Dynamo Frontend Benchmark

Dynamo frontend benchmarking

End-to-end harness for measuring and profiling the Dynamo frontend under load from a configurable client, served by mock workers so the backend isn't the variable under test. Bundled scripts are in scripts/; they all source env.sh, which requires DYN_REPO to point at your Dynamo checkout.

TL;DR workflow

export DYN_REPO=/path/to/dynamo          # checkout with built .venv
# 0. one-time: request plane up, venv built, FlameGraph cloned (see Setup)
sudo bash scripts/isolate.sh             # optional but recommended: CPU isolation
BLOCK_SIZE=512 FRONTEND_LD_PRELOAD=$DYN_REPO/bench/jemalloc/libjemalloc.so \
  bash scripts/start.sh                  # frontend (pinned) + N mockers
WARMUP_REQUESTS=512 bash scripts/run_aiperf.sh   # one measured run
python3 scripts/extract_throughput.py $DYN_REPO/bench/results/aiperf-*  # robust numbers
bash scripts/stop.sh                     # teardown + etcd drain

For an A/B: teardown + restart between every run, interleave arms, take the median of 3+. For profiling: profile_oncpu.sh (non-root) and capture_offcpu.sh (sudo).

What this measures (and what it doesn't)

• Frontend: HTTP (axum/hyper), tokenization (fastokens or HF), KV-router block hashing + radix-tree scheduling, request dispatch, SSE response relay.
• Mock workers (dynamo.mocker): simulate generation with --speedup-ratio (e.g. 1e6 = ~instant) and KV-cache block bookkeeping. Not a real vLLM worker — no GPU compute. Use them to remove backend variance, not to model production backends.
• Closed-loop client (aiperf): fixed --concurrency, so throughput ≈ concurrency / request_latency (Little's law). This is the single most important fact for interpreting results (see Pitfalls).

Setup (one-time)

1. Request plane — Dynamo needs etcd (:2379) + NATS with JetStream (:4222):
   • etcd is often a systemd service (survives reboot). Check: etcdctl endpoint health.
   • NATS is usually a user binary that does NOT auto-start on reboot. Start: nohup nats-server -js > /tmp/nats.log 2>&1 & then confirm ss -ltn | grep 4222.
2. Build the bindings into a venv: uv venv && source .venv/bin/activate && (cd lib/bindings/python && maturin develop --uv --release). Rust changes require rebuilding this; never run a build concurrently with a benchmark — it steals cores and contaminates results.
3. aiperf: pip install aiperf (the GenAI-perf successor) in some venv; set AIPERF.
4. FlameGraph: git clone https://github.com/brendangregg/FlameGraph and set FLAMEGRAPH_DIR.
5. jemalloc (optional, for the frontend): get a libjemalloc.so and pass it via FRONTEND_LD_PRELOAD to start.sh. Big alloc-churn reductions vs glibc.
6. perf access for on-CPU profiling: sudo sysctl kernel.perf_event_paranoid=-1 kernel.kptr_restrict=0. Off-CPU (sched tracepoints / BPF) still needs root even with paranoid=-1 (tracefs event files are root-only).

Topology & config (env.sh)

• FRONTEND_CORES (e.g. 0-3), OTHER_CORES (e.g. 4-23): frontend is pinned with taskset; mockers + client share OTHER_CORES. Keep FRONTEND_CORES small so frontend CPU effects are observable, but give OTHER_CORES enough headroom that the client doesn't starve the mockers (see Pitfalls).
• BLOCK_SIZE: frontend --kv-cache-block-size and mocker --block-size MUST match. Affects both sides — see "Block size" below.
• DYN_TOKENIZER = fastokens (PCRE2+rayon, fast) or default (HF tokenizers).
• DYN_TOKENIZER_CACHE / _BYTES: L1 prefix cache (helps with shared system prompts).

