chunkloris: actix-web (h2c)

part of the chunkloris per-chunk amplification survey. this page is the per-server record for actix-web (h2c) under http/2 (h2c) data frames.

at a glance

server: actix-web (h2c) 4.x
runtime: rustc 1.89
ecosystem: rust
concurrency model: event-loop
parser: actix-http h2
delivery granularity: per-frame
chunk-limit helper: none exposed by the framework
verdict: batches correctly — the implementation coalesces wire units before waking the application, either via an explicit per-stream frame credit, a pipelined reader, or a similar batching primitive. mode b cpu cost is in the band you would expect from a per-recv() batched delivery.
scaling exponent (mode a): 0.86 (wall time vs N, log-log slope across common cells)
scaling exponent (mode b): 0.99

measurements

all cells run on a 1-vcpu docker container. cpu cost is derived from the target container’s cgroup v2 cpu.stat usage_usec delta around each cell.

mode	N	wall (s)	server cpu %	µs / frame	basis	ok
`A-h2-bridge`	50,000	0.201	16.4	0.660	server-cpu-cgroup	✓
`A-h2-bridge`	100,000	0.355	15.4	0.549	server-cpu-cgroup	✓
`A-h2-bridge`	250,000	0.804	14.1	0.454	server-cpu-cgroup	✓
`B-h2-paced-100us`	50,000	5.468	4.6	5.027	server-cpu-cgroup	✓
`B-h2-paced-100us`	100,000	10.830	4.1	4.428	server-cpu-cgroup	✓
`B-h2-paced-100us`	250,000	27.066	3.8	4.143	server-cpu-cgroup	✓

parser path — source citations

Actix HTTP/2 support — ? → source
Actix h2c binding — ? → source

what this means

the implementation batches wire units before waking the application (either via an explicit per-stream frame credit, a pipelined reader, or an equivalent primitive). the cpu cost under paced mode b is in the band you would expect from a per-recv() batched delivery.

what to do today

if this is an h2 origin, prefer a frontend that terminates h2 into h1 with proxy_request_buffering on upstream.
consider imposing a per-stream DATA-frame credit (count, not bytes) before forwarding the body to the application handler.
HTTP/2 byte-level flow control (WINDOW_UPDATE) does not bound the number of frames; configure stream-frame-rate limits where the implementation exposes them.

reproducer

the full reproducer for this server is in the paper repo. the docker container pins actix-web (h2c) 4.x and constrains the test container to a single cpu (--cpus=1). the prober script implements mode a (bridge-coalesced) and mode b (paced 100 µs) per the methodology section.

see the draft pdf for the full per-framework discussion.