[pbs-devel] [PATCH proxmox-backup 2/3] partial fix #6049: datastore: use config fast-path in Drop

[Fabian Grünbichler]

On November 11, 2025 1:29 pm, Samuel Rufinatscha wrote:
> The Drop impl of DataStore re-read datastore.cfg to decide whether
> the entry should be evicted from the in-process cache (based on
> maintenance mode’s clear_from_cache). During the investigation of
> issue #6049 [1], a flamegraph [2] showed that the config reload in Drop
> accounted for a measurable share of CPU time under load.
> 
> This patch makes Drop O(1) on the fast path by reusing the maintenance-

I am not sure what the O(1) is refering to? This patch implements a
faster cache lookup in front of the (slow) config parsing variant, but
that doesn't really align well with what the "Big O" notation tries to
express ;)

The parsing below still scales with the number of datastores in the
config, after all. It can just be skipped sometimes :)

> mode decision captured at lookup time and stored with the cached
> datastore entry. When the last reference goes away we:
> - decrement active-operation counters, and
> - evict only if the cached decision mandates eviction.
> 
> If the cache tag is absent or not fresh, a subsequent slow-path lookup
> will be performed.
> 
> Testing
> 
> Compared flamegraphs before and after: prior to this change
> (on top of patch 1), stacks originating from Drop included
> pbs_config::datastore::config(). After the change, those vanish from
> the drop path.
> 
> An end-to-end benchmark using `/status?verbose=0` with 1000 datastores,
> 5 requests per store, and 16-way parallelism shows a further
> improvement:
> 
> | Metric                  | After commit 1 | After commit 2 | Δ (abs) | Δ (%)   |
> |-------------------------|:--------------:|:--------------:|:-------:|:-------:|
> | Total time              | 11s            | 10s            | −1s     | −9.09%  |
> | Throughput (all rounds) | 454.55         | 500.00         | +45.45  | +10.00% |
> | Cold RPS (round #1)     | 90.91          | 100.00         | +9.09   | +10.00% |
> | Warm RPS (rounds 2..N)  | 363.64         | 400.00         | +36.36  | +10.00% |
> 
> Optimizing Drop improves overall throughput by ~10%. The gain appears
> in both cold and warm rounds, and the flamegraph confirms the config
> reload no longer sits on the hot path.
> 
> Links
> 
> [1] Bugzilla: https://bugzilla.proxmox.com/show_bug.cgi?id=6049
> [2] cargo-flamegraph: https://github.com/flamegraph-rs/flamegraph
> 
> Fixes: #6049
> Signed-off-by: Samuel Rufinatscha <s.rufinatscha at proxmox.com>
> ---
>  pbs-datastore/src/datastore.rs | 31 +++++++++++++++++++++++++++----
>  1 file changed, 27 insertions(+), 4 deletions(-)
> 
> diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
> index 18eebb58..da80416a 100644
> --- a/pbs-datastore/src/datastore.rs
> +++ b/pbs-datastore/src/datastore.rs
> @@ -200,15 +200,38 @@ impl Drop for DataStore {
>              // remove datastore from cache iff
>              //  - last task finished, and
>              //  - datastore is in a maintenance mode that mandates it
> -            let remove_from_cache = last_task
> -                && pbs_config::datastore::config()
> +
> +            // first check: check if last task finished
> +            if !last_task {
> +                return;
> +            }
> +
> +            let cached_tag = self.inner.cached_config_tag.as_ref();
> +            let last_gen_num = cached_tag.and_then(|c| c.last_generation);
> +            let gen_num = ConfigVersionCache::new()
> +                .ok()
> +                .map(|c| c.datastore_generation());
> +
> +            let cache_is_fresh = match (last_gen_num, gen_num) {
> +                (Some(a), Some(b)) => a == b,
> +                _ => false,
> +            };

this is just last_gen_num == gen_num and checking that either is Some.
if we make the tag always contain a generation instead of an option, we
can simplify this code ;)

> +
> +            let mm_mandate = if cache_is_fresh {
> +                cached_tag
> +                    .and_then(|c| c.last_maintenance_mode.as_ref())
> +                    .is_some_and(|m| m.clear_from_cache())
> +            } else {
> +                pbs_config::datastore::config()
>                      .and_then(|(s, _)| s.lookup::<DataStoreConfig>("datastore", self.name()))
>                      .is_ok_and(|c| {
>                          c.get_maintenance_mode()
>                              .is_some_and(|m| m.clear_from_cache())
> -                    });
> +                    })
> +            };
>  
> -            if remove_from_cache {
> +            // second check: check maintenance mode mandate
> +            if mm_mandate {
>                  DATASTORE_MAP.lock().unwrap().remove(self.name());
>              }
>          }
> -- 
> 2.47.3
> 
> 
> 
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