that the kernel has to backup all registers that it uses in hardware interrupt, because they need to have their original values when the interrupted program continues. So if the kernel uses SIMD registers, it has to backup a lot more data, which noticably decreases performance. To avoid this performance loss, we want to disable the `sse` and `mmx` features (the `avx` feature is disabled by default).
However, the large SIMD registers lead to problems in OS kernels. The reason is that the kernel has to backup all registers that it uses to memory on each hardware interrupt, because they need to have their original values when the interrupted program continues. So if the kernel uses SIMD registers, it has to backup a lot more data (512–1600 bytes), which noticably decreases performance. To avoid this performance loss, we want to disable the `sse` and `mmx` features (the `avx` feature is disabled by default).
We can do that through the the `features` field in our target specification. To disable the `mmx` and `sse` features we add them prefixed with a minus:

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blog/content/second-edition/posts/02-minimal-rust-kernel/index.md
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@@ -164,7 +164,7 @@ We're writing a kernel, so we'll need to handle interrupts at some point. To do
The `features` field enables/disables target features. We disable the `mmx` and `sse` features by prefixing them with a minus and enable the `soft-float` feature by prefixing it with a plus.
The `mmx` and `sse` features determine support for [Single Instruction Multiple Data (SIMD)] instructions, which can often speed up programs significantly. However, the large SIMD registers lead to performance problems in OS kernels, because the kernel has to back them up on each hardware interrupt. To avoid this, we disable SIMD for our kernel (not for applications running on top!).
The `mmx` and `sse` features determine support for [Single Instruction Multiple Data (SIMD)] instructions, which can often speed up programs significantly. However, using the large SIMD registers in OS kernels leads to performance problems. The reason is that the kernel needs to restore all registers to their original state before continuing an interrupted program. This means that the kernel has to save the complete SIMD state to main memory on each system call or hardware interrupt. Since the SIMD state is very large (512–1600 bytes) and interrupts can occur very often, these additional save/restore operations considerably harm performance. To avoid this, we disable SIMD for our kernel (not for applications running on top!).
[Single Instruction Multiple Data (SIMD)]: https://en.wikipedia.org/wiki/SIMD