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CLEANUP: stream: explain why we queue the stream at the head of the server list

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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.
This commit is contained in:
Ubuntu 2021-03-01 06:21:47 +00:00 committed by Willy Tarreau
parent 2f67e54dca
commit 6fa9225628
1 changed files with 6 additions and 0 deletions
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6
include/haproxy/stream.h
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@ -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) 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); MT_LIST_ADD(&srv->actconns, &sess->by_srv);
HA_ATOMIC_STORE(&sess->srv_conn, srv); HA_ATOMIC_STORE(&sess->srv_conn, srv);
} }