Re: [pve-devel] [PATCH v2 proxmox-backup-qemu 05/11] access: use bigger cache and LRU chunk reader

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

On 16/03/2021 21:17, Thomas Lamprecht wrote:
> On 03.03.21 10:56, Stefan Reiter wrote:
>> Values chosen by fair dice roll, seems to be a good sweet spot on my
>> machine where any less causes performance degradation but any more
>> doesn't really make it go any faster.
>>
>> Keep in mind that those values are per drive in an actual restore.
>>
>> Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
>> ---
>>
>> Depends on new proxmox-backup.
>>
>> v2:
>> * unchanged
>>
>>   src/restore.rs | 5 +++--
>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/src/restore.rs b/src/restore.rs
>> index 0790d7f..a1acce4 100644
>> --- a/src/restore.rs
>> +++ b/src/restore.rs
>> @@ -218,15 +218,16 @@ impl RestoreTask {
>>   
>>           let index = client.download_fixed_index(&manifest, &archive_name).await?;
>>           let archive_size = index.index_bytes();
>> -        let most_used = index.find_most_used_chunks(8);
>> +        let most_used = index.find_most_used_chunks(16); // 64 MB most used cache
> 
> 
> 
>>   
>>           let file_info = manifest.lookup_file_info(&archive_name)?;
>>   
>> -        let chunk_reader = RemoteChunkReader::new(
>> +        let chunk_reader = RemoteChunkReader::new_lru_cached(
>>               Arc::clone(&client),
>>               self.crypt_config.clone(),
>>               file_info.chunk_crypt_mode(),
>>               most_used,
>> +            64, // 256 MB LRU cache
> 
> how does this work with low(er) memory situations? Lots of people do not over
> dimension their memory that much, and especially the need for mass-recovery could
> seem to correlate with reduced resource availability (a node failed, now I need
> to restore X backups on my <test/old/other-already-in-use> node, so multiple
> restore jobs may run in parallel, and they all may have even multiple disks,
> so tens of GiB of memory just for the cache are not that unlikely.

This is a seperate function from the regular restore, so it currently 
only affects live-restore. This is not an operation you would usually do 
under memory constraints anyway, and regular restore is unaffected if 
you just want the data.

Upcoming single-file restore too though, I suppose, where it might make 
more sense...

> 
> How is the behavior, hard failure if memory is not available? Also, some archives
> may be smaller than 256 MiB (EFI disk??) so there it'd be weird to have 256 cache
> and get 64 of most used chunks if that's all/more than it would actually need to
> be..

Yes, if memory is unavailable it is a hard error. Memory should not be 
pre-allocated however, so restoring this way will only ever use as much 
memory as the disk size (not accounting for overhead).

> 
> There may be the reversed situation too, beefy fast node with lots of memory
> and restore is used as recovery or migration but network bw/latency to PBS is not
> that good - so bigger cache could be wanted.

The reason I chose the numbers I did was that I couldn't see any real 
performance benefits by going higher, though I didn't specifically test 
with slow networking.

I don't believe more cache would improve the situation there though, 
this is mostly to avoid random access from the guest and the linear 
access from the block-stream operation to interfere with each other, and 
allow multiple smaller guest reads within the same chunk to be served 
quickly.

> 
> Maybe we could get the available memory and use that as hint, I mean as memory
> usage can be highly dynamic it will never be perfect, but better than just ignoring
> it..

If anything, I'd make it user-configurable - I don't think a heuristic 
would be a good choice here.

This way we could also set it smaller for single-file restore for 
example - on the other hand, that adds another parameter to the already 
somewhat cluttered QEMU<->Rust interface.

> 
>>           );
>>   
>>           let reader = AsyncIndexReader::new(index, chunk_reader);
>>
>