Running a throughput benchmark — methodology

The harness encodes hard-won protocol. Follow it or results drift:

1. Full teardown + fresh restart between every run (stop.sh then start.sh). The KV router and tokenizer cache accumulate state across runs; reusing an instance inflates later runs.
2. Drain etcd to 0 workers between runs (stop.sh does this; verify with count_workers). Dead frontends/mockers leave lease-backed keys that expire, but verify the slate is clean before starting.
3. Warmup (WARMUP_REQUESTS=512) to prime the prefix cache + warm the allocator before the measured phase. The first run after a fresh build is still a cold-start outlier — discard it.
4. jemalloc on the frontend via FRONTEND_LD_PRELOAD for stable allocator behavior.
5. A/B: same binary serves both arms when the difference is a runtime flag; otherwise rebuild between arms (never during a run). Interleave arms (A,B,A,B,…) to cancel drift, run 3+ each, compare medians (means get dragged by the cold first run).

run_aiperf.sh knobs (env overrides): CONCURRENCY, REQUEST_COUNT, WARMUP_REQUESTS. Default workload: shared-system-prompt 48000 + user-context 12000 (≈60k-token prompts), output-tokens-mean 500, conversation-turn-mean 4.

Profiling

On-CPU (where compute goes) — non-root

bash scripts/profile_oncpu.sh --frontend --conc 2048   # or --mocker, or --pid N --cores 0-3
python3 scripts/analyze_folded.py <out>/oncpu.folded

• Uses perf record -F 99 --call-graph dwarf. DWARF is required: release .sos have no frame pointers, so -g (FP unwinding) truncates Rust stacks.
• Also samples the target's cores (mpstat) and process CPU (pidstat) so you can see if it saturates. analyze_folded.py prints top self-time leaves.

Off-CPU (what blocked threads wait on) — REQUIRES sudo

sudo DYN_REPO=$DYN_REPO bash scripts/capture_offcpu.sh --frontend --conc 2048
python3 scripts/analyze_folded.py <out>/offcpu_bcc.folded --offcpu

• Captures two ways: offcputime-bpfcc -df (duration-weighted, user+kernel, folded) and perf -e sched:sched_switch --call-graph dwarf (backup, reliable Rust user frames). bcc's folded format uses a literal - frame to separate user (root→leaf) from kernel stacks; the innermost user frame before - is what called into the blocking syscall — analyze_folded.py --offcpu aggregates by it.
• Interpreting categories: futex/park = tokio workers idle (no runnable task) OR mutex; epoll = waiting on network/backend; __lll_lock_wait = glibc malloc-arena contention; rayon = fastokens pool idle/spin. Lock contention in app code shows as parking_lot/Mutex/RwLock frames — if those are ~0%, the process is idle-waiting, not internally serialized.

Analysis cheatsheet

• Throughput: extract_throughput.py <artifact_dir> — recompute from raw JSONL (do NOT trust the finalizer; see Pitfalls). Closed-loop sanity check: throughput ≈ concurrency / mean_latency.
• Cores busy (avg): from mpstat per-core %idle → busy = 100 - idle; or cpu_ms_per_req × req_per_s / 1000. Per-request CPU = Δ(utime+stime from /proc/<pid>/stat)/CLK_TCK ÷ requests.
• Latency decomposition: request_latency ≈ TTFT + (output_tokens × ITL). If TTFT dominates and explodes under load → queueing upstream of generation.

Pitfalls & gotchas (read this)

Benchmark methodology

• Closed-loop, not open-loop. Fixed concurrency means you measure concurrency / latency, NOT the server's max throughput. Idle frontend cores usually mean the system is latency-bound (each request spends most of its life waiting between streamed tokens), not that the frontend is slow. To push the frontend toward saturation: raise concurrency AND lower per-request latency (smaller block size → more frontend KV work; shorter outputs).
• Congestion collapse at high concurrency. Pushing concurrency too high can lower throughput (latency explodes faster than concurrency rises). Sweep concurrency to find the knee; don't assume "more load = more throughput".
• Client/server core contention. aiperf is CPU-heavy (client-side tokenizes every prompt, manages every stream across ~25 procs). Co-located with the mockers on OTHER_CORES, it can saturate those cores and starve the mockers — making a "collapse" that's really the load generator running out of CPU. Always check the CPU split (pidstat mocker vs mpstat on OTHER_CORES); if cores are pegged but the mocker is low, the client is the bottleneck.
• Cold-start first run is systematically slow even with warmup — discard it.
• Don't build while benchmarking. Compiles steal cores and ruin the run.

aiperf

• The finalizer hangs/deadlocks on large runs ("processing records…"). The per-request profile_export.jsonl is written incrementally — kill the finalizer and use extract_throughput.py. Don't wait for profile_export_aiperf.json.
• Orphan processes. aiperf's controller spawns many workers; killing the parent can orphan them. Worse: if you ran a capture with sudo, aiperf ran as root and a non-root pkill can't reap it — use sudo pkill -9 -f aiperf. Stray aiperf workers hold ZMQ/mmap resources and make the next run stall.
• --benchmark-duration N (time-based) avoids the giant fixed --request-count
  • finalizer problem for profiling loads.

