Configurable 'npoints' for ngx_http_upstream_hash

[Koby Nachmany]

Hello!

I have a use case for an even, consistent balancing of a caching layer
upstream cluster. I.e using the "Ketama algorithm"
Current consistent hashing implementation in ngx_http_upstream_hash is
hard-coded to '160' vbuckets  and real world results show a 20% variance in
balancing, which is not acceptable in our case.

Following is a patch (Thanks to agentz) that will allow a configurable
vbuckets configuration param. Default will remain the same = 160.
Please consider pushing this "upstream" . No pun intended ;)

Koby N

--- a/src/http/modules/ngx_http_upstream_hash_module.c 2015-07-15
00:46:06.000000000 +0800
+++ b/src/http/modules/ngx_http_upstream_hash_module.c 2015-10-11
22:26:47.952670175 +0800
@@ -23,6 +23,7 @@ typedef struct {


 typedef struct {
+    ngx_uint_t                          npoints;
     ngx_http_complex_value_t            key;
     ngx_http_upstream_chash_points_t   *points;
 } ngx_http_upstream_hash_srv_conf_t;
@@ -66,7 +67,7 @@ static char *ngx_http_upstream_hash(ngx_
 static ngx_command_t  ngx_http_upstream_hash_commands[] = {

     { ngx_string("hash"),
-      NGX_HTTP_UPS_CONF|NGX_CONF_TAKE12,
+      NGX_HTTP_UPS_CONF|NGX_CONF_TAKE123,
       ngx_http_upstream_hash,
       NGX_HTTP_SRV_CONF_OFFSET,
       0,
@@ -296,7 +297,10 @@ ngx_http_upstream_init_chash(ngx_conf_t
     us->peer.init = ngx_http_upstream_init_chash_peer;

     peers = us->peer.data;
-    npoints = peers->total_weight * 160;
+
+    hcf = ngx_http_conf_upstream_srv_conf(us,
ngx_http_upstream_hash_module);
+
+    npoints = peers->total_weight * hcf->npoints;

     size = sizeof(ngx_http_upstream_chash_points_t)
            + sizeof(ngx_http_upstream_chash_point_t) * (npoints - 1);
@@ -355,7 +359,7 @@ ngx_http_upstream_init_chash(ngx_conf_t
         ngx_crc32_update(&base_hash, port, port_len);

         prev_hash.value = 0;
-        npoints = peer->weight * 160;
+        npoints = peer->weight * hcf->npoints;

         for (j = 0; j < npoints; j++) {
             hash = base_hash;
@@ -391,7 +395,6 @@ ngx_http_upstream_init_chash(ngx_conf_t

     points->number = i + 1;

-    hcf = ngx_http_conf_upstream_srv_conf(us,
ngx_http_upstream_hash_module);
     hcf->points = points;

     return NGX_OK;
@@ -657,6 +660,19 @@ ngx_http_upstream_hash(ngx_conf_t *cf, n
     } else if (ngx_strcmp(value[2].data, "consistent") == 0) {
         uscf->peer.init_upstream = ngx_http_upstream_init_chash;

+        if (cf->args->nelts > 3) {
+            hcf->npoints = ngx_atoi(value[3].data, value[3].len);
+
+            if (hcf->npoints == (ngx_uint_t) NGX_ERROR) {
+                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
+                                   "invalid npoints parameter \"%V\"",
&value[3]);
+                return NGX_CONF_ERROR;
+            }
+
+        } else {
+            hcf->npoints = 160;
+        }
+
     } else {
         ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                            "invalid parameter \"%V\"", &value[2]);
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