I have Dovecot 0.99.9.1 compiled from the source, without SSL, running on a Red Hat 9.0 system - EXT3 file system, Pentium Pro 233 MHz, Intel motherboard, 128M RAM, IBM 40Gig 7200 RPM HD. I find reading and searching to be nice and fast, and comparing it with Courier IMAP on a Celeron 824 MHz, I think Dovecot is generally not much slower, which means it may well be faster than Courier in some or many ways if it was running on the same machine.
My only problem is when writing mailboxes with large numbers of messages. I use Mailir - the same arrangement as Courier IMAP. (I also find that Netscape 4.77 can't use mailboxes which are subfolders of others, but that is probably a Netscape config problem.)
I am using Netscape 4.77 (Windows 2000 1.3 GHz Celeron, via 100Mbps switched Ethernet to the Linux machines) to do these tests since I find Netscape 7.02 to be much slower. Netscape 7.02 seems to want to resynch its notion of the mailbox repeatedly as it is writing to it. Without looking at the IMAP traffic (I don't know how) I would say that Netscape 4.77 is not doing this at all. Indeed, after the write, looking at the mailbox causes Netscape 4.77 to read the headers from the IMAP server - so I feel sure that its way of writing the messages is purely "write".
Writing a mailbox with a small number of large messages, such as 20 x 100k messages is fine. There is no obvious speed problem or excessive memory use.
When writing 2000 x 1k messages, it is totally different. Here are some times for writing from a Netscape 4.77 local mailbox to Dovecot and to Courier IMAP.
Message Total Time CPU max Memory RSS size number size Seconds Approx Megs - max
Dovecot
100k 100 10M 23 15% ~2 100k 20 2M ~5 ? 1.2 10k 200 2M 20 15% 4 ! 1k 2000 2M 230 60 32 !!!!
Courier IMAP
100k 100 10M 14 5% ~1 100k 20 2M ~2 ? ? 10k 200 2M 9 5% ~0.6 1k 2000 2M 88 2% ~0.6
This 2000 message test is small compared to some of the mailboxes I have.
I used a real-live mailbox, 6878 messages - 44 Megabytes in a single Mbox for a more demanding test. Using Netscape 4.77 to copy this to a Dovecot Maildir took a very long time - over 30 minutes. The process was fast at first and got slower and slower. As the number of messages in the destination Maildir increased, the memory usage went up and the speed reduced. It seems that RSS memory usage (as reported by "top -d 0.3" hit some kind of limit at about 49 Megs after 4000 messages and then went down a little to about 48 or less. CPU usage was 30 to 82%, fluctuating according to the moment top looked. At the end, I think it was only writing 1.5 messages a second.
RSS is the "total amount of physical memory used by the task". The "SIZE" was even bigger - "code plus data plus stack". This got up to 114 Megs by the end of the writing process!
After the write had finished, I made Netscape 4.77 view the destination mailbox. This caused Dovecot to send it all the headers, and again Dovecot had these huge memory usage figures. It seemed to read the mailbox at a decent speed - 20 seconds or so.
But if I made Dovecot look at another mailbox, and then again at this big target mailbox (say with another client - Netscape 7), it would read the big mailbox at Dovecot's usual high speed - without the high memory usage and I think without high CPU usage as well.
Without knowing anything about how Dovecot works, I imagine that there is some kind of caching algorithm for writes, and that this is retained until Dovecot (or at least the instance serving this client) is asked to look at another Maildir. It seems that reading is not slowed by this process much or at all - but writing gets progressively slower as the cache gets bigger.
The memory usage - even 49 Megs for RSS - seems excessive. It reached that about halfway through writing the mailbox, so the entire message contents at that state would have been about 22 Megs. But the "SIZE" memory usage is even larger - perhaps limited by the available RAM.
This slow speed for writing Maildirs with large numbers of messages is currently a barrier to me using Dovecot. I am about to go back and try again to get Courier IMAP to do what I want . . .
The Courier IMAP documentation and what I regard as the difficult "configure -> build -> install -> wonder why it doesn't run" process has cost me so much time that I would be very happy to use something simpler and easier (for me, at least) to install.
Thanks for developing this new IMAP server!
- Robin