http2: increase default window sizes

[mcollina]

Increase the default HTTP/2 stream window from 64KB (65535) to 4MB
(4194304) and the default local connection window to 32MB (33554432).

The default 64KB window limits throughput on high-latency connections
to window_size / RTT. With a 250ms RTT, throughput is limited to
256KB/s. The new defaults improve throughput to 16MB/s (128Mbps)
for the stream window and 128MB/s (1Gbps) for the connection window.

Fixes #38426

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/http2
• @nodejs/net
• @nodejs/sqlite

[pimterry]

There's a pre-existing issue that this exposes: you can't configure this in advance. You can only control the connection window with session.setLocalWindowSize once you have a session, but at that point this initial WINDOW_UPDATE has already been sent, and shrinking the window isn't effective as shown in the tests here (it just blocks further updates, it can't reduce what's already been sent). The stream window is configurable in advance (it's a SETTINGS parameter) but this connection window isn't.

That already existed implicitly but it was kind of OK when the initial window was small, but it's a real problem with a very large initial window I think, since there's basically no way to override this to make it any smaller again if you need to.

Can we add client & server options for this, so it can be configured before the first WINDOW_UPDATE is sent somehow?

[pimterry]

Nit: would be nicer to extract this as a constant

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.14%. Comparing base (ace91de) to head (bd4cbd3).
⚠️ Report is 68 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_http2.cc	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64623      +/-   ##
==========================================
- Coverage   90.23%   90.14%   -0.09%     
==========================================
  Files         741      741              
  Lines      241681   242143     +462     
  Branches    45547    45560      +13     
==========================================
+ Hits       218075   218279     +204     
- Misses      15141    15373     +232     
- Partials     8465     8491      +26     

Files with missing lines	Coverage Δ
lib/internal/http2/core.js	94.87% <100.00%> (+0.06%)	⬆️
src/node_http2.h	92.26% <ø> (+0.21%)	⬆️
src/node_http2.cc	81.82% <0.00%> (-0.35%)	⬇️

... and 93 files with indirect coverage changes

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[mcollina]

Both addressed:

1. Extracted DEFAULT_SETTINGS_LOCAL_CONNECTION_WINDOW_SIZE = 33554432 as a named constant in src/node_http2.h and used it in src/node_http2.cc.
2. The pre-existing issue about not being able to configure the connection window in advance is noted — the initial WINDOW_UPDATE is sent at session creation before setLocalWindowSize can be called. This is a known limitation of the current API.

[RafaelGSS]

Wouldn't it be semver-major PRs that contain breaking changes and should be released in the next major version. ?

[mcollina]

yes