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The recv loop only exited on EAGAIN/error/close, so a peer sending at or above the JS drain rate kept recvmmsg returning data forever and one UDP socket monopolized the thread: timers, every other socket, the timeout sweep and the pre/post callbacks starved until the sender slowed. Remotely inducible against any exposed dgram port since JS per-packet handling is far slower than kernel enqueue at line rate. libuv caps a UDP dispatch at 32 datagrams for exactly this (uv__udp_recvmsg: "Prevent loop starvation when the data comes in as fast as (or faster than) we can read it"); 4 batches of LIBUS_UDP_RECV_COUNT(8) matches that. Readable is level-triggered or persistent on all three backends, so leftover data redelivers next tick. Fixture: receiver burning ~2ms per datagram under a child-process flood; a 20ms interval fired 0 times in 2s before (the watchdog inside the data handler is what terminates the unfixed run: even Bun.sleep's timer starves), 30 times after.
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Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 3 remain after this review. WalkthroughUDP receive processing now limits each readable socket event to 32 datagrams. The change updates Windows receive batching and Linux error handling, documents receive-buffer behavior, and adds UDP and QUIC tests that measure per-iteration packet counts. ChangesUDP receive budget
Priority: ➖ Normal Merge Risk: ⚪ Minimal · up to The receive cap yields between event-loop turns while queued datagrams remain eligible for later delivery, and the Linux guard avoids closing a socket simply because a batch reached the limit. No concrete merge blocker is established. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Beyond the inline nits, I also checked whether exiting the recv loop on the batch limit (instead of EAGAIN) can leave recv_would_block_only/recv_error_surfaced at 0 and trip the Linux EPOLLERR close path at the bottom of the UDP case — it can't: a pending sk_err on an adopted connected socket makes the first recvmmsg fail (setting recv_error_surfaced), and when IP_RECVERR is enabled the MSG_ERRQUEUE drain has already set it, so the batch-limit exit never reaches us_udp_socket_close.
Extended reasoning...
The core loop.c change is a two-line bound on the UDP recv dispatch and matches libuv's 32-datagram cap; the load-bearing claim that leftover data redelivers on the next tick (level-triggered epoll, persistent kqueue EVFILT_READ, re-armed libuv poll) checks out against the surrounding code. The two inline findings are test-diagnostic nits, not correctness issues with the fix. Deferring rather than approving because this is core event-loop C touching all three platform backends.
…ut per file convention
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Updated 8:42 AM PT - Sep 29th, 2026
✅ @robobun, your commit 4d930f3750aaf12662e8aa7c1673cddbe03f6b69 passed in 🧪 To try this PR locally: bunx bun-pr 37103That installs a local version of the PR into your bun-37103 --bun |
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Both earlier nits are addressed — the fixture now races firstPacket against flooder.exited with stderr piped, and the test asserts stderr/stdout before exitCode per the file convention. The C change itself is two lines and mirrors libuv's 32-datagram cap; I verified epoll UDP polls are level-triggered (only the eventfd uses EPOLLET) and kqueue's EVFILT_READ is registered without EV_ONESHOT/EV_CLEAR, so leftover data does redeliver. Deferring only because this is the core native dispatch path — worth a maintainer glance, particularly for the Windows/libuv backend and QUIC's UDP consumers.
Extended reasoning...
Overview
Bounds the UDP readable dispatch in packages/bun-usockets/src/loop.c at 4 iterations of the recvmmsg loop (× LIBUS_UDP_RECV_COUNT=8 → 32 datagrams), matching libuv's uv__udp_recvmsg starvation guard. Adds a subprocess fixture and test in test/js/bun/udp/ that floods a receiver burning ~2ms/datagram and asserts a 20ms interval still fires.
Security risks
The change mitigates a remotely-inducible event-loop starvation (any exposed UDP port). The bound itself cannot introduce a new failure mode: it only causes the loop to break earlier and return to the poller, which re-fires on the next tick since UDP readable interest is level-triggered/persistent. No new attack surface.
