[PATCH 05 of 11] SSL: single allocation in session cache on 32-bit platforms

Maxim Dounin mdounin at mdounin.ru
Fri Aug 26 03:01:12 UTC 2022


# HG changeset patch
# User Maxim Dounin <mdounin at mdounin.ru>
# Date 1661481950 -10800
#      Fri Aug 26 05:45:50 2022 +0300
# Node ID e88baee178eed529c6170678e373f5e2e0883c37
# Parent  f4ae0f4ee928cf20346530e96f1431314ecd0171
SSL: single allocation in session cache on 32-bit platforms.

Given the present typical SSL session sizes, on 32-bit platforms it is
now beneficial to store all data in a single allocation, since rbtree
node + session id + ASN1 representation of a session takes 256 bytes of
shared memory (36 + 32 + 150 = about 218 bytes plus SNI server name).

Storing all data in a single allocation is beneficial for SNI names up to
about 40 characters long and makes it possible to store about 4000 sessions
in one megabyte (instead of about 3000 sessions now).  This also slightly
simplifies the code.

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -3789,9 +3789,9 @@ ngx_ssl_session_cache_init(ngx_shm_zone_
  * Typical length of the external ASN1 representation of a session
  * is about 150 bytes plus SNI server name.
  *
- * On 32-bit platforms we allocate separately an rbtree node,
- * a session id, and an ASN1 representation, they take accordingly
- * 64, 32, and 256 bytes.
+ * On 32-bit platforms we allocate an rbtree node, a session id, and
+ * an ASN1 representation in a single allocation, it typically takes
+ * 256 bytes.
  *
  * On 64-bit platforms we allocate separately an rbtree node + session_id,
  * and an ASN1 representation, they take accordingly 128 and 256 bytes.
@@ -3804,7 +3804,8 @@ static int
 ngx_ssl_new_session(ngx_ssl_conn_t *ssl_conn, ngx_ssl_session_t *sess)
 {
     int                       len;
-    u_char                   *p, *id, *cached_sess, *session_id;
+    u_char                   *p, *session_id;
+    size_t                    n;
     uint32_t                  hash;
     SSL_CTX                  *ssl_ctx;
     unsigned int              session_id_length;
@@ -3863,23 +3864,13 @@ ngx_ssl_new_session(ngx_ssl_conn_t *ssl_
     /* drop one or two expired sessions */
     ngx_ssl_expire_sessions(cache, shpool, 1);
 
-    cached_sess = ngx_slab_alloc_locked(shpool, len);
-
-    if (cached_sess == NULL) {
-
-        /* drop the oldest non-expired session and try once more */
-
-        ngx_ssl_expire_sessions(cache, shpool, 0);
-
-        cached_sess = ngx_slab_alloc_locked(shpool, len);
-
-        if (cached_sess == NULL) {
-            sess_id = NULL;
-            goto failed;
-        }
-    }
-
-    sess_id = ngx_slab_alloc_locked(shpool, sizeof(ngx_ssl_sess_id_t));
+#if (NGX_PTR_SIZE == 8)
+    n = sizeof(ngx_ssl_sess_id_t);
+#else
+    n = offsetof(ngx_ssl_sess_id_t, session) + len;
+#endif
+
+    sess_id = ngx_slab_alloc_locked(shpool, n);
 
     if (sess_id == NULL) {
 
@@ -3887,7 +3878,7 @@ ngx_ssl_new_session(ngx_ssl_conn_t *ssl_
 
         ngx_ssl_expire_sessions(cache, shpool, 0);
 
-        sess_id = ngx_slab_alloc_locked(shpool, sizeof(ngx_ssl_sess_id_t));
+        sess_id = ngx_slab_alloc_locked(shpool, n);
 
         if (sess_id == NULL) {
             goto failed;
@@ -3896,30 +3887,25 @@ ngx_ssl_new_session(ngx_ssl_conn_t *ssl_
 
 #if (NGX_PTR_SIZE == 8)
 
