merge r2889, r2918, and r2921:

fix a pool growing and CPU usage for multi-requests keepalive connections:

*) keepalive_requests
*) try to reuse last 4 free large allocation links
*) do not test a pool block space if we can not allocated from
   the block 4 times

src/core/ngx_connection.h

@@ -135,6 +135,8 @@ struct ngx_connection_s {
 
     ngx_atomic_uint_t   number;
 
+    ngx_uint_t          requests;
+
     unsigned            buffered:8;
 
     unsigned            log_error:3;     /* ngx_connection_log_error_e */

src/core/ngx_palloc.c

@@ -25,6 +25,7 @@ ngx_create_pool(size_t size, ngx_log_t *log)
     p->d.last = (u_char *) p + sizeof(ngx_pool_t);
     p->d.end = (u_char *) p + size;
     p->d.next = NULL;
+    p->d.failed = 0;
 
     size = size - sizeof(ngx_pool_t);
     p->max = (size < NGX_MAX_ALLOC_FROM_POOL) ? size : NGX_MAX_ALLOC_FROM_POOL;
@@ -189,6 +190,7 @@ ngx_palloc_block(ngx_pool_t *pool, size_t size)
 
     new->d.end = m + psize;
     new->d.next = NULL;
+    new->d.failed = 0;
 
     m += sizeof(ngx_pool_data_t);
     m = ngx_align_ptr(m, NGX_ALIGNMENT);
@@ -197,7 +199,7 @@ ngx_palloc_block(ngx_pool_t *pool, size_t size)
     current = pool->current;
 
     for (p = current; p->d.next; p = p->d.next) {
-        if ((size_t) (p->d.end - p->d.last) < NGX_ALIGNMENT) {
+        if (p->d.failed++ > 4) {
             current = p->d.next;
         }
     }
@@ -214,6 +216,7 @@ static void *
 ngx_palloc_large(ngx_pool_t *pool, size_t size)
 {
     void              *p;
+    ngx_uint_t         n;
     ngx_pool_large_t  *large;
 
     p = ngx_alloc(size, pool->log);
@@ -221,6 +224,19 @@ ngx_palloc_large(ngx_pool_t *pool, size_t size)
         return NULL;
     }
 
+    n = 0;
+
+    for (large = pool->large; large; large = large->next) {
+        if (large->alloc == NULL) {
+            large->alloc = p;
+            return p;
+        }
+
+        if (n++ > 3) {
+            break;
+        }
+    }
+
     large = ngx_palloc(pool, sizeof(ngx_pool_large_t));
     if (large == NULL) {
         ngx_free(p);

src/core/ngx_palloc.h

@@ -46,6 +46,7 @@ typedef struct {
     u_char               *last;
     u_char               *end;
     ngx_pool_t           *next;
+    ngx_uint_t            failed;
 } ngx_pool_data_t;
 
 

src/http/ngx_http_core_module.c

@@ -440,6 +440,13 @@ static ngx_command_t  ngx_http_core_commands[] = {
       0,
       NULL },
 
+    { ngx_string("keepalive_requests"),
+      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1,
+      ngx_conf_set_num_slot,
+      NGX_HTTP_LOC_CONF_OFFSET,
+      offsetof(ngx_http_core_loc_conf_t, keepalive_requests),
+      NULL },
+
     { ngx_string("satisfy"),
       NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1,
       ngx_conf_set_enum_slot,
@@ -1326,8 +1333,13 @@ ngx_http_update_location_config(ngx_http_request_t *r)
 
     r->request_body_in_single_buf = clcf->client_body_in_single_buffer;
 
-    if (r->keepalive && clcf->keepalive_timeout == 0) {
-        r->keepalive = 0;
+    if (r->keepalive) {
+        if (clcf->keepalive_timeout == 0) {
+            r->keepalive = 0;
+
+        } else if (r->connection->requests >= clcf->keepalive_requests) {
+            r->keepalive = 0;
+        }
     }
 
     if (!clcf->tcp_nopush) {
@@ -2914,6 +2926,7 @@ ngx_http_core_create_loc_conf(ngx_conf_t *cf)
     lcf->limit_rate = NGX_CONF_UNSET_SIZE;
     lcf->keepalive_timeout = NGX_CONF_UNSET_MSEC;
     lcf->keepalive_header = NGX_CONF_UNSET;
+    lcf->keepalive_requests = NGX_CONF_UNSET_UINT;
     lcf->lingering_time = NGX_CONF_UNSET_MSEC;
     lcf->lingering_timeout = NGX_CONF_UNSET_MSEC;
     lcf->resolver_timeout = NGX_CONF_UNSET_MSEC;
@@ -3114,6 +3127,8 @@ ngx_http_core_merge_loc_conf(ngx_conf_t *cf, void *parent, void *child)
                               prev->keepalive_timeout, 75000);
     ngx_conf_merge_sec_value(conf->keepalive_header,
                               prev->keepalive_header, 0);
+    ngx_conf_merge_uint_value(conf->keepalive_requests,
+                              prev->keepalive_requests, 100);
     ngx_conf_merge_msec_value(conf->lingering_time,
                               prev->lingering_time, 30000);
     ngx_conf_merge_msec_value(conf->lingering_timeout,

src/http/ngx_http_core_module.h

@@ -337,6 +337,7 @@ struct ngx_http_core_loc_conf_s {
 
     time_t        keepalive_header;        /* keepalive_timeout */
 
+    ngx_uint_t    keepalive_requests;      /* keepalive_requests */
     ngx_uint_t    satisfy;                 /* satisfy */
     ngx_uint_t    if_modified_since;       /* if_modified_since */
     ngx_uint_t    client_body_in_file_only; /* client_body_in_file_only */

src/http/ngx_http_request.c

@@ -259,6 +259,8 @@ ngx_http_init_request(ngx_event_t *rev)
         return;
     }
 
+    c->requests++;
+
     hc = c->data;
 
     if (hc == NULL) {