Why does converting 'interface{}' back to a slice result in extra heap allocation?

In PHP, converting the "interface{}" type to the slice type will result in additional heap allocation. This is because in PHP, an interface is an abstract data type, and a slice is a dynamic array type. When we convert an interface type to a slice type, PHP needs to allocate additional memory space for the slice type to store the elements of the slice. This additional heap allocation operation will cause additional memory overhead, which may cause performance issues for some memory-sensitive applications. Therefore, when performing type conversion, we should pay attention to this issue and try to avoid unnecessary additional heap allocation.

Question content

func benchmarkpool(b *testing.b) {
    b.reportallocs()
    p := sync.pool{new: func() interface{} {
        return make([]byte, 1024)
    }}
    for i := 0; i < b.n; i++ {
        bts := p.get().([]byte)
        p.put(bts)
    }
}

This benchmark gives the following output in go1.19.5.

benchmarkpool
benchmarkpool-10        47578498            24.47 ns/op       24 b/op          1 allocs/op

Things look different when using *[]byte:

func benchmarkpool(b *testing.b) {
    b.reportallocs()
    p := sync.pool{new: func() interface{} {
        bts := make([]byte, 1024)
        return &bts
    }}
    for i := 0; i < b.n; i++ {
        bts := p.get().(*[]byte)
        p.put(bts)
    }
}

BenchmarkPool
BenchmarkPool-10        142008002            8.581 ns/op           0 B/op          0 allocs/op

It seems that converting interface{} back to a slice results in additional heap allocations.

Why does go need this extra allocation? What are the design considerations behind this?

Solution

What causes the allocation is not the conversion of any to []byte, but the conversion of []byte to Conversion of any. p.Put(bts) Implicitly converts the parameter bts to any before passing it to (*sync.Pool).Put . The interface in GoGC 1.19 is implemented as a pair of pointers, one pointing to the type metadata and one pointing to the actual object, in this case the second pointer is escaped to the pool, causing the slice object to be allocated. This applies not only to slice types, but also to any other non-pointer type.

For pointers, such as *[]byte, the compiler performs optimizations that place their values ​​directly into the iface structure, thus removing the when converting to an interface. *[]byte Allocation of instances. Therefore, it is generally recommended to put pointers into a pool rather than the structure itself.

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