incubator-cassandra-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Edward Capriolo <>
Subject Re: mmaped files and swap
Date Wed, 13 Mar 2013 14:50:08 GMT
You really can not control what the OS-swaps out. java has other memory
usage outside the heap, and native memory. best to turn swap off. Swap is
kinda old school anyway at this point. It made sense when machines had 32MB

Keeping your read 95th percentile low is mostly about removing deviations
that cause requests to slow down, swap is one of the things that cause
fluctuation becuase it is not predictable.

On Wed, Mar 13, 2013 at 10:39 AM, Fredrik

> I've got a question regarding understanding the recomendation to disable
> swap.
> Since Cassandra uses mlockall to lock the heap in RAM what is the reason
> for disabling swap?
> My guess is that is has to do with memory mapped files but as of my
> understanding, accessing pages of
> memory mapped files, those pages are never put in swap since they're
> backed by files on disk and the OS
> writes those pages to the memory mapped file instead of swap.
> We've seen on Cassandra installations on Linux with swap enabled that
> parts of the java process is swaped out and increasing.
> So what's swaped out?
> Regards
> /Fredrik

View raw message