Case Study: Two Throughput Defects

Felix’s sustained loopback throughput was once capped at about 73 MB/s of payload regardless of configuration, and after that cap was lifted, roughly one benchmark run in three still landed 5–6× below the rest on identical settings. This page covers both defects: what they looked like, how they were diagnosed, and what fixed them. It also covers the investigative method, which carries over to other problems better than either fix does.

The mechanisms and the code that implements them are described in Concurrency internals. Current numbers are in Benchmarks.

Defect 1: a throughput ceiling exactly proportional to bytes

The symptom

The ceiling depended on bytes and nothing else:

Payload	Messages/s	Payload rate
2 KiB	35,707	73.1 MB/s
4 KiB	17,128	70.2 MB/s
8 KiB	8,675	71.1 MB/s
16 KiB	4,382	71.8 MB/s
32 KiB	2,185	71.6 MB/s

A 16× spread in payload size and a 16× spread in message rate produced the same byte rate. That result rules out every per-message, per-batch and per-frame explanation at once: encode cost, channel operations, publisher round trips, framing overhead. Any of those would hold messages per second constant, and here bytes per second were constant.

Meanwhile every individual stage measured fast: decode 27 µs, append 1 µs, QUIC write 2 µs. Twelve of sixteen cores sat idle. No queue, lock, or flow-control window was ever saturated, and the QUIC layer reported zero packet loss, zero congestion events, and zero blocked frames in either direction.

Every stage fast, nothing saturated and the rate fixed per byte: that points to a latency-bound dependency chain rather than a resource limit.

What it wasn’t

Fifteen hypotheses were measured and rejected before the real cause was found. Most are plausible enough that someone will propose them again, so here they are:

Hypothesis	Verdict
Transport / MTU / packet loss	MTU 16354, 0 lost, 0 congestion events
QUIC flow control	0 blocked frames, both directions
CPU saturation	12 of 16 cores idle
Redundant payload copy	0.2% of one core
Read strategy (`read_exact` vs `read_chunk`)	no measurable difference
Channel hops, locks, re-encode copies	a relay with 4 extra hops: no change
Egress topology (writer lanes → direct writer)	no change
Egress frame granularity (64 KiB → 1 MiB)	no change
Queue depths (core, lane, in-flight bytes)	flat
Publish worker sharding	more workers was worse
Publisher connection concurrency	flat from 1 to 16 connections
Per-event and per-batch client costs	ruled out by the byte-rate invariance above

Five structural refactors were also tested against this ceiling. None of them moved it.

The mechanism

Quinn’s driver tasks (the endpoint receive loop and each connection’s transmit/ACK/timer loop) do a bounded slice of work per poll and then reschedule themselves. Sustained throughput is therefore:

bounded work per poll ÷ scheduler re-poll latency

On a runtime shared with ~50 application tasks, that re-poll latency grows with load. Wakeups scale with datagram count, and datagram count is bytes ÷ MTU, so the cost lands per byte. That is why the ceiling looked like a payload-size problem rather than a scheduling one. In effect, Felix was clocked by its own scheduler.

The fix

Three changes, all in Felix, using only official quinn APIs:

Dedicated I/O runtimes. Quinn drivers run on a pool of single-threaded tokio runtimes, isolated from application tasks and assigned by endpoint role, so a driver’s self-wake is re-polled immediately instead of queueing behind application work or migrating cores.
Pump colocation. The two tasks that trade a wakeup with the transport per datagram or per write (the client’s subscription reader and the broker’s per-connection delivery writer) run on the same thread as their connection’s drivers, turning a cross-core kernel round trip into a task switch.
ACK-frequency tuning. Max ACK delay 25 ms → 2 ms, and one ACK per 20 packets instead of every other packet. Each reverse-path ACK costs a datagram plus its own wakeup chain.

Config	Before	After
4 KiB × batch 64, fanout 1	73 MB/s	~508 MB/s (median)
1 KiB × batch 64, fanout 1	75 MB/s	~461 MB/s
16 KiB × batch 64, fanout 1	72 MB/s	~503 MB/s

Latency was unaffected. These changes alter transport acking and task placement and leave request round trips alone.

Defect 2: one run in three, 5–6× slower

The symptom

With the ceiling fixed, identical fresh-process benchmark runs became bimodal: most delivered ~118–120 K msg/s (4 KiB × batch 64), but ~30% landed at 20–24 K. The slow mode was selected near startup, was sticky for the life of the process, and became much more likely under background CPU load.

Everything about it pointed at scheduling. It appeared during scheduling work. A real scheduling defect had just been fixed in the same area: endpoint-to-runtime assignment depended on creation history, which made every second in-process benchmark case slow, a deterministic alternation that had looked random. And the slow mode used 5.7× more system time and 5.7× more context switches per message at the same CPU utilization. That is one kernel round trip per item instead of one per batch, the usual shape of a wakeup lockstep.

