Re: [PATCH v2 proxmox 2/6] io: request zeroed memory instead of manually clearing it

[Robert Obkircher <r.obkircher@proxmox.com>]

On 12.08.26 10:25, Robert Obkircher wrote:
> A quick comparison using cargo bench showed no notable differences in
> release mode, but in debug mode 1 GiB allocations by themselves got
> 43879 times faster. This is because the standard library continued to
> call alloc_zeroed, matching release mode, while the previous version
> actually had to touch the memory. With an additional .fill(1) the
> speedup shrank to around 1.09x.
>
> Signed-off-by: Robert Obkircher <r.obkircher@proxmox.com>
> ---
>  proxmox-io/src/boxed.rs   | 7 ++-----
>  proxmox-io/src/vec/mod.rs | 6 +-----
>  2 files changed, 3 insertions(+), 10 deletions(-)
>
> diff --git a/proxmox-io/src/boxed.rs b/proxmox-io/src/boxed.rs
> index 7af51cfc..fb67bfd7 100644
> --- a/proxmox-io/src/boxed.rs
> +++ b/proxmox-io/src/boxed.rs
> @@ -12,9 +12,6 @@ pub fn uninitialized(len: usize) -> Box<[u8]> {
>  
>  /// Zero-initialized bytes, meant for large sizes to avoid busting the stack.
>  pub fn zeroed(len: usize) -> Box<[u8]> {
> -    let mut bytes = uninitialized(len);
> -    unsafe {
> -        std::ptr::write_bytes(bytes.as_mut_ptr(), 0, bytes.len());
> -    }
> -    bytes
> +    // vec! guarantees not to over-allocate, so shrink_to_fit inside into_boxed_slice does nothing
> +    vec![0; len].into_boxed_slice()
The comment is wrong about the over-allocation, but a sane allocator
wouldn't hand out a larger allocation if it could shrink it immediately.

I'm not sure if using Box::new_zeroed() with unsafe is worth it,
because the callers could easily switch to Vec.
>  }
> diff --git a/proxmox-io/src/vec/mod.rs b/proxmox-io/src/vec/mod.rs
> index b68fc750..7fe106e8 100644
> --- a/proxmox-io/src/vec/mod.rs
> +++ b/proxmox-io/src/vec/mod.rs
> @@ -69,11 +69,7 @@ pub fn clear(data: &mut [u8]) {
>  /// Create a newly allocated, zero initialized byte vector.
>  #[inline]
>  pub fn zeroed(len: usize) -> Vec<u8> {
> -    unsafe {
> -        let mut out = uninitialized(len);
> -        clear(&mut out);
> -        out
> -    }
> +    vec![0; len]
>  }
>  
>  /// Create a newly allocated byte vector of a specific size with "undefined" content.