[Dovecot] Better APPEND performance
Moving a large number of messages into an IMAP folder on a dovecot
server takes quite a long time. This is a popular operation for users
migrating their stored messages from their old mail servers through a
mail user agent, and it causes a poor first impression of dovecot.
I'd like your help to speed this up.
I have studied this problem in recent versions, including 1.1.[17-19]
and 1.2.[4-6] but I'll focus on 1.2.6, using Maildir++. It seems to
be caused by O(n^2) behavior in maintaining the UID list, where n is
the number of messages. The client selects a folder then sends
thousands of individual APPEND commands (no use of multiappend
unfortunately). These cause dovecot to behave as follows:
- For every other APPENDed message, dovecot appends the new UID to
the list quickly. No problem here, this is fast. - For every other other APPENDed message, dovecot scans the entire
UID list. This is an O(n) algorithm. Since it happens every n/2
times it causes O(n^2) behavior across n consecutive APPENDs.
More specifically,
- APPEND a message to a folder that already contains 30,000
messages. maildir_uidlist_refresh_fast_init() takes the fast path. - APPEND a second message to the same folder.
maildir_uidlist_refresh_fast_init() takes the slow path: it populates
the uidlist->files hash table with all the UIDs, checking for
duplicates. At the end of the command maildir_uidlist_deinit()
destroys the hash table. Note how this APPEND takes much longer than
#1. - APPEND a third message to the same folder. Just like #1.
- APPEND a fourth message to the same folder. Just like #2, and
since #2 destroyed the hash, it has to do all that work over again. - And so on.
What can be done to make maildir_uidlist_refresh_fast_init() choose
the fast path more often?
On Wed, 2009-10-07 at 17:53 -0500, Mike Abbott wrote:
- For every other APPENDed message, dovecot appends the new UID to
the list quickly. No problem here, this is fast.- For every other other APPENDed message, dovecot scans the entire
UID list. This is an O(n) algorithm. Since it happens every n/2
times it causes O(n^2) behavior across n consecutive APPENDs.
I'll look at this more closely later, but did you already try maildir_very_dirty_syncs=yes? Does this behavior happen also with it?
Timo Sirainen wrote:
On Wed, 2009-10-07 at 17:53 -0500, Mike Abbott wrote:
- For every other APPENDed message, dovecot appends the new UID to
the list quickly. No problem here, this is fast.- For every other other APPENDed message, dovecot scans the entire
UID list. This is an O(n) algorithm. Since it happens every n/2
times it causes O(n^2) behavior across n consecutive APPENDs.I'll look at this more closely later, but did you already try maildir_very_dirty_syncs=yes? Does this behavior happen also with it?
Hello Timo,
Also i have observed this behaviour. Although i think it's not the most urgent matter, it would really be nice if you could speed up massive message imports.
In our case, we don't use it that much for migration, but sometimes some POP users like to be able to backup their messages in the IMAP server.
Thanks in advance,
Hugo Monteiro.
-- ci.fct.unl.pt:~# cat .signature
Hugo Monteiro Email : hugo.monteiro@fct.unl.pt Telefone : +351 212948300 Ext.15307 Web : http://hmonteiro.net
Centro de Informática Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa Quinta da Torre 2829-516 Caparica Portugal Telefone: +351 212948596 Fax: +351 212948548 www.ci.fct.unl.pt apoio@fct.unl.pt
ci.fct.unl.pt:~# _
On Wed, 2009-10-07 at 17:53 -0500, Mike Abbott wrote:
What can be done to make maildir_uidlist_refresh_fast_init() choose
the fast path more often?
Pretty simple bug. Fixed: http://hg.dovecot.org/dovecot-1.2/rev/ebdba086e3b1
This makes the performance pretty good when appending to maildirs with large number of messages. In my desktop the append speed stays pretty constant at ~500 msgs/sec after 20k messages, while without the patch it crawls at ~30-40 msgs/sec.
On Wed, 2009-10-14 at 12:48 -0400, Timo Sirainen wrote:
On Wed, 2009-10-07 at 17:53 -0500, Mike Abbott wrote:
What can be done to make maildir_uidlist_refresh_fast_init() choose
the fast path more often?Pretty simple bug. Fixed: http://hg.dovecot.org/dovecot-1.2/rev/ebdba086e3b1
This makes the performance pretty good when appending to maildirs with large number of messages. In my desktop the append speed stays pretty constant at ~500 msgs/sec after 20k messages, while without the patch it crawls at ~30-40 msgs/sec.
This is also useful for maildir_very_dirty_syncs=yes, otherwise dovecot-uidlist never shrinks: http://hg.dovecot.org/dovecot-1.2/rev/7956cc1086e1
What can be done to make maildir_uidlist_refresh_fast_init() choose the fast path more often?
Pretty simple bug. Fixed: http://hg.dovecot.org/dovecot-1.2/rev/ebdba086e3b1
This makes the performance pretty good when appending to maildirs with large number of messages.
Sure does! Thanks, that fixes it!
participants (3)
-
Hugo Monteiro
-
Mike Abbott
-
Timo Sirainen