Core: idle connections now closed only once on exiting. Iterating through all connections takes a lot of CPU time, especially with large number of worker connections configured. As a result nginx processes used to consume CPU time during graceful shutdown. To mitigate this we now only do a full scan for idle connections when shutdown signal is received. Transitions of connections to idle ones are now expected to be avoided if the ngx_exiting flag is set. The upstream keepalive module was modified to follow this.

--- a/src/http/modules/ngx_http_upstream_keepalive_module.c
+++ b/src/http/modules/ngx_http_upstream_keepalive_module.c
@@ -302,6 +302,10 @@ ngx_http_upstream_free_keepalive_peer(ng
         goto invalid;
     }
 
+    if (ngx_terminate || ngx_exiting) {
+        goto invalid;
+    }
+
     if (ngx_handle_read_event(c->read, 0) != NGX_OK) {
         goto invalid;
     }