Re: [pbs-devel] [PATCH proxmox-backup-qemu 8/9] add shared_cache module

[Wolfgang Bumiller <w.bumiller@proxmox.com>]

On Wed, Jun 02, 2021 at 04:38:32PM +0200, Stefan Reiter wrote:
> Provides a shared AsyncLruCache of 256MB (w/ 4MB chunks) that can be
> used by multiple readers at the same time. It is dropped once no more
> readers exist, so the memory gets freed if all QEMU block/pbs instances
> disappear.
> 
> Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
> ---
>  src/lib.rs          |  7 ++++++-
>  src/shared_cache.rs | 36 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 42 insertions(+), 1 deletion(-)
>  create mode 100644 src/shared_cache.rs
> 
> diff --git a/src/lib.rs b/src/lib.rs
> index 05d7b58..aa167f7 100644
> --- a/src/lib.rs
> +++ b/src/lib.rs
> @@ -25,6 +25,7 @@ mod restore;
>  use restore::*;
>  
>  mod tools;
> +mod shared_cache;
>  
>  pub const PROXMOX_BACKUP_DEFAULT_CHUNK_SIZE: u64 = 1024*1024*4;
>  
> @@ -804,7 +805,11 @@ pub extern "C" fn proxmox_restore_connect_async(
>  pub extern "C" fn proxmox_restore_disconnect(handle: *mut ProxmoxRestoreHandle) {
>  
>      let restore_task = handle as * mut Arc<RestoreTask>;
> -    unsafe { Box::from_raw(restore_task) }; //drop(restore_task)
> +    let restore_task = unsafe { Box::from_raw(restore_task) };
> +    drop(restore_task);
> +
> +    // after dropping, cache may be unused (if no other handles open)
> +    shared_cache::shared_chunk_cache_cleanup();
>  }
>  
>  /// Restore an image (sync)
> diff --git a/src/shared_cache.rs b/src/shared_cache.rs
> new file mode 100644
> index 0000000..bebae5b
> --- /dev/null
> +++ b/src/shared_cache.rs
> @@ -0,0 +1,36 @@
> +use once_cell::sync::OnceCell;
> +use proxmox_backup::backup::ChunkCache;
> +use proxmox_backup::tools::async_lru_cache::AsyncLruCache;
> +use std::sync::{Arc, Mutex};
> +
> +const SHARED_CACHE_CAPACITY: usize = 64; // 256 MB
> +static SHARED_CACHE: OnceCell<Mutex<Option<ChunkCache>>> = OnceCell::new();

OnceCell *and* Option is a bit too much, `get_shared_chunk_chache()` can
just initialize it in `get_or_init()` directly.

> +
> +pub fn get_shared_chunk_cache() -> ChunkCache {
> +    let mut guard = SHARED_CACHE
> +        .get_or_init(|| Mutex::new(None))
> +        .lock()
> +        .unwrap();
> +    match &*guard {
> +        Some(cache) => Arc::clone(cache),
> +        None => {
> +            let cache = Arc::new(AsyncLruCache::new(SHARED_CACHE_CAPACITY));
> +            *guard = Some(Arc::clone(&cache));
> +            cache
> +        }
> +    }
> +}
> +
> +pub fn shared_chunk_cache_cleanup() {
> +    let mut guard = SHARED_CACHE
> +        .get_or_init(|| Mutex::new(None))
> +        .lock()
> +        .unwrap();
> +    if let Some(arc) = guard.as_ref() {
> +        let refcount = Arc::strong_count(arc);
> +        if refcount == 1 {
> +            // no one else is using the cache anymore, drop it
> +            let _drop = guard.take();
> +        }
> +    }
> +}
> -- 
> 2.30.2