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

Stan Hoeppner stan at hardwarefreak.com
Tue Mar 16 17:29:57 EET 2010

Timo Sirainen put forth on 3/16/2010 6:36 AM:
> On Tue, 2010-03-16 at 11:16 +0100, Steffen Kaiser wrote:
>> If I display a process map of an active imap process, I have:
>> mapped: 2656K    writeable/private: 460K    shared: 300K
>> Essentially you are talking about 460KB of KSM-able data.
>> IMHO, all modern Unix-alike system just "map" code pages read-only into 
>> the process and share them among all processes. No KSM required at all.
> Right. Code and mmaped pages (e.g. some of Dovecot index files) are
> shared already. There's very little (if any) data that KSM could
> de-duplicate.

Ok, I've boned up on virtual shared memory and mmap and I now have the
knowledge I previously lacked.  I also have a far better understanding of
RSS than I previously did, mainly that shared pages are included in RSS,
even though there is only one copy of a shared page.  Thus the sum of all
RSS values can be much greater than the physical RAM in the system, which is
opposite to my previous (mis)understanding or RSS.  And I also now
understand why KSM would have a very limited use case on a non KVM Linux
kernel, since virtual shared memory serves the same purpose, and does so
much more efficiently.

Feel free to slap me around with some trout for previously speaking to
things above my knowledge level at the time. ;)


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