A scheduling-based theory was tested directly (keeping the I/O threads runnable through the inter-datagram gap so cross-thread wakes become flag checks) and changed nothing: the slow-run rate was identical with and without it. A fix that does not move the failure rate falsifies the diagnosis, so the theory was dropped and the investigation went back to observation.

The diagnosis

Logging per-connection quinn::ConnectionStats on both ends (FELIX_CONN_STATS_MS) and comparing one fast against one slow run found the difference in a single field on a single connection, the publish connection carrying the offered load:

	Fast run	Slow run
Path MTU at end of run	16354	1200
UDP datagrams for ~240 MB	14,278	166,580
Bytes per datagram	15,744	1,549
Lost packets	0	674, in one burst

The slow run’s timeline shows MTU discovery succeeding and then being un-done:

t=0.25s   mtu=8792    discovery in progress
t=0.50s   mtu=16354   discovery complete, zero loss
t=0.75s   mtu=1200    one ~674-packet loss burst → collapse
t≥0.75s   mtu=1200    pinned for the rest of the run

At MTU 1200 the same byte stream costs ~13.6× the datagrams, and each datagram carries a syscall and a wakeup chain. That accounts for the 5.7× system time per message that had looked like a scheduling defect. An earlier check (“MTU reaches 16354 in both modes”) had been performed on a delivery connection; the collapse hits the connection carrying the load.

The mechanism

Three interlocking behaviors, all in the QUIC layer:

The loss burst is congestion. It has nothing to do with the MTU. At high rate the publish path keeps a multi-megabyte standing queue in the receiver’s UDP socket buffer, within a couple MB of its cap. A scheduling stall of a few milliseconds on the draining side overflows it, and the kernel drops a window of packets. Background CPU load makes stalls, and so the collapse, more likely. If the startup ramp gets through without a burst, steady state is loss-free. That is why the runs were bimodal and the mode was picked at startup.
Quinn’s black-hole detector cannot tell the difference. Every packet in the burst is full-MTU, because that is what a saturated sender’s traffic looks like, and “large packets vanish while small ones survive” is the pattern the detector watches for. It declares an MTU black hole and resets the path MTU to min_mtu (1200 by default), with a 60-second cooldown before discovery may run again.
Recovery is starved by the load itself. Quinn sends MTU probes only when a transmit poll finds nothing else to send. A backlogged publisher never has an empty transmit buffer, so after a collapse no probe is ever sent. The connection stays collapsed for as long as the load lasts, well past the 60-second cooldown.

flowchart LR
    A(["ramp-up at<br/>full MTU"]) e1@--> B["socket buffer<br/>overflow burst"]
    B e2@--> C{{"black-hole detector:<br/>all lost packets are full-MTU"}}
    C e3@--> D["path MTU resets<br/>to min_mtu (1200)"]
    D e4@--> E(["~13× datagrams per byte,<br/>probes starved by backlog"])
    E e5@--> D

    e1@{ animate: true }
    e2@{ animate: true }
    e3@{ animate: true }
    e4@{ animate: true }
    e5@{ animate: true }

    classDef step fill:#e8f0fe,stroke:#4a6fa5,color:#1a2b40
    classDef gate fill:#fdeaea,stroke:#b04a4a,color:#3d1414
    classDef bad fill:#fdf0e3,stroke:#b07d3a,color:#3d2a12
    class A,B step
    class C gate
    class D,E bad

The fix

Loopback is the benchmark’s topology and the one path where the interface itself guarantees a 16 KiB datagram. For loopback peers, connections now start at the loopback MTU and guarantee it, on both the connect and accept sides:

initial_mtu = 16336 (fits IPv4/IPv6 headers within the 16 KiB loopback interface MTU), removing the discovery ramp;
min_mtu = 16336 on macOS (4096 on every other platform, see below). This is the part that matters. The black-hole verdict resets to min_mtu, so with the floor at the real MTU there is nothing to collapse to, and a congestive loss burst is handled as ordinary congestion;
an initial congestion window scaled to the MTU per RFC 9002’s formula. Quinn’s default window is a flat 14,720 bytes, smaller than one 16 KiB segment, which otherwise deadlocks the handshake outright.

The guarantee is conditional on the socket buffers the OS actually granted (~1 MiB), which acts as a proxy for “this host has been tuned”: hosts where Linux silently clamps SO_RCVBUF to a stock net.core.rmem_max (~208 KB) keep the RFC-safe default path instead. An explicit FELIX_INITIAL_MTU disables the special case. For non-loopback paths, where min_mtu must never be raised, the black-hole cooldown drops from 60 s to 2 s (FELIX_MTU_BLACK_HOLE_COOLDOWN_MS), so a spurious collapse on an intermittently loaded connection heals at the next idle gap instead of being pinned for a minute.

