On 2/12/2014 12:07 PM, Greg Walters wrote:
Take a look at
http://blog.thetaphi.de/2012/07/use-lucenes-mmapdirectory-on-64bit.html as it's
a pretty decent explanation of memory mapped files. I don't believe that the
default configuration for solr is to use MMapDirectory but even if it does my
understanding is that the entire file won't be forcibly cached by solr. The
OS's filesystem cache should control what's actually in ram and the eviction
process will depend on the OS.
I only have a little bit to add. Here's the first thing that Uwe's blog
post (linked above) says:
"Since version 3.1, *Apache Lucene*and *Solr *use MMapDirectoryby
default on 64bit Windows and Solaris systems; since version 3.3 also for
64bit Linux systems."
The default in Solr 4.x is NRTCachingDirectory, which uses MMapDirectory
by default under the hood.
A summary about all this that should be relevant to the original question:
It's the *operating system* that handles memory mapping, including any
caching that happens. Assuming that you don't have a badly configured
virtual machine setup, I'm fairly sure that only real memory gets used,
never swap space on the disk. If something else on the system makes a
memory allocation, the operating system will instantly give up memory
used for caching and mapping. One of the strengths of mmap is that it
can't exceed available resources unless it's used incorrectly.
Thanks,
Shawn
