New UserspaceIOCPBuffers option to set SO_{SND,RCV}BUF to zero

When running with IOCP, we are in theory able to use userspace-
allocated buffers to avoid filling up the stingy amount of kernel
space allocated for sockets buffers.

The bufferevent_async implementation in Libevent provides this
ability, in theory.  (There are likely to be remaining bugs).  This
patch adds a new option that, when using IOCP bufferevents, sets
each socket's send and receive buffers to 0, so that we should use
this ability.

When all the bugs are worked out here, if we are right about bug 98,
this might solve or mitigate bug 98.

This option is experimental and will likely require lots of testing
and debugging.

diff --git a/changes/UserspaceIOCPBuffers b/changes/UserspaceIOCPBuffers
new file mode 100644 (file)
index 0000000..7115b5f
--- /dev/null
+++ b/changes/UserspaceIOCPBuffers
@@ -0,0 +1,7 @@
+  o Minor features:
+    - Experimental support for running on Windows with IOCP and no
+      kernel-space socket buffers. This feature is controlled by a new
+      UserspaceIOCPBuffers feature (off by default), which has no
+      effect unless Tor has been built with support for bufferevents,
+      is running on Windows, and has enabled IOCP.  This may, in the
+      long run, help solve or mitigate bug 98.

diff --git a/src/or/config.c b/src/or/config.c
index 1b9f9fb475d0c86938bf906c1ac36f6592ad1039..4710b5590bbe8f723426b008070719a206acef3d 100644 (file)
--- a/src/or/config.c
+++ b/src/or/config.c
@@ -403,6 +403,7 @@ static config_var_t _option_vars[] = {
   V(UseEntryGuards,              BOOL,     "1"),
   V(UseMicrodescriptors,         AUTOBOOL, "auto"),
   V(User,                        STRING,   NULL),
+  V(UserspaceIOCPBuffers,        BOOL,     "0"),
   VAR("V1AuthoritativeDirectory",BOOL, V1AuthoritativeDir,   "0"),
   VAR("V2AuthoritativeDirectory",BOOL, V2AuthoritativeDir,   "0"),
   VAR("V3AuthoritativeDirectory",BOOL, V3AuthoritativeDir,   "0"),

diff --git a/src/or/main.c b/src/or/main.c
index c1a7015e682adf846dbd3e134a362e31424cd0de..9f6d307a3e6baf020e7e36e1b21d1e44bb74ef45 100644 (file)
--- a/src/or/main.c
+++ b/src/or/main.c
@@ -196,6 +196,26 @@ free_old_inbuf(connection_t *conn)
 }
 #endif
 
+#ifdef MS_WINDOWS
+/** Remove the kernel-space send and receive buffers for <b>s</b>. For use
+ * with IOCP only. */
+static int
+set_buffer_lengths_to_zero(tor_socket_t s)
+{
+  int zero = 0;
+  int r = 0;
+  if (setsockopt(s, SOL_SOCKET, SO_SNDBUF, (void*)&zero, sizeof(zero))) {
+    log_warn(LD_NET, "Unable to clear SO_SNDBUF");
+    r = -1;
+  }
+  if (setsockopt(s, SOL_SOCKET, SO_RCVBUF, (void*)&zero, sizeof(zero))) {
+    log_warn(LD_NET, "Unable to clear SO_RCVBUF");
+    r = -1;
+  }
+  return r;
+}
+#endif
+
 /** Add <b>conn</b> to the array of connections that we can poll on.  The
  * connection's socket must be set; the connection starts out
  * non-reading and non-writing.
@@ -216,6 +236,14 @@ connection_add_impl(connection_t *conn, int is_connecting)
 #ifdef USE_BUFFEREVENTS
   if (connection_type_uses_bufferevent(conn)) {
     if (SOCKET_OK(conn->s) && !conn->linked) {
+
+#ifdef MS_WINDOWS
+      if (tor_libevent_using_iocp_bufferevents() &&
+          get_options()->UserspaceIOCPBuffers) {
+        set_buffer_lengths_to_zero(conn->s);
+      }
+#endif
+
       conn->bufev = bufferevent_socket_new(
                          tor_libevent_get_base(),
                          conn->s,

diff --git a/src/or/or.h b/src/or/or.h
index e4f9b9b2b6f38ca59508452cd9af9aea50e4795c..7fb7e9cb22cb8a8a04a47c8a37e964ca775d3dc1 100644 (file)
--- a/src/or/or.h
+++ b/src/or/or.h
@@ -3424,6 +3424,11 @@ typedef struct {
    * never use it.  If -1, we do what the consensus says. */
   int OptimisticData;
 
+  /** If 1, and we are using IOCP, we set the kernel socket SNDBUF and RCVBUF
+   * to 0 to try to save kernel memory and avoid the dread "Out of buffers"
+   * issue. */
+  int UserspaceIOCPBuffers;
+
 } or_options_t;
 
 /** Persistent state for an onion router, as saved to disk. */