Kir Kolyshkin 8eec9259e6 mount/mountinfo_linux: parser speed up ...

The mountinfo parser implemented via `fmt.Sscanf()` is slower than the one
using `strings.Split()` and `strconv.Atoi()`. This rewrite helps to speed it
up to a factor of 8x, here is a result from `go bench`:

> BenchmarkParsingScanf-4            300          22294112 ns/op
> BenchmarkParsingSplit-4           3000           2780703 ns/op

I tried other approaches, such as using `fmt.Sscanf()` for the first
three (integer) fields and `strings.Split()` for the rest, but it slows
things down considerably:

> BenchmarkParsingMixed-4           1000           8827058 ns/op

Note the old code uses `fmt.Sscanf` first, then a linear search for the
'-' field, then a split for the last 3 fields. The new code relies
on a single split.

One other thing is, the new code is more future proof as it skips
extra optional fields before the separator (currently there are none).

I have also added more comments to aid in future development.

Finally, the test data is fixed to not have white space before
the first field.

Based on a similar change in Moby,
 https://github.com/moby/moby/pull/36091

[v2: remove no-op break statement to silence staticcheck]
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

2018-05-07 11:38:37 -07:00

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