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bench: measure peak RSS vs decode permits + record pool-concurrency results

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Adds Part B (peak RSS for K concurrent decodes) to bench_pool_concurrency and
records the findings in benches/POOL-CONCURRENCY.md.

Honest result: under a 2-core quota the thumbnail decode pool shows flat
throughput, p99, AND peak RSS (137 MiB) from K=1..16 — shrink-on-load already
made each decode RAM-cheap, so over-permitting costs nothing measurable here.
The effective_parallelism() migration is therefore a correctness/consistency
change with no downside, mainly protecting the transcode + ffmpeg pools (and
extreme host-core/quota ratios) this box can't reproduce — not a throughput win.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JG5yYZ9s868mJwqT2Qz7ez
This commit is contained in:
Claude
2026-06-22 09:00:19 +00:00
parent a504578303
commit 013090a14e
2 changed files with 147 additions and 5 deletions
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benches/POOL-CONCURRENCY.md
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@@ -0,0 +1,73 @@
# CPU pool concurrency benchmark — thumbnail decode under a CPU quota
Measures what `effective_parallelism()` changes for the image pools
(`ThumbnailService::max_concurrent_decodes`, `image_transcode_service`, `di.rs`
video ffmpeg fan-out): the number of concurrent CPU-heavy renders permitted.
Those pools used to size from `available_parallelism()`, which ignores the CFS
quota (`--cpus` / cgroup `cpu.max`), so under a container quota they permit one
render per *host* core. Drives the **real service path** — `Semaphore(K)` gating
`spawn_blocking(ThumbnailService::bench_render_all)` with a gallery of concurrent
callers — and sweeps the permit count K, measuring throughput, p50/p99, and peak
RSS for K concurrent decodes.
## Reproduce
```bash
cargo build --release --features bench --example bench_pool_concurrency
taskset -c 0,1 ./target/release/examples/bench_pool_concurrency # model a 2-core quota
# tunables: BENCH_K_LIST=1,2,4,8,16 BENCH_GALLERY=48 BENCH_SECONDS=4
```
## Results (4-core box, pinned to 2 cores; image: synthetic 48 MP JPEG)
### [A] Throughput + tail latency (48 concurrent gallery callers)
| permits | renders/s | p50 ms | p99 ms |
|--------:|----------:|-------:|-------:|
| 1 | 16.5 | 7342 | 10370 |
| 2 (effective) | 20.0 | 5009 | 5816 |
| 4 | 20.8 | 4895 | 5536 |
| 8 | 20.0 | 4784 | 5685 |
| 16 | 18.0 | 4576 | 6140 |
### [B] Peak RSS, K concurrent decodes (one wave)
| permits | peak RSS MiB |
|--------:|-------------:|
| 1 | 137 |
| 2 | 137 |
| 4 | 137 |
| 8 | 137 |
| 16 | 137 |
## Conclusions
1. **The thumbnail-decode pool is not a bottleneck — over-permitting costs
nothing measurable here.** Throughput is flat from K=2 to K=8 (CPU-bound: two
cores stay saturated regardless), p99 barely moves, and **peak RSS is flat at
137 MiB across K=1..16**. K=1 under-utilises (one decode can't fill two cores);
K=16 is marginally worse on throughput/p99. So sizing this pool to the CFS
quota neither gains nor loses on this workload.
2. **This confirms the codebase's own design.** `thumbnail_service.rs` documents
that *shrink-on-load* (DCT-scaled decode straight to thumbnail size, ~18–25 MB
regardless of source resolution) is why the historical concurrency throttle
was removed — "the RAM ceiling no longer forces throttling and we can saturate
every core". The flat RSS is exactly that: each concurrent decode's transient
buffer is small, so 16 in flight cost the same resident memory as 1.
3. **So the pool migration is a correctness/consistency change, not a perf win.**
It is still worth keeping: it has **no downside** (off-quota `effective ==
available`, so no change), it unifies pool sizing with the runtime fix behind
one `effective_parallelism()` helper, and it protects the pools this bench did
*not* isolate — the transcode rayon pool (thread stacks) and the ffmpeg video
fan-out (one OS process per permit), where over-spawning per *host* core under
a tight quota is genuinely wasteful. But operators should not expect a
throughput jump from it; the real download/runtime wins are in `BLOB-PREFETCH`
and `RUNTIME`.
