bneradt commented on code in PR #13554:
URL: https://github.com/apache/trafficserver/pull/13554#discussion_r3854801149
##########
src/iocore/net/QUICNetVConnection.cc:
##########
@@ -688,6 +688,23 @@ void
QUICNetVConnection::_handle_write_ready()
{
if (quiche_conn_is_established(this->_quiche_con)) {
+ // Count real contention for THIS event before deciding its budget, rather
than
+ // sizing it from a previous event's count -- a stale count can be wrong
in either
+ // direction whenever contention swings between events, not just on the
first
+ // event. writable() is a pure, side-effect-free snapshot (verified against
+ // quiche's source), so draining it twice costs one extra O(n) collect and
n extra
+ // FFI calls, n bounded by this connection's stream limit -- cheap next to
the
+ // per-stream work that follows.
+ quiche_stream_iter *probe =
quiche_conn_writable(this->_quiche_con);
+ uint64_t probe_id = 0;
+ size_t writable_count = 0;
+ while (quiche_stream_iter_next(probe, &probe_id)) {
+ ++writable_count;
Review Comment:
**[P2] Count streams that actually have data to send.**
`quiche_conn_writable()` reports every unfinished stream with flow-control
capacity, not just streams with queued application data ([quiche 0.28
`StreamMap::writable()`](https://github.com/cloudflare/quiche/blob/0.28.0/quiche/src/stream/mod.rs#L628-L633),
[`Stream::is_writable()`](https://github.com/cloudflare/quiche/blob/0.28.0/quiche/src/stream/mod.rs#L786-L793)).
For example, with 20 open request streams but response data available on only
one, `writable_count` is 20, so the active stream receives the 16 KB floor
while the other 19 consume none of their budgets. This repeats every event and
effectively restores the original per-stream limitation. Please base the
divisor on ATS streams whose `QUICStream::has_data_to_send()` is true; ideally
collect that filtered snapshot once and reuse it for the send loop. The QMux
path has the same issue.
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