CLEANUP: stream: explain why we queue the stream at the head of the server list
In stream_add_srv_conn() MT_LIST_ADD() is used instead of MT_LIST_ADDQ(),
resulting in the stream being queued at the end of the server list. This
has no particular effect since we cannot dump the streams on a server,
and this is only used by "shutdown sessions" on a server. But it also
turns out to be significantly faster due to the shorter recovery from
the conflict with an adjacent MT_LIST_DEL(), thus it remains desirable
to use it, but at least it deserves a comment.
In addition to this, it's worth mentioning that this list should creates
extreme contention with threads while almost never used. It should be
made per-thread just like the global streams list.
diff --git a/include/haproxy/stream.h b/include/haproxy/stream.h
index d4d202b..8daf2ad 100644
--- a/include/haproxy/stream.h
+++ b/include/haproxy/stream.h
@@ -286,6 +286,12 @@
static inline void stream_add_srv_conn(struct stream *sess, struct server *srv)
{
+ /* note: this inserts in reverse order but we do not care, it's only
+ * used for massive kills (i.e. almost never). MT_LIST_ADD() is a bit
+ * faster than MT_LIST_ADDQ under contention due to a faster recovery
+ * from a conflict with an adjacent MT_LIST_DEL, and using it improves
+ * the performance by about 3% on 32-cores.
+ */
MT_LIST_ADD(&srv->actconns, &sess->by_srv);
HA_ATOMIC_STORE(&sess->srv_conn, srv);
}