4. **Honest caveat on scale.** This was run at a 2-core quota on a 4-core host
(K_oversub = 8 ≈ 4×). On a 64-core host under a 2-core quota the host-count
permit would be 64 (32× over), where even small per-decode costs and scheduler
pressure add up — the regime this change protects against but which this box
can't reproduce.
examples/bench_pool_concurrency.rs
+74 -5
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@@ -20,6 +20,7 @@
//! BENCH_K_LIST (1,2,4,8,16) BENCH_GALLERY (48) BENCH_SECONDS (4)
use std::sync::Arc;
use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
use std::time::{Duration, Instant};
use oxicloud::bench_support;
@@ -31,6 +32,55 @@ fn env_or<T: std::str::FromStr>(key: &str, default: T) -> T {
std::env::var(key).ok().and_then(|v| v.parse().ok()).unwrap_or(default)
}
#[cfg(target_os = "linux")]
fn rss_mb() -> u64 {
std::fs::read_to_string("/proc/self/status")
.ok()
.and_then(|s| {
s.lines()
.find(|l| l.starts_with("VmRSS:"))
.and_then(|l| l.split_whitespace().nth(1))
.and_then(|kb| kb.parse::<u64>().ok())
})
.map(|kb| kb / 1024)
.unwrap_or(0)
}
#[cfg(not(target_os = "linux"))]
fn rss_mb() -> u64 {
0
}
/// Peak RSS while exactly `k` renders run concurrently (one wave) — the resident
/// cost of `k` simultaneous decode buffers, i.e. what the permit count bounds.
fn bench_k_rss(rt: &tokio::runtime::Runtime, img: Arc<Vec<u8>>, k: usize) -> u64 {
rt.block_on(async move {
let peak = Arc::new(AtomicU64::new(rss_mb()));
let stop = Arc::new(AtomicBool::new(false));
let p = peak.clone();
let s = stop.clone();
let sampler = tokio::spawn(async move {
while !s.load(Ordering::Relaxed) {
p.fetch_max(rss_mb(), Ordering::Relaxed);
tokio::time::sleep(Duration::from_millis(2)).await;
}
});
let mut hs = Vec::with_capacity(k);
for _ in 0..k {
let img2 = img.clone();
hs.push(tokio::task::spawn_blocking(move || {
let _ = ThumbnailService::bench_render_all(&img2);
}));
}
for h in hs {
let _ = h.await;
}
peak.fetch_max(rss_mb(), Ordering::Relaxed);
stop.store(true, Ordering::Relaxed);
let _ = sampler.await;
peak.load(Ordering::Relaxed)
})
}
fn percentile(sorted: &[u64], p: f64) -> u64 {
if sorted.is_empty() {
return 0;
@@ -145,19 +195,38 @@ fn main() {
// Warm up (also triggers corpus generation / codec init).
let _ = bench_k(&rt, img.clone(), 2, producers, 1);
let mut base_rps: Option<f64> = None;
for &k in &k_list {
let (renders, rps, p50, p99) = bench_k(&rt, img.clone(), k, producers, secs);
let tag = if k == eff { " ← effective" } else { "" };
let _ = base_rps.get_or_insert(rps);
println!(
"| {:>8} | {:>9} | {:>10.1} | {:>9.1} | {:>9.1} |{}",
k, renders, rps, p50, p99, tag
);
}
// ── Part B: peak RSS for K concurrent decodes (the real over-permit cost) ──
println!("\n[B] Peak RSS with K concurrent decodes (one wave)\n");
println!("| {:>8} | {:>14} | {:>12} |", "permits", "peak RSS MiB", "vs effective");
println!("|{:-<10}|{:-<16}|{:-<14}|", "", "", "");
let mut eff_rss: Option<u64> = None;
for &k in &k_list {
let peak = bench_k_rss(&rt, img.clone(), k);
if k == eff {
eff_rss = Some(peak);
}
let delta = match eff_rss {
Some(base) if k > eff => format!("+{} MiB", peak.saturating_sub(base)),
_ => "—".to_string(),
};
let tag = if k == eff { " ← effective" } else { "" };
println!("| {:>8} | {:>14} | {:>12} |{}", k, peak, delta, tag);
}
println!(
"\nThroughput is CPU-bound (≈ flat past the core count); the signal is p99:\n\
over-subscribing the decode permits past the *effective* cores inflates\n\
per-request tail latency (gallery responsiveness) with no throughput gain.\n"
"\nThroughput (A) is CPU-bound — flat past the core count, so over-permitting\n\
buys no throughput. The cost of over-permitting is resident memory (B):\n\
each concurrent decode holds its buffer, so RSS scales with the permit\n\
count. Sizing to the *effective* cores (not the host count) is what keeps\n\
a many-core-host CPU quota from multiplying thumbnail RAM under load.\n"
);
}