Use smaller zlib objects when under memory pressure

We add a compression level argument to tor_zlib_new, and use it to
determine how much memory to allocate for the zlib object.  We use the
existing level by default, but shift to smaller levels for small
requests when we have been over 3/4 of our memory usage in the past
half-hour.

Closes ticket 11791.

1 files changed, 28 insertions, 5 deletions

diff --git a/src/or/directory.c b/src/or/directory.c
index e1f5964e1e..0ab3e6ae23 100644
--- a/src/or/directory.c
+++ b/src/or/directory.c
@@ -20,6 +20,7 @@
 #include "networkstatus.h"
 #include "nodelist.h"
 #include "policies.h"
+#include "relay.h"
 #include "rendclient.h"
 #include "rendcommon.h"
 #include "rephist.h"
@@ -2521,6 +2522,24 @@ client_likes_consensus(networkstatus_t *v, const char *want_url)
   return (have >= need_at_least);
 }
 
+/** Return the compression level we should use for sending a compressed
+ * response of size <b>n_bytes</b>. */
+static zlib_compression_level_t
+choose_compression_level(ssize_t n_bytes)
+{
+  if (! have_been_under_memory_pressure()) {
+    return HIGH_COMPRESSION; /* we have plenty of RAM. */
+  } else if (n_bytes < 0) {
+    return HIGH_COMPRESSION; /* unknown; might be big. */
+  } else if (n_bytes < 1024) {
+    return LOW_COMPRESSION;
+  } else if (n_bytes < 2048) {
+    return MEDIUM_COMPRESSION;
+  } else {
+    return HIGH_COMPRESSION;
+  }
+}
+
 /** Helper function: called when a dirserver gets a complete HTTP GET
  * request.  Look for a request for a directory or for a rendezvous
  * service descriptor.  On finding one, write a response into
@@ -2703,7 +2722,7 @@ directory_handle_command_get(dir_connection_t *conn, const char *headers,
                                smartlist_len(dir_fps) == 1 ? lifetime : 0);
     conn->fingerprint_stack = dir_fps;
     if (! compressed)
-      conn->zlib_state = tor_zlib_new(0, ZLIB_METHOD);
+      conn->zlib_state = tor_zlib_new(0, ZLIB_METHOD, HIGH_COMPRESSION);
 
     /* Prime the connection with some data. */
     conn->dir_spool_src = DIR_SPOOL_NETWORKSTATUS;
@@ -2791,7 +2810,8 @@ directory_handle_command_get(dir_connection_t *conn, const char *headers,
 
     if (smartlist_len(items)) {
       if (compressed) {
-        conn->zlib_state = tor_zlib_new(1, ZLIB_METHOD);
+        conn->zlib_state = tor_zlib_new(1, ZLIB_METHOD,
+                                    choose_compression_level(estimated_len));
         SMARTLIST_FOREACH(items, const char *, c,
                  connection_write_to_buf_zlib(c, strlen(c), conn, 0));
         connection_write_to_buf_zlib("", 0, conn, 1);
@@ -2840,7 +2860,8 @@ directory_handle_command_get(dir_connection_t *conn, const char *headers,
     conn->fingerprint_stack = fps;
 
     if (compressed)
-      conn->zlib_state = tor_zlib_new(1, ZLIB_METHOD);
+      conn->zlib_state = tor_zlib_new(1, ZLIB_METHOD,
+                                      choose_compression_level(dlen));
 
     connection_dirserv_flushed_some(conn);
     goto done;
@@ -2908,7 +2929,8 @@ directory_handle_command_get(dir_connection_t *conn, const char *headers,
       }
       write_http_response_header(conn, -1, compressed, cache_lifetime);
       if (compressed)
-        conn->zlib_state = tor_zlib_new(1, ZLIB_METHOD);
+        conn->zlib_state = tor_zlib_new(1, ZLIB_METHOD,
+                                        choose_compression_level(dlen));
       /* Prime the connection with some data. */
       connection_dirserv_flushed_some(conn);
     }
@@ -2983,7 +3005,8 @@ directory_handle_command_get(dir_connection_t *conn, const char *headers,
 
     write_http_response_header(conn, compressed?-1:len, compressed, 60*60);
     if (compressed) {
-      conn->zlib_state = tor_zlib_new(1, ZLIB_METHOD);
+      conn->zlib_state = tor_zlib_new(1, ZLIB_METHOD,
+                                      choose_compression_level(len));
       SMARTLIST_FOREACH(certs, authority_cert_t *, c,
             connection_write_to_buf_zlib(c->cache_info.signed_descriptor_body,
                                          c->cache_info.signed_descriptor_len,