Level of scrutiny
High — us_internal_dispatch_ready_poll is the core event-loop dispatch path in vendored native code, reached by every UDP consumer (Bun.udpSocket, node:dgram, QUIC). The change is mechanically trivial (declare int recv_batches = 4, add && --recv_batches to the while condition), and I confirmed the redelivery claim: epoll UDP polls are registered without EPOLLET (epoll_kqueue.c:742 is only the async eventfd), and kqueue EVFILT_READ has no EV_ONESHOT/EV_CLEAR (only EVFILT_WRITE does, epoll_kqueue.c:548,552). LIBUS_UDP_RECV_COUNT = 524288/65536 = 8, so 4×8=32 matches the libuv cap exactly. Still, native loop code with cross-platform poller semantics is the kind of change a maintainer should sign off on.
Other factors
Both nits from my prior review are addressed in commit 700fd13. The fixture's Promise.race loser rejection is handled by Promise.race's own reject handler, so no unhandled-rejection when the flooder is later killed. The test adds ~2s to the file, uses port: 0, is hermetic (loopback only, flooder self-expires at 15s), and follows the sibling subprocess-test convention. CI is still building; the PR description notes the test was deferred to CI for platform coverage.
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I arrived at the same change from a different direction and am standing down in favor of this PR. Three things from that investigation that may be useful here: 1. This is reachable by an ordinary QUIC transfer, not only by a flood. For 2. Linux EPOLLERR interaction. With the budget, the loop can now end without having reached the if (error && !recv_error_surfaced && !recv_would_block_only && !u->closed)now fires when 33 or more datagrams happen to be queued, whereas main keeps reading to 3. A deterministic test for the bound, if it helps with the proof: For what it is worth, I also measured idle-process throughput on a debug build before and after the cap (16 MiB |
…v-budget # Conflicts: # test/js/bun/udp/udp_socket.test.ts
The UDP case of us_internal_dispatch_ready_poll read until recvmmsg returned EAGAIN. A peer that keeps the queue non-empty kept the loop inside that one event, and no timer, immediate or other poll ran until the peer stopped. Count datagrams, as libuv does, and ask recvmmsg for no more than the count that is left, so that an event hands over at most 32. The poll is level-triggered: what is left in the queue raises the next event. A read that stops on its count has not seen EAGAIN and has not seen an error. The Linux close on EPOLLERR leaves that case to the next event. The Windows arm of bsd_recvmmsg reads max_packets datagrams. The tests count the datagrams of each iteration of the event loop.
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changed the title
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Status: new diff pushed (2292940). It replaces the first diff of this PR. How I reproduced it, on release builds of main a4f1429 (Linux x64):
What changed against the first diff:
One decision is open, and it belongs to a maintainer. @dylan-conway, 8133dd1 asks for "a bound that is not a tunable". This PR has the tests and uses libuv's 32 as one constant. Is a fixed count acceptable, or did you mean a bound that the loop derives from something it measures? The Notes of the description give the cost of a flat 32 (a receive ceiling per loop iteration, and more kernel loss on an HTTP/3 listener under load) and one other form. Not run: macOS, FreeBSD, and the final form of the tests on Windows. |
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline findings, I also checked two things and ruled them out: the Windows bsd_recvmmsg path returns i on a partial read and max_packets on a full one, so recv_budget -= npackets counts real datagrams on every backend (including the shared_fd one-per-call clamp); and the new recv_budget > 0 gate on the residual-EPOLLERR close only defers, never suppresses, the close for connected sockets without IP_RECVERR — the pending error still surfaces on the first recv that finds the queue empty, which is the same condition the base branch needed.
Extended reasoning...
