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.
>
> > On Friday, June 11, 2021 at 7:59:27 PM UTC-4 tapi...@gmail.com wrote:
> >>
> >> On Friday, June 11, 2021 at 2:00:03 PM UTC-4 Ian Lance Taylor wrote:
> >>>
> >>> On Fri, Jun 11, 2021 at 9:38 AM tapi...@gmail.com <tapi...@gmail.com>
> wrote:
> >>> >
> >>> > 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?
> >>>
> >>> 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
> >>
> >>
> >> 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.
> >
> > --
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>
>
>
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