[Libguestfs] [PATCH libnbd 5/9] golang: aio_buffer.go: Add Slice()

[Eric Blake]

On Sun, Jan 30, 2022 at 01:33:33AM +0200, Nir Soffer wrote:
> AioBuffer.Bytes() cannot be used for coping images from NBD to other

copying

> APis because it copies the entire image. Add a new Slice() function,
> creating a slice backed by the underling buffer.

underlying

> 
> Using Slice() is efficient, but less safe, like Get(). The returned
> slice must be used only before calling Free(). This should not be an
> issue with typical code.
> 
> Testing show that Slice() is much faster than Bytes() for typical 256k
> buffer:
> 
> BenchmarkAioBufferBytes-12       	   86616	     16529 ns/op
> BenchmarkAioBufferSlice-12       	1000000000	         0.4630 ns/op
> 
> I modified the aio_copy example to use AioBuffer and complied 2

compiled

> +++ b/golang/libnbd_620_aio_buffer_test.go
> @@ -44,20 +44,35 @@ func TestAioBuffer(t *testing.T) {
>  	/* Modifying returned slice does not modify the buffer. */
>  	for i := 0; i < len(b); i++ {
>  		b[i] = 42
>  	}
>  
>  	/* Bytes() still returns zeroes. */
>  	if !bytes.Equal(buf.Bytes(), zeroes) {
>  		t.Fatalf("Expected %v, got %v", zeroes, buf.Bytes())
>  	}
>  
> +	/* Creating a slice without copying the underlhing buffer. */

underlying

> +	s := buf.Slice()
> +	if !bytes.Equal(s, zeroes) {
> +		t.Fatalf("Expected %v, got %v", zeroes, s)
> +	}
> +
> +	/* Modifing the slice modifies the underlying buffer. */

Modifying

>  }
> +
> +// Benchmark creating a slice without copying the underling buffer.

underlying

> +func BenchmarkAioBufferSlice(b *testing.B) {
> +	buf := MakeAioBuffer(bufferSize)
> +	defer buf.Free()
> +	var r int
> +
> +	b.ResetTimer()
> +	for i := 0; i < b.N; i++ {
> +		r += len(buf.Slice())
> +	}
> +}
> -- 
> 2.34.1
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org