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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.
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@ -286,6 +286,12 @@ static inline void stream_inc_http_fail_ctr(struct stream *s)
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static inline void stream_add_srv_conn(struct stream *sess, struct server *srv)
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{
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/* note: this inserts in reverse order but we do not care, it's only
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* used for massive kills (i.e. almost never). MT_LIST_ADD() is a bit
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* faster than MT_LIST_ADDQ under contention due to a faster recovery
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* from a conflict with an adjacent MT_LIST_DEL, and using it improves
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* the performance by about 3% on 32-cores.
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*/
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MT_LIST_ADD(&srv->actconns, &sess->by_srv);
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HA_ATOMIC_STORE(&sess->srv_conn, srv);
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}
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