The change bounds one UDP readable dispatch in packages/bun-usockets/src/loop.c at 32 datagrams, fixes the Windows bsd_recvmmsg return value to honor max_packets, and adds fixtures/tests under test/js/bun/udp and test/js/node/quic plus a shared loop-iteration helper. It touches a remotely reachable network input path but no auth, crypto or parsing surface. Inline findings are being posted (including an exit-time datagram loss behavior change and an unbounded MSG_ERRQUEUE drain), so a human look is already signalled; this note only records the two adjacent concerns examined and ruled out.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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packages/bun-usockets/src/loop.c— pre-existing, security-relevant: a remote peer can still starve the whole event loop of a Bun UDP receiver by flooding it with ICMP errors, even after this change bounds the data path. The MSG_ERRQUEUE drain at loop.c:993 iswhile (!u->closed)with no budget, and each pass enters JS throughon_recv_error, so a queue refilled faster than JS drains it never exits. Fix: give the error-queue drain the same kind of per-event cap as the data loop (a fixed count ofrecvmsg(MSG_ERRQUEUE)calls per dispatch), relying on level-triggered EPOLLERR to redeliver what is left; this covers both the IPv4 and IPv6 cmsg paths since they share the one loop.Why this was flagged
Every socket Bun creates through Bun.udpSocket or node:dgram passes on_recv_error (src/runtime/socket/udp_socket.rs:710 and :722), so udp.c:238 turns on LIBUS_UDP_LINUX_RECVERR and bsd.c:1665 sets IP_RECVERR. On Linux the kernel queues an error skb for any ICMP unreachable whose embedded header quotes the socket's local address and port; an unconnected socket matches any remote, so the ICMP can be forged from anywhere and is not rate-limited on receipt. When EPOLLERR is reported, loop.c:990-1012 runs
while (!u->closed)calling recvmsg(MSG_ERRQUEUE) and, per error, u->on_recv_error, which enters JS via to_js and call_error_handler (udp_socket.rs:116-120). Nothing counts iterations; the loop ends only when the queue is momentarily empty or the socket closes. A sender that refills the queue faster than one JS callback drains it keeps the loop inside this dispatch, so timers, immediates and other sockets stop, exactly the starvation the PR fixes for the data path at loop.c:1033-1089. The base branch has the same loop, so this is pre-existing, but it is the sibling site of the class…Verification: pre-existing (same class as the starvation this PR fixes, left unbounded in the same dispatch function). Trigger: on Linux, a remote host floods a Bun UDP receiver with forged ICMP errors (e.g. port unreachable) whose embedded IP/UDP header quotes the socket's local address/port, at a rate at or above the per-error JS handling rate. Mechanism verified in… | pre-existing (security-relevant remote…
firstUsable now has a budget of 20 s across all attempts and hands the seconds that are left to the scenario. A run whose datagrams never arrive ends with the budget, not after 20 deadlines of 10 s. The fixture imports bun:internal-for-testing at module scope. The two new tests use the runner's timeout like the other tests of the fixture.
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On the error-queue drain (loop.c:993): it stays unbounded in this PR on purpose, and #44218 tracks it. A count in front of that drain loses inbound data, because the kernel charges queued error reports to the receive buffer of the socket. With 64 reports and 16 datagrams per iteration for 64 iterations, main receives 1024 of 1024 datagrams and a count of 32 receives 80. The Notes of the description have the measurement. The bound of that loop needs its own shape and its own test. The four inline threads are handled in 6d78a04: the retries of the fixtures now have a budget of 20 s in wall time, the fixture imports |
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Review comments at @test/_util/loop-iterations.ts:
- Line 64: Update the remaining-time calculation in the loop-iteration retry
flow to pass the actual nonnegative time left before the deadline, rather than
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| * timer, immediate or other poll runs until it stops. The poll is | ||
| * level-triggered on every backend, so what is left raises the | ||
| * next event. */ | ||
| int recv_budget = LIBUS_UDP_MAX_RECV_PER_EVENT; |
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🔴 Operators of Bun.serve({ http3: true }) and node:quic servers get kernel-side packet loss under concurrent load that the base branch does not have. The cap at packages/bun-usockets/src/loop.c:1033 is per socket, and a QUIC listener is one socket for every connection, so 32 packets per loop iteration is the whole server's inbound ceiling. Fix: keep the bound where on_data runs user JS, but let QUIC sockets drain what the base drained, e.g. a per-socket budget that quic.c raises, a time-bounded continuation past 32, or a larger SO_RCVBUF on QUIC sockets. The PR's Downsides put the loss at 6.5 % to 16.2 % for 64 uploads; the author calls it accepted, but us_quic_udp_on_data runs no user code.
Why this was flagged
An HTTP/3 server started with Bun.serve({ http3: true }) listens through one QUIC UDP socket for all its connections (packages/bun-uws/src/Http3Context.h:100 calls us_quic_socket_context_listen, packages/bun-usockets/src/quic.c:1017 creates the single us_udp_socket_t); node:quic listen() does the…
Verification: normal — acknowledged in diff: the PR description's "Downsides" section states "An HTTP/3 listener is one socket. 64 concurrent uploads: kernel loss 6.5 % to 16.2 %, CPU per MiB +10 %" and the Notes leave "the form of the bound" as an open question; the mechanism the note describes is accurate and nothing in the code bounds it further. Triggering condition: an HTTP/3 (`Bun.serve({ http3:…
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This cost is real and the Downsides and the Notes of the description state it with the numbers. The bound applies to the QUIC sockets on purpose, and the choice of its form is the open question for a maintainer at the end of the description.
