crypto/rand: Read performance improvements

[CAFxX]

Wrapping crypto/rand.Reader in a bufio.Reader amortizes the syscall overhead significantly, especially for smaller reads (number of bytes per read after the /). This is obviously a quick and dirty test - I don't think it's a good idea to bluntly wrap it like this, but it's just to show that there are potential gains to be had by buffering reads from getrandom:

name                 old time/op  new time/op  delta
CryptoRandRead/1-4    921ns ± 0%    23ns ± 0%  -97.51%  (p=0.008 n=5+5)
CryptoRandRead/4-4    922ns ± 0%    40ns ± 0%  -95.64%  (p=0.008 n=5+5)
CryptoRandRead/8-4    922ns ± 0%    64ns ± 0%  -93.05%  (p=0.008 n=5+5)
CryptoRandRead/16-4   923ns ± 0%   111ns ± 0%  -87.97%  (p=0.008 n=5+5)
CryptoRandRead/32-4   923ns ± 0%   205ns ± 0%  -77.76%  (p=0.008 n=5+5)
CryptoRandRead/64-4  1.27µs ± 0%  0.39µs ± 0%  -68.95%  (p=0.016 n=5+4)

While on topic, it may be worthwhile to point out that short reads from getrandom don't need to be discarded, as they can be still used:

go/src/crypto/rand/rand_batched.go

Lines 39 to 42 in 9195948

	func getRandomBatch(p []byte) (ok bool) {
	n, err := unix.GetRandom(p, 0)
	return n == len(p) && err == nil
	}

Environment:

cafxx@instance-1:~$ uname -a
Linux instance-1 5.0.0-1010-gcp #10-Ubuntu SMP Tue Jun 25 10:29:28 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
cafxx@instance-1:~$ go version
go version go1.12.7 linux/amd64

Benchmark

package main

import (
    "crypto/rand"
    "io"
    "testing"
    "strconv"
    "bufio"
)

var sz = []int{1, 4, 8, 16, 32, 64}
var cr, bcr io.Reader

func init() {
    cr = rand.Reader
    bcr = bufio.NewReader(rand.Reader)
}

func BenchmarkCryptoRandRead(b *testing.B) {
    rand.Reader = cr
    b.Run("raw", benchmark)
    rand.Reader = bcr
    b.Run("buffered", benchmark)
}

func benchmark(b *testing.B) {
    defer func() {
        rand.Reader = cr
    }()
    for _, s := range sz {
        buf := make([]byte, s)
        b.Run(strconv.Itoa(s), func (b *testing.B) {
            for i := 0; i < b.N; i++ {
                if n, err := rand.Read(buf); n != s || err != nil {
                    b.Fatal("error in rand.Read()")
                }
            }
        })
        _ = buf
    }
}

[takeyourhatoff]

See: #16593

[julieqiu]

Is this a dupe of #16593?

[CAFxX]

Yup will add a comment there.