Patch: Solaris performance tweak

[Steven Grimm]

We've been playing with memcached on different OSes to compare 
performance. It turns out Solaris 10 does not deal with large 
scatter-gather sendmsg() calls in as low-latency a way as Linux. (They 
are optimizing for throughput rather than latency, but latency is often 
more important in the case of memcached.) So we made the following 
change, which has a negligible performance impact on Linux and is a 
measurable improvement on Solaris.

Note that we are NOT running this in production yet. It should be safe 
-- it's very simple, as you'll see -- but caveat emptor.

This is against the 1.2.0-rc1 source but you should be able to apply it 
to the Facebook branch too if you like.

-Steve

-------------- next part --------------
--- memcached.c.orig	2006-08-18 01:16:37.000000000 -0700
+++ memcached.c	2006-08-18 01:19:00.000000000 -0700
@@ -352,13 +352,20 @@
     struct msghdr *m;
     int i;
     int leftover;
+    int limit_to_mtu;
 
     do {
         m = &c->msglist[c->msgused - 1];
 
+        /*
+         * Limit UDP packets, and the first payloads of TCP replies, to
+         * UDP_MAX_PAYLOAD_SIZE bytes.
+         */
+        limit_to_mtu = c->udp || (1 == c->msgused);
+
         /* We may need to start a new msghdr if this one is full. */
         if (m->msg_iovlen == IOV_MAX ||
-                c->udp && c->msgbytes >= UDP_MAX_PAYLOAD_SIZE) {
+                limit_to_mtu && c->msgbytes >= UDP_MAX_PAYLOAD_SIZE) {
             add_msghdr(c);
             m = &c->msglist[c->msgused - 1];
         }
@@ -367,7 +374,7 @@
             return -1;
 
         /* If the fragment is too big to fit in the datagram, split it up */
-        if (c->udp && len + c->msgbytes > UDP_MAX_PAYLOAD_SIZE) {
+        if (limit_to_mtu && len + c->msgbytes > UDP_MAX_PAYLOAD_SIZE) {
             leftover = len + c->msgbytes - UDP_MAX_PAYLOAD_SIZE;
             len -= leftover;
         } else {