Why the same bound: us_quic_udp_on_data runs no JS, but each packet goes through lsquic_engine_packet_in (decrypt, frame parse, stream buffering) before the event ends, and loop_post then runs process_conns over all of it. The starvation was reproduced on node:quic, not only on node:dgram: a 4 MiB body next to a loop that blocks 20 ms between turns read 181 to 555 packets in one turn on main (comment above, Aug 11). A budget that quic.c raises brings that back. Node's QUIC endpoint receives through libuv's uv_udp_t and so has the same count per event.
What reduces the loss on a listener without moving the bound: a larger receive buffer for the QUIC sockets (quic.c sets none, so the listener has the default 208 KiB for all its connections). The Notes list it as the follow-up that comes after the bound, because on main a larger buffer makes one event longer. The other form in the Notes, 32 as a floor and then a short time bound, also removes the ceiling for a cheap handler. Both are small changes on top of this one. I leave this thread open for the maintainer's answer on the form.
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Measurements for the open question on the form of the bound I measured the count on release builds in parallel with this PR. The native change I arrived at for the data loop is the one that is here now (a count of datagrams, the exact clamp, All numbers: Linux x64, kernel 7.0, release builds, loopback. The host load average was between 250 and 800 in every round, so each table has an A/A row (the same binary twice) as its noise floor. Ratios are the median of 12 paired and interleaved rounds with [p25..p75]. CPU is user + system time of the process. 1. The QUIC sockets: 32 costs, 1024 does not
A count against no count, 1 MiB uploads,
This PR's head against the head plus one commit that gives the QUIC sockets 1024 (branch below). Two ordinary release builds, no environment switch.
The direction is the same in every cell: less CPU with 1024. The size is smaller than in the first table, and the CPU ranges overlap the A/A ranges in every cell. The datagram counts are lower in 4 of 5 cells. At 16 and 64 connections and for the small GETs their ranges overlap the A/A ranges by 0.01 or less. I cannot say why the two tables differ in size. The description's own number for 64 uploads is +10% CPU per MiB. What 1024 gives up. The description measures a loop lag p99 of 10.1 ms on main and 1.8 ms with 32 for an HTTP/3 listener under 64 uploads. With 1024 an event reads what the buffer holds, as on main, so that gain goes away. I did not measure loop lag. A flood still stops after 1024 datagrams. The branch. One commit on top of 4d930f3: farm/7c2d84da/udp-recv-budget...robobun/69061535/udp-quic-recv-budget
This does not answer the question about "a bound that is not a tunable". It makes the question larger: the count becomes a field with a setter in place of one constant. The setter is C only, its one caller passes a compile-time constant, and JS has no option. @dylan-conway, this is data for that decision. It is not a request to change the PR. 2.
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| receiver | rate, other work per loop turn | Bun 1.4.2 (no count) | this PR (32) | count of 128 | node v26.3.0 |
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Bun.udpSocket |
100 per ms, 0.5 ms | 1.8% [1.0..5.3] | 48.8% [47.7..49.2] | 0.6% | |
Bun.udpSocket |
20 per ms, 2 ms | 0.5% [0.0..3.7] | 27.4% [25.5..30.4] | 0.2% | |
node:dgram |
100 per ms, 0.5 ms | 0.8% [0.3..1.4] | 50.1% [49.0..52.6] | 57.2% [56.0..59.7] | |
node:dgram |
20 per ms, 2 ms | 2.0% [0.2..6.5] | 29.0% [27.4..31.5] | 34.0% [32.4..34.8] |
The column for 128 is from an earlier build with the same data loop in which Bun.udpSocket started at 128 (12 rounds). Main (9f70da0) lost 0.6% and 0.1% in those runs.
The Downsides already name this ceiling. What the table adds: for node:dgram the loss is close to what node has. For Bun.udpSocket it is node's count on a Bun API, and 128 had no loss in these two cells. A count is still a ceiling: a rate above 128 per loop turn loses at 128 too. With the field of the branch, Bun.udpSocket can start at 128 while node:dgram and node:quic ask for 32. That part is not on the branch. It needs a private option from dgram.ts to udp_socket.rs, and it changes the expected counts of the Bun.udpSocket tests and the line in udp.mdx.