-    id = sess_id->sess_id;
-
-#else
-
-    id = ngx_slab_alloc_locked(shpool, session_id_length);
-
-    if (id == NULL) {
+    sess_id->session = ngx_slab_alloc_locked(shpool, len);
+
+    if (sess_id->session == NULL) {
 
         /* drop the oldest non-expired session and try once more */
 
         ngx_ssl_expire_sessions(cache, shpool, 0);
 
-        id = ngx_slab_alloc_locked(shpool, session_id_length);
-
-        if (id == NULL) {
+        sess_id->session = ngx_slab_alloc_locked(shpool, len);
+
+        if (sess_id->session == NULL) {
             goto failed;
         }
     }
 
 #endif
 
-    ngx_memcpy(cached_sess, buf, len);
-
-    ngx_memcpy(id, session_id, session_id_length);
+    ngx_memcpy(sess_id->session, buf, len);
+    ngx_memcpy(sess_id->id, session_id, session_id_length);
 
     hash = ngx_crc32_short(session_id, session_id_length);
 
@@ -3929,9 +3915,7 @@ ngx_ssl_new_session(ngx_ssl_conn_t *ssl_
 
     sess_id->node.key = hash;
     sess_id->node.data = (u_char) session_id_length;
-    sess_id->id = id;
     sess_id->len = len;
-    sess_id->session = cached_sess;
 
     sess_id->expire = ngx_time() + SSL_CTX_get_timeout(ssl_ctx);
 
@@ -3945,10 +3929,6 @@ ngx_ssl_new_session(ngx_ssl_conn_t *ssl_
 
 failed:
 
-    if (cached_sess) {
-        ngx_slab_free_locked(shpool, cached_sess);
-    }
-
     if (sess_id) {
         ngx_slab_free_locked(shpool, sess_id);
     }
@@ -4045,9 +4025,8 @@ ngx_ssl_get_cached_session(ngx_ssl_conn_
 
             ngx_rbtree_delete(&cache->session_rbtree, node);
 
+#if (NGX_PTR_SIZE == 8)
             ngx_slab_free_locked(shpool, sess_id->session);
-#if (NGX_PTR_SIZE == 4)
-            ngx_slab_free_locked(shpool, sess_id->id);
 #endif
             ngx_slab_free_locked(shpool, sess_id);
 
@@ -4135,9 +4114,8 @@ ngx_ssl_remove_session(SSL_CTX *ssl, ngx
 
             ngx_rbtree_delete(&cache->session_rbtree, node);
 
+#if (NGX_PTR_SIZE == 8)
             ngx_slab_free_locked(shpool, sess_id->session);
-#if (NGX_PTR_SIZE == 4)
-            ngx_slab_free_locked(shpool, sess_id->id);
 #endif
             ngx_slab_free_locked(shpool, sess_id);
 
@@ -4184,9 +4162,8 @@ ngx_ssl_expire_sessions(ngx_ssl_session_
 
         ngx_rbtree_delete(&cache->session_rbtree, &sess_id->node);
 
+#if (NGX_PTR_SIZE == 8)
         ngx_slab_free_locked(shpool, sess_id->session);
-#if (NGX_PTR_SIZE == 4)
-        ngx_slab_free_locked(shpool, sess_id->id);
 #endif
         ngx_slab_free_locked(shpool, sess_id);
     }
diff --git a/src/event/ngx_event_openssl.h b/src/event/ngx_event_openssl.h
--- a/src/event/ngx_event_openssl.h
+++ b/src/event/ngx_event_openssl.h
@@ -134,14 +134,14 @@ typedef struct ngx_ssl_sess_id_s  ngx_ss
 
 struct ngx_ssl_sess_id_s {
     ngx_rbtree_node_t           node;
-    u_char                     *id;
     size_t                      len;
-    u_char                     *session;
     ngx_queue_t                 queue;
     time_t                      expire;
+    u_char                      id[32];
 #if (NGX_PTR_SIZE == 8)
-    void                       *stub;
-    u_char                      sess_id[32];
+    u_char                     *session;
+#else
+    u_char                      session[1];
 #endif
 };
 



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