Profiling

• Off-CPU needs root. Tracepoints (sched:sched_switch) and BPF (offcputime) require root even at perf_event_paranoid=-1 (tracefs event files are root-readable only). On-CPU perf -F.. -g works non-root at paranoid≤1.
• Native perf --off-cpu is often NOT compiled in (needs BUILD_BPF_SKEL=1); it silently no-ops with a warning. Use offcputime-bpfcc / bpftrace instead.
• No frame pointers in release builds → BPF user-stack walking truncates. Prefer perf --call-graph dwarf; bcc still gives good kernel stacks + partial user frames. analyze_folded.py handles the bcc - separator.
• Async-runtime off-CPU is dominated by worker park (futex) which is benign idle, not contention. Look for app-level lock frames (parking_lot/Mutex) to find real serialization. A blocked async task ≠ a blocked thread.

Topology / environment

• Block size must match frontend and mocker. And very large block sizes break the current mocker: at BLOCK_SIZE=2048 requests are received but the mocker never emits a token (40s hang → client cancel, output_tokens=0, "Failed to publish response"). 512 and 1024 work; 64 is realistic. Smoke-test a single request after any block-size change.
• Block size is a lever, not just a detail. Smaller blocks → more blocks per prompt → more frontend KV-routing work (radix tree, hashing) AND more mocker block bookkeeping. At bs=64 a 60k-token prompt is 940 blocks and the mocker's KV bookkeeping can dominate (48% of its CPU); at bs=512 (~117 blocks) it drops to ~3%. Pick the block size deliberately for what you're stressing.
• CPU isolation doesn't survive reboot (isolate.sh sets runtime cgroup cpusets on system.slice). Re-run sudo bash scripts/isolate.sh after every reboot. unisolate.sh reverts. Check: cat /sys/fs/cgroup/system.slice/cpuset.cpus.effective.
• NATS doesn't auto-start after reboot (user binary); etcd usually does (systemd). After a reboot, restart NATS before start.sh.
• jemalloc is frontend-only here (via FRONTEND_LD_PRELOAD); the mocker runs on glibc, so its alloc churn can show glibc-arena lock contention (__lll_lock_wait under __libc_free/Vec::finish_grow) in off-CPU. Preload jemalloc on the mocker too if that matters.
• DYN_RUNTIME_NUM_WORKER_THREADS and DYN_RUNTIME_MAX_BLOCKING_THREADS are applied to every runtime the bindings build, including the one the pyo3 async bridge builds for itself. Thread counts are still worth checking in /proc/<pid>/task: if the bridge builds its runtime before a DistributedRuntime is created, the process ends up with two runtimes and twice the threads the configuration describes (a warning says so).

Known result (calibration): with mock workers, the Dynamo frontend is rarely the bottleneck — it's latency/IO-bound, sitting ~60–85% of its pinned cores with ~0 internal lock contention. Frontend micro-opts therefore show flat e2e throughput on this setup; their value is CPU-efficiency/headroom. To make the frontend the bottleneck, use small block size + high concurrency, or real backends, or move the client off-box.

Script reference (scripts/)

• env.sh — config; set DYN_REPO; everything else overridable.
• start.sh — launch frontend (pinned, optional FRONTEND_LD_PRELOAD/FASTOKENS_*)
  • NUM_WORKERS mockers; port preflight, etcd worker-count verify.
• stop.sh — teardown both + drain etcd to 0.
• run_aiperf.sh — one measured run (CONCURRENCY/REQUEST_COUNT/WARMUP_REQUESTS).
• isolate.sh / unisolate.sh — CPU isolation (sudo; Lite by default, --full for max).
• smoke.sh — single-request sanity check (use after any topology/block-size change).
• profile_oncpu.sh — on-CPU perf + flamegraph (non-root): --frontend/--mocker/--pid.
• capture_offcpu.sh — off-CPU bcc + perf (sudo): --frontend/--mocker/--pid.
• analyze_folded.py — top self-time (on-CPU) or innermost-frame + category (off-CPU).
• extract_throughput.py — robust throughput/latency from raw aiperf JSONL.