I have the rather stupid benchmark to test the efficiency of memory allocation:
package main
import (
"time"
"fmt"
)
func main() {
size := 1024 * 1024 * 1024
t := time.Now()
for j := 0; j < 100; j += 1 {
a := make([]int, size)
for i := 0; i < size; i += 1 {
a[i] = i
}
a = nil
}
t1 := time.Now()
fmt.Printf("Duration: %1d", t1.Sub(t).Seconds())
}
It takes roughly 2-3 minutes on Mac Pro with 16GB of RAM, the memory usage is consistent at 5-8 GB for the process.
The very similar code in Java takes 3 GB of memory and completes in 30 seconds.
What am I missing here?
What am I missing here?
In Java, size of the int type is fixed 4 bytes. In Go int is an architecture dependent type, on 32-bit architectures it's 32 bits (4 bytes), and on 64-bit architectures it's 64 bits (8 bytes).
Most likely you're running it on a 64-bit arch. Which means the size of the Go slice / array you allocate is 8 * 1 GB = 8 GB, while in Java it's only 4 * 1 GB = 4 GB.
Moreover, since you're using int in your loop, Java only has to increment and set 4-byte values, while in Go you're incrementing and setting 8-byte values (type of i and j will be int).
Change your Go code to use int32, and give it a go again.
Also note that your memory usage measurement is flawed, as just the array size in Java is 4 GB and 8 GB in Go, so that 3 GB for Java and 5-8 GB in Go is not the total memory usage!
Also note that []int in Go is a slice and not an array, they are not the same. Slices in Go are struct-like headers containing a pointer to a backing array (see reflect.SliceHeader for details), so there is an implicit indirection step involved using them. For details see Why have arrays in Go? Also related: Array vs Slice: accessing speed
One last note: your code does not measure memory allocation, as that is just the tiny part of the app's execution time. The majority (like 99.99999%) of the execution time is to increment the loop variable a billion times and to fill the arrays with a billion elements.