http: use intrusive lists in ConnectionsList by mcollina · Pull Request #65296 · nodejs/node · GitHub

Every HTTP message was performing multiple erase and insert operations on the two std::set instances in ConnectionsList, which are ordered by a key (last_message_start_) that mutates on every message. In a hello-world server profile this showed up as ~2.2% of total CPU cycles spent in red-black tree rebalancing plus tree-node allocations, all on the per-request hot path.

This PR replaces both sets with intrusive doubly-linked lists:

A node is linked to itself when it is not in a list, so Remove() is idempotent and the destructor unlinks automatically, which also removes the old "Pop from the lists BEFORE resetting last_message_start_" footgun.
Membership in the "all connections" list no longer changes per message, so the Pop+Push pair in on_message_begin/on_message_complete is gone entirely.
Updating the active list is now a single O(1) unlink + tail append with no allocations.
Appending to the tail keeps the active list ordered by last_message_start_ because uv_hrtime() is monotonic, so expired() semantics are unchanged. The arrays returned by all()/idle()/active() are now in insertion order instead of comparator order; none of their consumers depend on ordering.

Benchmark results (Linux, i7-7700, server and benchmarker pinned to separate cores):

                                                                                           confidence improvement accuracy (*)   (**)  (***)
http/simple.js duration=5 chunkedEnc=0 c=50 chunks=1 len=4 type='bytes' benchmarker='wrk'          **      4.06 %       ±2.41% ±3.23% ±4.25%
http/simple.js duration=5 chunkedEnc=0 c=500 chunks=1 len=4 type='bytes' benchmarker='wrk'                 2.35 %       ±2.45% ±3.29% ±4.34%

A separate interleaved wrk A/B (hello-world server, -t2 -c50, 5 alternating pairs) shows +4.9% (26,599 → 27,914 req/s), with the patched binary winning every pair. A perf profile of the patched binary confirms the _Rb_tree symbols are gone and malloc traffic in the parser path is roughly halved.

This PR was prepared with the help of AI assistance.

Review requested:

@nodejs/http
@nodejs/net

Codecov Report

❌ Patch coverage is 86.27451% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.12%. Comparing base (347e266) to head (0cc64e0).
⚠️ Report is 269 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_http_parser.cc	86.27%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65296      +/-   ##
==========================================
- Coverage   90.30%   90.12%   -0.19%     
==========================================
  Files         759      752       -7     
  Lines      248294   251826    +3532     
  Branches    46860    47356     +496     
==========================================
+ Hits       224220   226954    +2734     
- Misses      15516    16225     +709     
- Partials     8558     8647      +89

Files with missing lines	Coverage Δ
src/node_http_parser.cc	84.72% <86.27%> (-0.26%)	⬇️

... and 174 files with indirect coverage changes

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Have you benchmarked against an unordered_set or just a plain vector?

I suspect those would be more cache friendly and avoid pointer chasing.

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----------------------------------- PR info ------------------------------------
Title      http: use intrusive lists in ConnectionsList (#65296)
Author     Matteo Collina <matteo.collina@gmail.com> (@mcollina)
Branch     mcollina:http-connectionslist-intrusive -> nodejs:main
Labels     c++, http_parser, author ready, needs-ci, commit-queue
Commits    1
 - http: use intrusive lists in ConnectionsList
Committers 1
 - Matteo Collina <hello@matteocollina.com>
PR-URL: https://github.com/nodejs/node/pull/65296
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/65296
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
--------------------------------------------------------------------------------
   ℹ  This PR was created on Sat, 15 Aug 2026 04:05:15 GMT
   ✔  Approvals: 2
   ✔  - Robert Nagy (@ronag) (TSC): https://github.com/nodejs/node/pull/65296#pullrequestreview-4943029706
   ✔  - Paolo Insogna (@ShogunPanda) (TSC): https://github.com/nodejs/node/pull/65296#pullrequestreview-4949195971
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   ℹ  Last Full PR CI on 2026-08-16T15:24:03Z: https://ci.nodejs.org/job/node-test-pull-request/75886/
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https://github.com/nodejs/node/actions/runs/32009841931

Every HTTP message was performing multiple erase and insert operations on two std::set instances ordered by a mutating key, showing up as ~2% of CPU cycles in a hello-world server profile due to red-black tree rebalancing and node allocations. Replace both sets with intrusive doubly-linked lists. Membership in the list of all connections no longer changes per message, and updating the active connections list is now O(1) with no allocations. Appending to the tail keeps the active list ordered by last_message_start_ because uv_hrtime() is monotonic. Signed-off-by: Matteo Collina <hello@matteocollina.com>

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----------------------------------- PR info ------------------------------------
Title      http: use intrusive lists in ConnectionsList (#65296)
Author     Matteo Collina <matteo.collina@gmail.com> (@mcollina)
Branch     mcollina:http-connectionslist-intrusive -> nodejs:main
Labels     c++, http_parser, author ready, needs-ci, commit-queue
Commits    1
 - http: use intrusive lists in ConnectionsList
Committers 1
 - Matteo Collina <hello@matteocollina.com>
PR-URL: https://github.com/nodejs/node/pull/65296
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: James M Snell <jasnell@gmail.com>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/65296
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: James M Snell <jasnell@gmail.com>
--------------------------------------------------------------------------------
   ℹ  This PR was created on Sat, 15 Aug 2026 04:05:15 GMT
   ✔  Approvals: 3
   ✔  - Robert Nagy (@ronag) (TSC): https://github.com/nodejs/node/pull/65296#pullrequestreview-4943029706
   ✔  - Paolo Insogna (@ShogunPanda) (TSC): https://github.com/nodejs/node/pull/65296#pullrequestreview-4969028193
   ✔  - James M Snell (@jasnell) (TSC): https://github.com/nodejs/node/pull/65296#pullrequestreview-4951932271
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   ℹ  Last Full PR CI on 2026-08-17T08:31:16Z: https://ci.nodejs.org/job/node-test-pull-request/75919/
   ⚠  Commits were pushed after the last Full PR CI run:
   ⚠  - http: use intrusive lists in ConnectionsList
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--------------------------------------------------------------------------------
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https://github.com/nodejs/node/actions/runs/32254812825

CI: https://ci.nodejs.org/job/node-test-pull-request/76274/

Landed in 0449521

Every HTTP message was performing multiple erase and insert operations on two std::set instances ordered by a mutating key, showing up as ~2% of CPU cycles in a hello-world server profile due to red-black tree rebalancing and node allocations. Replace both sets with intrusive doubly-linked lists. Membership in the list of all connections no longer changes per message, and updating the active connections list is now O(1) with no allocations. Appending to the tail keeps the active list ordered by last_message_start_ because uv_hrtime() is monotonic. Signed-off-by: Matteo Collina <hello@matteocollina.com> PR-URL: #65296 Reviewed-By: Robert Nagy <ronagy@icloud.com> Reviewed-By: Paolo Insogna <paolo@cowtech.it> Reviewed-By: James M Snell <jasnell@gmail.com>

nodejs-github-bot added c++ Issues and PRs that require attention from people who are familiar with C++. http_parser Issues and PRs related to the HTTP Parser dependency or the http_parser binding. needs-ci PRs that need a full CI run. labels Aug 15, 2026

mcollina force-pushed the http-connectionslist-intrusive branch from e7c6580 to 41044a2 Compare August 15, 2026 04:07

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2026-08-26, Version 26.8.0 (Current) #65551

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