3. node:quic at 32 when the process has other work
One stream between two processes. From the earlier build (node:quic at 32, same data loop) against main 9f70da0, 10 paired rounds (8 for the idle cubic row).
| transfer | time, 32 / main | A/A |
|---|---|---|
| 16 MiB, both idle | x0.871 [0.728..1.024] | x1.022 |
| 4 MiB, receiver works 20 ms per loop turn | x1.030 [0.979..1.052] | x0.986 |
| 4 MiB, sender works 20 ms per loop turn | x0.950 [0.896..1.118] | x0.988 |
16 MiB, both idle, cc: "cubic" |
x0.993 [0.933..1.163] | x1.037 |
4 MiB, receiver works 20 ms per loop turn, cc: "cubic" |
x2.580 [2.343..2.777] | x1.010 |
4 MiB, sender works 20 ms per loop turn, cc: "cubic" |
x3.541 [2.696..5.225] | x0.994 |
- In the idle 16 MiB transfer the busiest loop turn on main read 61 packets on the receiver and 859 on the sender. With 32 it is 32 and 32. The longest loop gap of the sender goes from 730 ms to 127 ms.
- The two last rows are a cost that the Downsides do not have yet.
4. The error-queue drain
My version had a count of 32 in front of the error-queue drain, with an SO_ERROR read at the stop. I dropped it for the reason in #44218. My tests queued one burst of 100 reports (32,32,32,4) and no sustained rate, so they did not see the loss of inbound datagrams. I did not measure the sustained case myself.
Method and the other rows
Builds.
- First table, counts 32, 64 and 128: main a4f1429 with the count of the receive loop read from an environment variable. That build had no clamp of the last batch, so an event handed over 32 to 39 datagrams at 32.
- First table, 4 MiB rows: main 9f70da0 with the per-socket count, the count and the receive buffer of the QUIC sockets read from the environment. The server counts the datagrams it sends and the events that stop at the count.
- Second table: 4d930f3 (head of this PR) and e53d4e1 (the branch). Datagrams are the
OutDatagramscounters of/proc/net/snmpand/proc/net/snmp6around the run, so they count both directions. perf,straceandvalgrindare not installed in the container, andperf_event_openreturnsEPERM.
More rows of the first table (count against no count, total CPU per request):
| cell | A/A | 32 | 64 | 128 | 256 | 512 | 1024 |
|---|---|---|---|---|---|---|---|
| 1 MiB uploads, 4 connections | x0.954 | x1.001 | x1.082 | x0.980 | |||
| 1 MiB uploads, 32 connections | x0.994 | x1.031 | x0.937 | x0.968 | |||
| 1 MiB uploads, 8 connections, second build | x0.998 | x1.041 | x0.988 | x1.017 | x1.041 | ||
| 1 MiB uploads, 16 connections, second build | x1.046 | x1.122 | x1.007 | x1.057 | x1.017 | ||
| 1 MiB uploads, 32 connections, second build | x1.040 | x0.985 | x0.985 | x1.034 | x0.990 | ||
| 1 MiB downloads, 8 connections | x0.914 | x0.937 | x0.924 | x0.974 | |||
| GET of 2 bytes, 256 connections, second build | x1.019 | x0.933 | x1.157 | x1.019 | x1.027 |
The second build has one row that disagrees with the first: 128 at 16 connections is x1.122 [1.037..1.207] there (58 of 1597 events stopped) and x1.005 in the first build.
Kernel loss in the second table (RcvbufErrors over OutDatagrams, all sockets of the run, median): 8 uploads 2.2% head and 3.1% branch. 16 uploads 5.5% and 4.4%. 64 uploads 10.5% and 9.7%. Small GETs 7.2% and 4.3%. The A/A runs of the head gave 2.5%, 6.4%, 10.0% and 8.5%.
Instructions in the UDP case for one readable event, single step in gdb on release builds: main 122, the earlier version with the same field, clamp and counter 142 (1 or 8 datagrams). 154 and 182 for 9 datagrams. No new call. I did not count the head or the branch.
Debug ASAN build. The host was too loaded to judge the suites there: tests of every file timed out at the default 5 s, the 32 tests of this PR among them (dgram.test.ts 4 of 5, quic-endpoint.test.ts 1). Alone, the two udp_socket.test.ts tests took 4.1 and 4.3 s.
