Dovecot version: 1.1-rc8, 1.1-rc9 System: Linux-2.6.2x User-Agent: claws-mail-3.3 and 3.4
When appending messages (e.g. copy from mailclient local folder or second server to dovecot imap folder) dovecot answers with: <tag> OK [APPENDUID <n> <m>] Append completed\r\n
This answer often reaches the client as two or more TCP packets, the client (when not using TCP_NODELAY on its socket) ACKs with some delay.
When copying lots of (small) messages this delay has a very negative impact on copy time even though there are lots of network and system resources available.
Sample sniffer output on client host: ... 1 0.0000 claws dovecot IMAP Request: 14639 APPEND "Test" (\Seen) {853} 2 0.0033 dovecot claws IMAP Response: +OK 3 0.0034 claws dovecot IMAP Request: <message> 4 0.0096 dovecot claws IMAP Response: 14639 5 0.0467 claws dovecot TCP 33453 > imap [ACK] 6 0.0481 dovecot claws IMAP Response: OK [APPENDUID 1204729411 2] Append Completed. 7 0.0481 claws dovecot TCP 33453 > imap [ACK] ...
The delay of about 35ms between packet 4 and its ACK in packet 5 causes copies from client to server to be extremely slow. (Client and server on same 100Mb LAN, same result if client and server are on same host)
The attached patch makes dovecot send the whole answer in a single packet, thus not triggering the delay issue.
As far as I found out the delay is generated by Naggle algorithm, this algorithm being used by Linux TCP stack for small packets with the aim of improving payload/overhead share.
Bruno Prémont RESTENA Foundation