119 lines
3.1 KiB
Go
119 lines
3.1 KiB
Go
package main
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import (
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"fmt"
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"runtime"
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"runtime/debug"
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)
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// ---------------------------------------------------------
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// EXERCISE: Observe the memory allocations
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//
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// In this exercise, your goal is to observe the memory allocation
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// differences between arrays and slices.
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//
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// You will create, assign arrays and slices then you will print
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// the memory usage of your program on each step.
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//
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// Please follow the instructions inside the code.
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//
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//
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// EXPECTED OUTPUT
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//
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// Note that, your memory usage numbers may vary. These are on my
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// own system. However, the size of the arrays and slices should be
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// the same on your own system as well (if you're on 64-bit machine).
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//
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//
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// [initial memory usage]
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// > Memory Usage: 104 KB
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// [after declaring an array]
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// > Memory Usage: 78235 KB
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// [after copying the array]
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// > Memory Usage: 156365 KB
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// [inside passArray]
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// > Memory Usage: 234495 KB
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// [after slicings]
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// > Memory Usage: 234497 KB
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// [inside passSlice]
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// > Memory Usage: 234497 KB
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//
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// Array's size : 80000000 bytes.
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// Array2's size: 80000000 bytes.
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// Slice1's size: 24 bytes.
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// Slice2's size: 24 bytes.
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// Slice3's size: 24 bytes.
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//
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//
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// HINTS
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//
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// I've declared a few functions to help you.
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//
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// report function:
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// - Prints the memory usage.
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// - Just call it with a message that matches to the expected output.
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//
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// passArray function:
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// - Accepts a [size]int array, so you can pass it your array.
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// - It automatically prints the memory usage.
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//
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// passSlice function:
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// - Accepts an int slice, so you can pass it one of your slices.
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// - It automatically prints the memory usage.
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//
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// ---------------------------------------------------------
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const size = 1e7
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func main() {
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// stops the gc: prevents cleaning up the memory
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debug.SetGCPercent(-1)
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// run the program to see what this prints
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report("initial memory usage")
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// 1. allocate an array with 10 million int elements
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// this array's size is equal to ~80MB
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// hint: use the `size` constant
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//
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// 2. print the memory usage
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// 3. copy the array to a new array (just assign)
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// 4. print the memory usage
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// 5. pass the array to the passArray function
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// 6. convert the one of the arrays to a slice (by slicing)
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// 7. slice only the first 1000 elements of the array
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// 8. slice only the elements of the array between 1000 and 10000
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// 9. print the memory usage
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// 10. pass the one of the slices to the passSlice function
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// 11. print the sizes of the arrays and slices
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// hint: use the unsafe.Sizeof function
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}
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// observe that passing an array affects the memory usage dramatically
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//
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// passes [size]int array — about 80MB!
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func passArray(items [size]int) {
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items[0] = 100
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report("inside passArray")
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}
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// observe that passing a slice doesn't affect the memory usage
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//
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// only passes 24-bytes of slice header
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func passSlice(items []int) {
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items[0] = 100
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report("inside passSlice")
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}
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func report(msg string) {
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var m runtime.MemStats
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runtime.ReadMemStats(&m)
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fmt.Printf("[%s]\n", msg)
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fmt.Printf("\t> Memory Usage: %v KB\n", m.Alloc/1024)
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}
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