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

[Stefan Reiter <s.reiter@proxmox.com>]

On 6/4/21 2:16 PM, Wolfgang Bumiller wrote:
> 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.
> 

Then how do we drop the ChunkCache? I used OnceCell since 
std::sync::Mutex doesn't seem to have a 'const new', not because of 
'Option' semantics.

>> +
>> +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