[Dovecot] severe performance problem (mail cache related?)

[Dan Price]

On Fri 16 Feb 2007 at 02:49AM, Dan Price wrote:
> > So.. Hmm. Could the problem simply be that the mmap-copy-growing is too
> > slow? If the user really has some 25MB cache file, that could be it. I
> > think I could change the code so that it mmap()s immediately enough
> > memory to fit the whole cache file. Probably a good idea to do anyway.

One thing I don't totally get is why you mmap anon here, if this is
really about a cache file?  It seems that you could mmap the cache file
directly, perhaps with MAP_PRIVATE if you need to make in-memory-only
changes.

> I agree that mmaping the whole index.cache in one shot is the way to
> go-- asking the VM system to do work in little chunks is (at least
> on Solaris) a real performance killer.

Another idea I had was that dovecot could take advantage of larger
pages--especially for its anon mappings-- this cuts TLB footprint and
is usually "a good thing."

So for example, on Solaris (SPARC) you can get 8K, 64K, 512K and 4M
pages.  On Solaris AMD64 it's 4K and 2M.

This is available via getpagesizes(3c).

For now, I'm going to try to change file_cache_set_size() to just
artificially inflate page_size up to 4M-- at least that should
make these resizes less frequent.

Thanks again for your help-- the fact that you take the time to
support the community is a major factor in my decision to deploy
dovecot.

        -dp

-- 
Daniel Price - Solaris Kernel Engineering - dp at eng.sun.com - blogs.sun.com/dp