The "worker_aio_requests" directive.

The default value is 32 AIO simultaneous requests per worker. Previously
they were hardcoded to 1024, and it was too large, since Linux allocated
them early on io_setup(), but not on request itself. So with default value
of /proc/sys/fs/aio-max-nr equal to 65536 only 64 worker processes could
be run simultaneously. 32 AIO requests are enough for modern disks even if
server runs only 1 worker.

src/event/modules/ngx_epoll_module.c

diff --git a/src/event/modules/ngx_epoll_module.c b/src/event/modules/ngx_epoll_module.c
index df5bf89..31c10c6 100644
--- a/src/event/modules/ngx_epoll_module.c
+++ b/src/event/modules/ngx_epoll_module.c
@@ -86,6 +86,7 @@
 
 typedef struct {
     ngx_uint_t  events;
+    ngx_uint_t  aio_requests;
 } ngx_epoll_conf_t;
 
 
@@ -133,6 +134,13 @@
       offsetof(ngx_epoll_conf_t, events),
       NULL },
 
+    { ngx_string("worker_aio_requests"),
+      NGX_EVENT_CONF|NGX_CONF_TAKE1,
+      ngx_conf_set_num_slot,
+      0,
+      offsetof(ngx_epoll_conf_t, aio_requests),
+      NULL },
+
       ngx_null_command
 };
 
@@ -207,7 +215,7 @@
 
 
 static void
-ngx_epoll_aio_init(ngx_cycle_t *cycle)
+ngx_epoll_aio_init(ngx_cycle_t *cycle, ngx_epoll_conf_t *epcf)
 {
     int                 n;
     struct epoll_event  ee;
@@ -232,7 +240,7 @@
         goto failed;
     }
 
-    if (io_setup(1024, &ngx_aio_ctx) == -1) {
+    if (io_setup(epcf->aio_requests, &ngx_aio_ctx) == -1) {
         ngx_log_error(NGX_LOG_EMERG, cycle->log, ngx_errno,
                       "io_setup() failed");
         goto failed;
@@ -294,7 +302,7 @@
 
 #if (NGX_HAVE_FILE_AIO)
 
-        ngx_epoll_aio_init(cycle);
+        ngx_epoll_aio_init(cycle, epcf);
 
 #endif
     }
@@ -794,6 +802,7 @@
     }
 
     epcf->events = NGX_CONF_UNSET;
+    epcf->aio_requests = NGX_CONF_UNSET;
 
     return epcf;
 }
@@ -805,6 +814,7 @@
     ngx_epoll_conf_t *epcf = conf;
 
     ngx_conf_init_uint_value(epcf->events, 512);
+    ngx_conf_init_uint_value(epcf->aio_requests, 32);
 
     return NGX_CONF_OK;
 }