Why the guaranteed size is smaller off macOS

Everything above was measured on macOS. Benchmarking Linux later showed the same guarantee stalling delivery completely there: zero events delivered on every sustained batch run. The cause is a platform difference this fix had not accounted for.

Linux UDP GSO carries a whole sendmsg batch inside one IP datagram, so MTU × segments must stay under 65,535. quinn batches up to 10 datagrams, which makes 6,550 bytes the real ceiling. A guaranteed 16,336 needs 163,360 bytes per batch, the kernel refuses it, and quinn only falls back to non-offloaded sends on EIO/EINVAL, so the rejection repeats on every batch and the connection never recovers. macOS is immune because it has no GSO at all: one syscall per datagram, no aggregate, no ceiling.

The guaranteed size is therefore capped at 4,096 off macOS, which holds margin even if quinn’s batch size grows, and which measured fastest on Linux regardless (1.17 GB/s at 4 KiB × batch 64 × fanout 1).

On Linux, the black-hole “misdiagnosis” this section treats as a bug is the only thing that recovers a GSO-oversized connection: collapsing the MTU to 1200 makes the batch fit again. Guaranteeing min_mtu removed that escape hatch. The pin did not cause the loss; it prevented the recovery.

The result

Fresh-process runs, 4 KiB × batch 64 × fanout 1, same machine:

	Before	After
Slow runs	5 of 16, at ~21 K msg/s	0 of 20
Median	~118 K msg/s	~122 K msg/s (~500 MB/s)
Worst ÷ median	5.9×	1.11×

Reverting the fix on the same binary (via FELIX_INITIAL_MTU=1200 and the stock 60 s cooldown) reproduces the slow runs, so the attribution is causal rather than machine drift.

What generalizes

Check that the benchmark measures what you think. The original numbers were inflated by a measurement artifact: any run whose total volume fits inside the 64 MiB QUIC send window is absorbed by buffers before backpressure appears, so the harness reports buffer-fill rate rather than sustained throughput. A 1 KiB run appeared to do 358 MB/s and actually sustained 74.9 MB/s.

flowchart LR
    subgraph absorbed["Run smaller than the send window"]
        direction TB
        A1(["publish 31 MB"]) a1@--> A2["send buffer<br/><small>64 MiB, never fills</small>"]
        A2 a2@--> A3(["reports buffer-fill rate<br/><small>backpressure never appears</small>"])
    end

    subgraph sustained["Run larger than the send window"]
        direction TB
        S1(["publish 312 MB"]) s1@--> S2["send buffer fills"]
        S2 s2@--> S3["backpressure reaches<br/>the publisher"]
        S3 s3@--> S4(["reports sustained rate"])
    end

    a1@{ animation: fast }
    a2@{ animation: fast }
    s1@{ animate: true }
    s2@{ animate: true }
    s3@{ animate: true }

    classDef step fill:#e8f0fe,stroke:#4a6fa5,color:#1a2b40
    classDef bad fill:#fdeaea,stroke:#b04a4a,color:#3d1414
    classDef ok fill:#e9f5ec,stroke:#4a8a5e,color:#16301f
    class A2,S2,S3 step
    class A3 bad
    class A1,S1,S4 ok

The benchmark harness now warns when a throughput run is undersized, and the matrix scales message counts per payload so this cannot silently return.

Build an honest baseline. A minimal store-and-forward relay, the smallest thing that does the broker’s job over two QUIC hops, reached 540 MB/s. That showed the architecture could do about 7× what Felix was achieving, so the gap had to be in Felix’s code rather than in QUIC, the extra hop, or the OS.

Let invariance do the eliminating. The byte-rate invariance across a 16× payload range ruled out more hypotheses in one measurement than the preceding five refactors did.

Distrust a fix that arrives without a mechanism, and a mechanism whose fix changes nothing. Five topology changes were implemented and measured before anyone could explain why the first ceiling existed. All five were wasted. The second defect ran the same trap in reverse: a plausible scheduling theory survived until its direct fix failed to move the failure rate, which falsified it in one experiment.

Treat a bimodal distribution as a state machine. Two clean modes with nothing in between means something discrete latches early and persists. The useful question is what state distinguishes the modes (here, one field, current_mtu, on one connection) rather than why is the benchmark noisy. Averaging across modes, or publishing medians without investigating the spread, would have reported the defect as the product’s performance.

Inspect the connection carrying the load. The MTU had been checked and found healthy, but on a delivery connection. Collecting statistics on every connection until the anomaly could be pinned to one of them turned four rounds of dead ends into a one-line diagnosis.