It looks making a slice with size 32768+1 will allocate with one memory page as needed

[tapi...@gmail.com]

package allocate

import "testing"
import "os"
import "fmt"

func init() {
    fmt.Println("OS page size:", os.Getpagesize())
}

var r1 []byte

func BenchmarkCount1(b *testing.B) {
    for i := 0; i < b.N; i++ {
        r1 = make([]byte, 32768+1)
    }
}

var r2 []byte

func BenchmarkCount2(b *testing.B) {
    for i := 0; i < b.N; i++ {
        r2 = make([]byte, 40)
    }

}

The output:

OS page size: 32768

BenchmarkCount1-4         166443          7312 ns/op       40960 B/op           1 allocs/op
BenchmarkCount2-4       31465389            36.88 ns/op          48 B/op           1 allocs/op

In fact, a memory block with size 36864 is enough to carry the elements of a byte slice with size 32768+1. Why to allocate one more page for them?

[Ian Lance Taylor]

Memory blocks larger than 32768 bytes flip over to the "large
allocation" model. See _MaxSmallSize in runtime/sizeclasses.go (a
generated file). This requires an extra span. I haven't tried to
work it out in detail, but it probably has something to do with that.

Ian

[tapi...@gmail.com]

Yes, that is why I chose a size 32768+1.

I understand that the large memory block will be a multiple of page size.

What I haven't get is why one more page is allocated.

I will try to read the source to get the answer.

BTW, sorry, I missed a "more" in the thread title.

[tapi...@gmail.com]

It looks the _PageSize constant used in allocating large memory block is declared as 8192 in code, but os.Getpagesize() returns 4096.

const (

    ...

    _PageShift      = 13
    _PageSize = 1 << _PageShift

    ...

)

Is this intended?

[Ian Lance Taylor]

On Fri, Jun 11, 2021 at 11:21 PM tapi...@gmail.com <tapi...@gmail.com> wrote:
>
> It looks the _PageSize constant used in allocating large memory block is declared as 8192 in code, but os.Getpagesize() returns 4096.
>
> const (
> ...
> _PageShift = 13
> _PageSize = 1 << _PageShift
> ...
> )
>
> Is this intended?

Yes. _PageSize is the page size that the memory allocator uses.
os.Getpagesize returns the page size that the system uses to map
pages. In the runtime that is known as physPageSize. There is no
necessary relationship between the two, although heapArenaBytes should
be a multiple of both.

Ian

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[tapi...@gmail.com]

On Saturday, June 12, 2021 at 12:04:21 PM UTC-4 Ian Lance Taylor wrote:

On Fri, Jun 11, 2021 at 11:21 PM tapi...@gmail.com <tapi...@gmail.com> wrote:
>
> It looks the _PageSize constant used in allocating large memory block is declared as 8192 in code, but os.Getpagesize() returns 4096.
>
> const (
> ...
> _PageShift = 13
> _PageSize = 1 << _PageShift
> ...
> )
>
> Is this intended?

Yes. _PageSize is the page size that the memory allocator uses.
os.Getpagesize returns the page size that the system uses to map
pages. In the runtime that is known as physPageSize. There is no
necessary relationship between the two, although heapArenaBytes should
be a multiple of both.

Ian

Thanks for the clarification.