[pve-devel] [RFC qemu-server] fix #6935: vmstatus: fallback to RSS in case of KSM usage

[Aaron Lauterer]

On  2025-11-25  16:20, Thomas Lamprecht wrote:
> Am 25.11.25 um 15:20 schrieb Fabian Grünbichler:
>> On November 25, 2025 3:08 pm, Thomas Lamprecht wrote:
>>> Just to be sure: The stats from memory.current or memory.stat inside the
>>> /sys/fs/cgroup/qemu.slice/${vmid}.scope/ directory is definitively not
>>> enough for our usecases?
>>
>> well, if we go for RSS they might be, for PSS they are not, since that
>> doesn't exist there?
> 
> Would need to take a closer look to tell for sure, but from a quick check
> it indeed seems to not be there.
> 
>> having the live view and the metrics use different semantics seems kinda
>> confusing tbh..
> 
> more than jumping between metrics over time silently? ;-) The live view can
> be easily annotated with a different label or the like if the source is
> another, not so easy for metrics.
> 
> The more I think about this the more I'm in favor of just deprecating this
> again completely, this page table walking can even cause some latency spikes
> in the target process, IMO just not worth it. If the kernel can give us this
> free, or at least much cheaper, in the future, then great, but until then it's
> not really an option. If, we can make this opt-in. The best granularity here
> probably would be through guest config, but for starters a cluster-wide
> datacenter option could be already enough for the setups that are fine with
> this performance trade-off in general.


If I may add my 2 cents here. How much do we lose by switching 
completely to fetching RSS (or the cgroupv2 equivalent)? For the metrics 
and live view.
AFAIU the resulting memory accounting will be a bit higher, as shared 
libraries will be fully accounted for for each cgroup and not 
proportionally as with PSS.

I am not sure if we want to introduce additional config options (global 
per DC, or per guest) to change the behavior. As that is probably even 
more confusing for not that much gain.

And its not like PSS doesn't come with its own set of weirdness. E.g., 
if we run 10 VMs, and stop all but one, the last will see an increase in 
memory consumption as it is the sole user of shared libraries.

> 
> 
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