Release builds. One test that this PR does not touch fails at times on both builds: server.stop(true) after an await inside an H3 handler sends CONNECTION_CLOSE in serve-http3.test.ts. The fetch rejects with HTTP3StreamReset before the test waits for it, so the run reports an unhandled rejection. Alone, 25 runs: 2 failures on the head, 4 on the branch. The Notes of the description cite #44146 for an unhandled rejection of this kind. I did not run main. Nothing else failed on the release builds.
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Problem
messagelistener and one flooding sender, node v26.3.0 fires a 6 s timer. Bun never does.us_internal_dispatch_ready_pollreads until EAGAIN (packages/bun-usockets/src/loop.c:1029).Fix
test/js/bun/udp/andtest/js/node/quic/quic-endpoint.test.ts. All fail on main.Background
send()from an earlier callback takes the pending ICMP error. Main kept such a socket open because its read ran to EAGAIN.recvmmsgcalls (Worker / worker_threads: WebCore-shaped lifetimes, joined threads, one ordered VM teardown #37075) for want of a test. A count for the error-queue drain loses inbound data (udp: an error handler that sends again to a closed local port holds the event loop (Linux) #44218).Downsides
epoll_pwait2calls, was 101.Notes
Open question for a maintainer: the form of the bound. 8133dd1 says the cap "will come back with a reproducing test and a bound that is not a tunable". This PR brings the tests. Its bound is libuv's count as one constant,
LIBUS_UDP_MAX_RECV_PER_EVENT, with no option. Two forms came out of the review:node:dgramdelivers a backlog as node does. The cost is the receive ceiling of the Downsides.The constant also applies to the QUIC sockets (HTTP/3 and
node:quic). A change of form or value is small: the constant, the loop condition, and the expected counts of the tests.Where the report comes from. No user reported this. A fuzzing run found it. #37075 lists it as a known follow-up ("A UDP socket whose receive buffer never drains ... keeps its event loop from running anything else"). The first diff of this PR counted
recvmmsgcalls (4). That gave a cluster-shared socket 4 datagrams per event and closed an adopted connected socket without an event from 25 queued datagrams.Not in this PR, with trackers. The same function has three more loops that a peer can keep running:
loop.c:522): usockets: bound the accepts of one listener readiness event #44198loop.c:791): usockets: one TCP connection whose reads stay full holds the event loop #44220loop.c:993): udp: an error handler that sends again to a closed local port holds the event loop (Linux) #44218. A count in front of that drain stops it, but the kernel charges queued reports to the receive buffer. With 64 reports and 16 datagrams per iteration for 64 iterations, main receives 1024 of 1024 datagrams and a count of 32 receives 80.#34037 carries the same UDP loop and close condition in
src/usockets/udp.rsand needs the same change. #35922 edits lines around the Linux close. Therecv_budget > 0term is still needed with it.Repro.
node:dgramreceiver whosemessagelistener works 3 ms, one child process that sends without pause, a 100 ms interval and a 6 s timer. Release builds, 2 to 3 runs each. main: a bail-out insidemessageends the run at 15 s, 0 of 150 ticks. This PR: the timer fires at 6008 to 6071 ms, 32 to 35 of 60 ticks. node v26.3.0: 6027 to 6095 ms, 31 of 60 ticks.sendMany(). main delivers100in one iteration. This PR and node deliver32,32,32,4.Tests. The fixtures queue a backlog before the loop polls and count datagrams by the iteration number of the loop (
getEventLoopStats().iteration). They end on a condition or on a deadline in wall time. A run whose backlog did not reach the bound is set up again, at most 20 times and within 20 s in all. The tests assert the total and the largest count of one iteration.udp_socket.test.ts: of a backlog of 10010032,32,32,4udp_socket.test.ts: when the data handler queues more on its own socket (7 queued, then 100)10732,32,32,11dgram.test.ts: of a backlog of 10010032,32,32,4dgram.test.ts: of each socket (two sockets, 100 each)20064,64,64,8dgram.test.ts: of an adopted descriptor, stale EPOLLERR, 40 queued (Linux)40, open32,8, opendgram.test.ts: of a socket with a full receive buffer, stale EPOLLERR (Linux)136, open32,32,32,32,8, opendgram.test.ts: cluster, a shared socket (POSIX)10032,32,32,4quic-endpoint.test.ts: hands over at most 32 packets (largest iteration of a 1 MiB upload)Each part of the change has a test that fails without it (debug builds with one line changed):
recv_budget > 0in the close: both stale EPOLLERR tests get 32 datagrams and a closed socket, with nocloseand noerrorevent.max_packets: the test with the handler that queues more gets39,32,32,4.The fixture of the first diff (
udp-flood-starvation-fixture.ts, on this branch only) counted timer ticks over 2 s of wall time. It is gone.Platforms.
