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>
This commit is contained in:
Kir Kolyshkin
parent
d8aef117f9
commit
8eec9259e6
@ -23,27 +23,10 @@ import (
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"fmt"
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"io"
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"os"
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"strconv"
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"strings"
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)
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const (
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/* 36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue
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(1)(2)(3) (4) (5) (6) (7) (8) (9) (10) (11)
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(1) mount ID: unique identifier of the mount (may be reused after umount)
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(2) parent ID: ID of parent (or of self for the top of the mount tree)
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(3) major:minor: value of st_dev for files on filesystem
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(4) root: root of the mount within the filesystem
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(5) mount point: mount point relative to the process's root
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(6) mount options: per mount options
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(7) optional fields: zero or more fields of the form "tag[:value]"
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(8) separator: marks the end of the optional fields
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(9) filesystem type: name of filesystem of the form "type[.subtype]"
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(10) mount source: filesystem specific information or "none"
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(11) super options: per super block options*/
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mountinfoFormat = "%d %d %d:%d %s %s %s %s"
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)
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// Self retrieves a list of mounts for the current running process.
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func Self() ([]Info, error) {
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f, err := os.Open("/proc/self/mountinfo")
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@ -56,41 +39,83 @@ func Self() ([]Info, error) {
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}
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func parseInfoFile(r io.Reader) ([]Info, error) {
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var (
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s = bufio.NewScanner(r)
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out = []Info{}
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)
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s := bufio.NewScanner(r)
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out := []Info{}
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for s.Scan() {
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if err := s.Err(); err != nil {
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return nil, err
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}
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var (
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p = Info{}
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text = s.Text()
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optionalFields string
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)
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/*
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36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue
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(1)(2)(3) (4) (5) (6) (7) (8) (9) (10) (11)
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(1) mount ID: unique identifier of the mount (may be reused after umount)
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(2) parent ID: ID of parent (or of self for the top of the mount tree)
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(3) major:minor: value of st_dev for files on filesystem
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(4) root: root of the mount within the filesystem
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(5) mount point: mount point relative to the process's root
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(6) mount options: per mount options
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(7) optional fields: zero or more fields of the form "tag[:value]"
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(8) separator: marks the end of the optional fields
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(9) filesystem type: name of filesystem of the form "type[.subtype]"
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(10) mount source: filesystem specific information or "none"
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(11) super options: per super block options
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*/
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if _, err := fmt.Sscanf(text, mountinfoFormat,
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&p.ID, &p.Parent, &p.Major, &p.Minor,
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&p.Root, &p.Mountpoint, &p.Options, &optionalFields); err != nil {
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return nil, fmt.Errorf("Scanning '%s' failed: %s", text, err)
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text := s.Text()
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fields := strings.Split(text, " ")
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numFields := len(fields)
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if numFields < 10 {
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// should be at least 10 fields
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return nil, fmt.Errorf("Parsing '%s' failed: not enough fields (%d)", text, numFields)
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}
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// Safe as mountinfo encodes mountpoints with spaces as \040.
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index := strings.Index(text, " - ")
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postSeparatorFields := strings.Fields(text[index+3:])
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if len(postSeparatorFields) < 3 {
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return nil, fmt.Errorf("Error found less than 3 fields post '-' in %q", text)
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p := Info{}
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// ignore any numbers parsing errors, as there should not be any
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p.ID, _ = strconv.Atoi(fields[0])
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p.Parent, _ = strconv.Atoi(fields[1])
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mm := strings.Split(fields[2], ":")
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if len(mm) != 2 {
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return nil, fmt.Errorf("Parsing '%s' failed: unexpected minor:major pair %s", text, mm)
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}
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p.Major, _ = strconv.Atoi(mm[0])
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p.Minor, _ = strconv.Atoi(mm[1])
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if optionalFields != "-" {
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p.Optional = optionalFields
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}
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p.Root = fields[3]
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p.Mountpoint = fields[4]
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p.Options = fields[5]
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// one or more optional fields, when a separator (-)
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i := 6
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for ; i < numFields && fields[i] != "-"; i++ {
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switch i {
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case 6:
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p.Optional = fields[6]
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default:
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/* NOTE there might be more optional fields before the separator
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such as fields[7]...fields[N] (where N < separatorIndex),
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although as of Linux kernel 4.15 the only known ones are
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mount propagation flags in fields[6]. The correct
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behavior is to ignore any unknown optional fields.
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*/
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}
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}
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if i == numFields {
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return nil, fmt.Errorf("Parsing '%s' failed: missing separator ('-')", text)
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}
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// There should be 3 fields after the separator...
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if i+4 > numFields {
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return nil, fmt.Errorf("Parsing '%s' failed: not enough fields after a separator", text)
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}
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// ... but in Linux <= 3.9 mounting a cifs with spaces in a share name
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// (like "//serv/My Documents") _may_ end up having a space in the last field
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// of mountinfo (like "unc=//serv/My Documents"). Since kernel 3.10-rc1, cifs
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// option unc= is ignored, so a space should not appear. In here we ignore
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// those "extra" fields caused by extra spaces.
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p.FSType = fields[i+1]
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p.Source = fields[i+2]
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p.VFSOptions = fields[i+3]
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p.FSType = postSeparatorFields[0]
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p.Source = postSeparatorFields[1]
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p.VFSOptions = strings.Join(postSeparatorFields[2:], " ")
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out = append(out, p)
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}
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return out, nil
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