[Dovecot] Design: Asynchronous I/O for single/multi-dbox

Timo Sirainen tss at iki.fi
Tue Mar 16 19:44:34 EET 2010

On Tue, 2010-03-16 at 13:30 -0400, Frank Cusack wrote:

> > Well, you just mentioned the benefits :) Less memory usage, less context
> > switches (of any kind). (You aren't assuming I'd do something like one
> > thread per connection, right? That's not going to happen.)
> But as I stated, dovecot is likely to be I/O limited, not CPU or memory
> limited.  Who cares if there is 10% less memory usage when the limit is
> still I/O?  Feel free to trout-slap me (to borrow from Stan) if you
> expect dovecot to be memory limited.

It is I/O limited, but less memory usage still can help. Some setups use
e.g. NFS storage with multiple Dovecot servers accessing it. They could
reduce the number of Dovecot servers if a single server could process
more connections.

Also all setups benefit from less memory usage, because that means
there's more memory left for disk cache -> less disk I/O.

> > That's kind of the point. You could have just a few IMAP processes where
> > each can handle hundreds of connections. Currently that's not a very good
> > idea, because a single connection that waits on disk I/O blocks all the
> > other connections in the same process from doing anything.
> Or you could have a whole bunch of IMAP processes, one per connection.
> All you're doing is multiplexing the I/O at a different part of your
> system.  Each connection doesn't share any state or other data with
> other "related" connections 

Actually if multiple connections are accessing the same mailbox in the
same process, they can (and already do!) share index data.

> > Kernel can only parallelize requests from different processes.
> Right, but again so what?  Since each connection is a different process,
> all I/O is parallelized in dovecot's case.  It works well because
> connections aren't related to each other.

I think I answered to this in the other mail: access to high-latency
storage (which can also reduce command reply latency somewhat for
low-latency storage).
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