32,32,32,4(canary a4f1429:100), handler that queues more32,32,32,11, two sockets64,64,64,8.udp_socket.test.tsanddgram.test.tspassed. The final form of the tests was not run on Windows.EVFILT_READwithEV_ADDonly (kqueue_changeinepoll_kqueue.c), so data that stays queued raises the filter again.Measurements. Four numbers below are marked "earlier build". They come from an earlier form of this diff, whose data loop has the same source as the final one. They were not measured again on the final build. Every other number is from the final build.
Release builds of the merge base and of this PR, linked from the same objects. Only the usockets C files differ. Linux x64. The host had a load average of 600 to 800, so times are noisy. Counts are exact.
strace,perfandvalgrindare not available in the container (perf_event_paranoidis 4). Syscall counts come from a ptrace counter.Syscalls per 100 bursts of N datagrams (
epoll_pwait2/recvmmsg):Datagrams lost in the kernel when a sender on the same loop queues K datagrams per iteration (64 bytes each, default receive buffer):
The count is for each datagram that the socket reads. A datagram that the consumer discards, for example one from an address on the
receiveBlockListofnode:dgram, takes a place of the 32 too.HTTP/3 listener, 64 client processes with one connection each, 1 MiB uploads for 8 s, server on 2 cores, 4 runs of each build in the order main, PR, PR, main. Median and range:
quic.csets no receive buffer size, so the listener has the default 208 KiB for all its connections. A larger buffer for the QUIC sockets is a follow-up. It has to come after the bound, because on main a larger buffer makes one event longer.Other numbers:
epoll_pwait21,futexabout 1.8). 50 bursts of 192:futex195 to 646. WithMIMALLOC_SCAVENGER=0: 78 to 100. Thefutexcalls belong to the scavenger hand-off of every loop tick, not to this change.perf, novalgrind).us_internal_dispatch_ready_pollgrows from 4230 to 4329 bytes.sizeof the release binary: text 80,686,631 bytes on both.Bun.servetofetchin one process: ACK datagrams 145 to 388, iterations of the receiving thread 144 to 388, bytes equal. CPU per request +3.6 % (paired median of 20 interleaved rounds of 5 requests). Main against main: median difference 27 %.struct us_udp_socket_tis unchanged.Suites run.
udp_socket.test.ts232 pass.dgram.test.ts67 pass.quic-endpoint.test.ts8 pass.udp_socket_recv_flags.test.ts2 pass.quic-sni.test.tsandquic-stream.test.tspass.test-dgram-*.jsandtest-cluster-dgram-*.jsoftest/js/node/test/parallelpass.fetch-http3-client.test.ts65 pass,fetch-http3-adversarial.test.ts27 pass.serve-http3.test.ts, release, final build: 3 runs, 72 pass and 0 fail in each (12 to 30 s). main: 4 runs, 72 pass and 0 fail in each (13 to 40 s). A fourth run of the final build had not finished when this was written.serve-http3.test.ts, release, an earlier form of this diff with the same data loop, 11 interleaved runs of each build while the host load was above 600: 31 failed tests on main, 33 with the change, never the same set twice. All are 5 s timeouts, except one unhandled rejection in a test that expects that rejection (test(serve-http3): attach the fetch handlers before the wait for STOPPED #44146).node-dgram.test.js: the IPv6 membership test fails on main and with this PR in this container.What a caller can see.
bsd_recvmmsgignoredmax_packets. It now reads that many datagrams, so the count is exact there too.Self-review. 24 concerns were raised.
docs/runtime/networking/udp.mdx. Anode:quictest covers the endpoint's socket.us_internal_dispatch_ready_pollthat states one rule for all loops: this PR bounds one of them, and the others have their own changes. A test with a blocked sender: it would assert a cost, not a behaviour that has to hold.Earlier work. The count per datagram and the close term come from e852af2. #29473 proposed a count of 32 for the error-queue drain.
no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/udp/udp_socket.test.ts, test/js/bun/udp/dgram.test.ts