On Fri, Jun 11, 2021 at 11:21 PM tapi...@gmail.com <tapir....@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
> 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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