btf: Optimize string table for globally increasing offsets #1210
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Nov 7, 2023
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After doing some observations while parsing VMLinux, it seems that the
lookup access pattern for strings is mostly globally increasing with
with some patches of seemingly unpridicable offsets.
This is a sample of the access pattern, numbers are the index into the
string table, not the offset:
```
1112
1113
1114
348
78
1115
1116
372
1117
1118
1119
```
This commit adds logic to track this globally increasing offset and
checks if the if the offset at the next expected index matches the
offset we are looking for. If it does, we can skip the binary search.
In VMLinux this fast path is taken about 50% of the time, resulting in
about a 7% speedup.
```
goos: linux
goarch: amd64
pkg: github.com/cilium/ebpf/btf
cpu: 11th Gen Intel(R) Core(TM) i7-11800H @ 2.30GHz
│ before.txt │ after.txt │
│ sec/op │ sec/op vs base │
ParseVmlinux-16 48.52m ± 1% 44.79m ± 1% -7.69% (n=100)
│ before.txt │ after.txt │
│ B/op │ B/op vs base │
ParseVmlinux-16 31.45Mi ± 0% 31.45Mi ± 0% ~ (p=0.127 n=100)
│ before.txt │ after.txt │
│ allocs/op │ allocs/op vs base │
ParseVmlinux-16 534.1k ± 0% 534.1k ± 0% ~ (p=0.130 n=100)
```
Signed-off-by: Dylan Reimerink <dylan.reimerink@isovalent.com>
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I get this on my machine: The laptop is plugged in and I ran |
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After doing some observations while parsing VMLinux, it seems that the lookup access pattern for strings is mostly globally increasing with with some patches of seemingly unpridicable offsets.
This is a sample of the access pattern, numbers are the index into the string table, not the offset:
This commit adds logic to track this globally increasing offset and checks if the if the offset at the next expected index matches the offset we are looking for. If it does, we can skip the binary search. In VMLinux this fast path is taken about 50% of the time, resulting in about a 7% speedup.