MINOR: backend: add hash-balance-factor option for hash-type consistent

0 will mean no balancing occurs; otherwise it represents the ratio
between the highest-loaded server and the average load, times 100 (i.e.
a value of 150 means a 1.5x ratio), assuming equal weights.

Signed-off-by: Andrew Rodland <andrewr@vimeo.com>

doc/configuration.txt

@@ -2205,6 +2205,32 @@ balance url_param <param> [check_post]
   See also : "dispatch", "cookie", "transparent", "hash-type" and "http_proxy".
 
 
+balance-factor <factor>
+  Specify the balancing factor for bounded-load consistent hashing
+  May be used in sections :   defaults | frontend | listen | backend
+                                 yes   |    no    |   no   |   yes
+  Arguments :
+    <factor> is the control for the maximum number of concurrent requests to
+             send to a server, expressed as a percentage of the average number
+	     of concurrent requests across all of the active servers.
+
+  Specifying a "balance-factor" for a server with "hash-type consistent"
+  enables an algorithm that prevents any one server from getting too many
+  requests at once, even if some hash buckets receive many more requests than
+  others. Setting <factor> to 0 (the default) disables the feature. Otherwise,
+  <factor> is a percentage greater than 100. For example, if <factor> is 150,
+  then no server will be allowed to have a load more than 1.5 times the average.
+  If server weights are used, they will be respected.
+
+  If the first-choice server is disqualified, the algorithm will choose another
+  server based on the request hash, until a server with additional capacity is
+  found. A higher <factor> allows more imbalance between the servers, while a
+  lower <factor> means that more servers will be checked on average, affecting
+  performance. Reasonable values are from 125 to 200.
+
+  See also : "balance" and "hash-type".
+
+
 bind [<address>]:<port_range> [, ...] [param*]
 bind /<path> [, ...] [param*]
   Define one or several listening addresses and/or ports in a frontend.
@@ -3358,7 +3384,7 @@ hash-type <method> <function> <modifier>
   default function is "sdbm", the selection of a function should be based on
   the range of the values being hashed.
 
-  See also : "balance", "server"
+  See also : "balance", "balance-factor", "server"
 
 
 http-check disable-on-404

include/types/lb_chash.h

@@ -30,6 +30,7 @@ struct lb_chash {
 	struct eb_root act;	/* weighted chash entries of active servers */
 	struct eb_root bck;	/* weighted chash entries of backup servers */
 	struct eb32_node *last;	/* last node found in case of round robin (or NULL) */
+	int balance_factor;	/* load balancing factor * 100, 0 if disabled */
 };
 
 #endif /* _TYPES_LB_CHASH_H */

src/cfgparse.c

@@ -1983,6 +1983,7 @@ void init_default_instance()
 	defproxy.maxconn = cfg_maxpconn;
 	defproxy.conn_retries = CONN_RETRIES;
 	defproxy.redispatch_after = 0;
+	defproxy.lbprm.chash.balance_factor = 0;
 
 	defproxy.defsrv.check.inter = DEF_CHKINTR;
 	defproxy.defsrv.check.fastinter = 0;
@@ -2825,6 +2826,7 @@ int cfg_parse_listen(const char *file, int linenum, char **args, int kwm)
 
 		if (curproxy->cap & PR_CAP_BE) {
 			curproxy->lbprm.algo = defproxy.lbprm.algo;
+			curproxy->lbprm.chash.balance_factor = defproxy.lbprm.chash.balance_factor;
 			curproxy->fullconn = defproxy.fullconn;
 			curproxy->conn_retries = defproxy.conn_retries;
 			curproxy->redispatch_after = defproxy.redispatch_after;
@@ -5958,6 +5960,19 @@ stats_error_parsing:
 			}
 		}
 	}
+	else if (strcmp(args[0], "hash-balance-factor") == 0) {
+		if (*(args[1]) == 0) {
+			Alert("parsing [%s:%d] : '%s' expects an integer argument.\n", file, linenum, args[0]);
+			err_code |= ERR_ALERT | ERR_FATAL;
+			goto out;
+		}
+		curproxy->lbprm.chash.balance_factor = atol(args[1]);
+		if (curproxy->lbprm.chash.balance_factor != 0 && curproxy->lbprm.chash.balance_factor <= 100) {
+			Alert("parsing [%s:%d] : '%s' must be 0 or greater than 100.\n", file, linenum, args[0]);
+			err_code |= ERR_ALERT | ERR_FATAL;
+			goto out;
+		}
+	}
 	else if (strcmp(args[0], "unique-id-format") == 0) {
 		if (!*(args[1])) {
 			Alert("parsing [%s:%d] : %s expects an argument.\n", file, linenum, args[0]);