From 6fa922562893cc5347da3568cf11a22afe5b1b60 Mon Sep 17 00:00:00 2001 From: Ubuntu Date: Mon, 1 Mar 2021 06:21:47 +0000 Subject: [PATCH] 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. --- include/haproxy/stream.h | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/include/haproxy/stream.h b/include/haproxy/stream.h index d4d202b2f..8daf2adcc 100644 --- a/include/haproxy/stream.h +++ b/include/haproxy/stream.h @@ -286,6 +286,12 @@ static inline void stream_inc_http_fail_ctr(struct stream *s) 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); }