Python vs. Go performance ... reading a set of values from a file

cache-with-dict.py

import time

def readFile(filename):
    res = {}
    for line in open(filename).read().split("\n"):
        if line:
            res[line] = True
    return res

if __name__ == '__main__':
    start_time = time.time()
    res = readFile("hash.db")
    duration = time.time() - start_time
    print "reading %d entries in %f seconds" % (len(res), duration)

cache.go

package main

import (
    "fmt"
    "os"
    "time"
    "bytes"
)


type set map[string]bool

func readFile(filename string) set {
    file, err := os.Open(filename)
    if err != nil {
        panic(err)
    }
    stat, err := file.Stat()
    if err != nil {
        panic(err)
    }
    buffer := make([]byte, stat.Size())
    _,err = file.Read(buffer)
    if err != nil {
        panic(err)
    }

    res := make(set)

    for _,value  := range bytes.Split(buffer, []byte("\n")) {
        s := string(value)
        if s != "" {
            res[s] = true
        }
    }
    return res
}

func main() {
    start_time := time.Now()
    res := readFile("hash.db")
    duration := time.Now().Sub(start_time)
    fmt.Printf("read %d entries in %s\n", len(res), duration)
}

cache.py

import time

def readFile(filename):
    res = set()
    for line in open(filename).read().split("\n"):
        if line:
            res.add(line)
    return res

if __name__ == '__main__':
    start_time = time.time()
    res = readFile("hash.db")
    duration = time.time() - start_time
    print "reading %d entries in %f seconds" % (len(res), duration)

create-file.py

import hashlib

def makeSet(size):
    res = set()
    for i in range(size):
        value = hashlib.sha1(str(i)).hexdigest()
        res.add(value)
    return res

def makeFile(filename, size):
    data = makeSet(size)
    open(filename,"w").write("\n".join(data) + '\n')
    

if __name__ == '__main__':
    makeFile("hash.db", 1000 * 1000)

GO output: read 1000000 entries in 790.16865ms
Python output: reading 1000000 entries in 0.470706 seconds
Python output with dict: reading 1000000 entries in 0.554953 seconds

The Python code is twice as fast as the Go code. After some debugging, it seems that the performance problem in the Go code is the

map[string]bool

data structure. Is there a better way to implement this in Go?

Btw: please note that the Go program spends most of the time in

res[s] = true

Changing the file read routine to a buffered io and ReadLine does not improve the performance

A couple things you can try (not sure how much they will help):

• Use a map[string]struct{}. For a large set, this should reduce the overall size noticeably.
• If you can test on the current go1.3, get rid of s and use map[string(value)], which is now free (no allocation).

Also noting that most of the time is in the map assignment may be a red herring, unless you compare that directly to the amount of time that python spends in set assignment.

Ok ... the hint on the mailing list was give a size hint to make: So the cache-improved.go is doing this and is now on the same performance level as the Python implementation.

Go improved output: read 1000000 entries in 468.880726ms

package main

import (
    "fmt"
    "os"
    "time"
    "bytes"
)


type set map[string]struct{}

func readFile(filename string) set {
    file, err := os.Open(filename)
    if err != nil {
        panic(err)
    }
    stat, err := file.Stat()
    if err != nil {
        panic(err)
    }
    buffer := make([]byte, stat.Size())
    _,err = file.Read(buffer)
    if err != nil {
        panic(err)
    }

    splitBuffer := bytes.Split(buffer, []byte("\n"))
    res := make(set, len(splitBuffer))

    for _,value  := range splitBuffer {
        s := string(value)
        if s != "" {
            res[s] = struct{}{}
        }
    }
    return res
}

func main() {
    start_time := time.Now()
    res := readFile("hash.db")
    duration := time.Now().Sub(start_time)
    fmt.Printf("read %d entries in %s\n", len(res), duration)
}