The POP support in movemail runs afoul of a combination of otherwise
reasonable optimizations in most TCP stacks, Nagle and delayed acks.
It sends (for example) "RETR 1234" and then there's a delay before it
sends "\r\n", because we make two write() calls. It can add something
like 0.4 seconds per message retrieved, which adds up with a lot of
messages.
Is this approach okay, or should I go change the code to include the
\r\n at all the call sites instead?
Should this wait until after the release?
Ken
--- lib-src/pop.c 1 Sep 2003 15:45:03 -0000 1.34
+++ lib-src/pop.c 3 Jul 2005 22:04:03 -0000
@@ -1404,10 +1393,39 @@ sendline (server, line)
#define SENDLINE_ERROR "Error writing to POP server: "
int ret;
- ret = fullwrite (server->file, line, strlen (line));
- if (ret >= 0)
- { /* 0 indicates that a blank line was written */
- ret = fullwrite (server->file, "\r\n", 2);
+ if (line == pop_error && strlen (line) < sizeof (pop_error) - 5)
+ {
+ /* This minor "abstraction" violation can save a fraction of a
+ second per message sent in a fast, reliable TCP network
+ environment with delayed acks and Nagle algorithm. (Movemail
+ writes line without \r\n, client kernel sends it, server
+ kernel delays ack to see if it can combine it with data,
+ movemail writes \r\n, client kernel waits because it has
+ unacked data, client kernel eventually times out and sends.)
+
+ On a NetBSD box, this delay is 0.2 seconds per message; if
+ you've got a few dozen messages or so, it adds up, and if
+ they're small and the server is close, it can be a
+ significant fraction of the execution time.
+
+ Turning off Nagle would probably change this to two packets
+ in rapid succession for most implementations. If we can make
+ it just one write call, we'll likely get one packet and keep
+ everybody happier.
+
+ Fortunately, most of the formatted calls (e.g., all those
+ including message numbers) use pop_error as the buffer
+ into which the command is written. */
+ strcat (line, "\r\n");
+ ret = fullwrite (server->file, line, strlen (line));
+ }
+ else
+ {
+ ret = fullwrite (server->file, line, strlen (line));
+ if (ret >= 0)
+ { /* 0 indicates that a blank line was written */
+ ret = fullwrite (server->file, "\r\n", 2);
+ }
}
if (ret < 0)
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