Re: [RFC] Re-enable alternative memory allocator (tcmalloc) for pve-qemu?

[Dietmar Maurer <dietmar@proxmox.com>]

> The mechanism is well-understood: librbd creates many small temporary
> objects per I/O operation, and glibc's allocator is slow under that
> fragmentation pattern.

Such things are usually easy to fix using by using a custom pool 
allocator, freeing
all allocated objects at once. But I don't know the Ceph code, so I am 
not sure if it is
possible to apply such pattern to the Ceph code ...

> But both allocators provide explicit "release everything now" APIs:
>
> - gperftools: MallocExtension::instance()->ReleaseFreeMemory()
>    (calls ReleaseToSystem(LONG_MAX) internally)
> - jemalloc: mallctl("arena.<i>.purge", ...)

The problem with that is that it can halt/delay the whole application 
while freeing memory?
We need to make sure that there is no noticeable delay.

> ## Questions for the team
>
> 1. Does anyone remember why tcmalloc was removed after only 8 days
>     in 2015 (2.3-4)? The changelog just says "remove tcmalloc" with
>     no explanation.
Not really, sorry.
> 2. Fiona: given the backup memory reclamation work in 5f9cb29, would
>     you see a problem with replacing malloc_trim with the
>     allocator-specific release API in the backup completion path?
>
> 3. Has the proxmox-backup-qemu library's allocation pattern changed
>     since 2020 in ways that might affect this? (e.g., different buffer
>     management, Rust allocator changes)

I am not aware of such changes.