See http://en.wikipedia.org/wiki/Swappiness
On 13 March 2013 19:56, Fredrik Stigbäck <fredrik.l.stigb...@sitevision.se>wrote: > Well, we've seen a Cassandra process swap out 500 MB on a Linux OS > with plenty of RAM, so I was just curious as why the OS thinks it > should use the swap at all. > > 2013/3/13 karim duran <karim.du...@gmail.com>: > > I agree with Edward Capriolo, > > Even when swap is enabled on your system, swaping rarely occurs on OS > > today...(except for very loaded systems). > > > > But, take care that some 32 bits system kernels allows only 2^32 bits > memory > > mapped file length ( ~ 2 Go ). > > It could be a limitation for NoSQL databases. It's the case for MongoDB > on > > 32 bits OS. > > > > I don't know how to avoid swaping if Cassandra exceeds these limitation > when > > this case occurs. > > > > > > 2013/3/13 Edward Capriolo <edlinuxg...@gmail.com> > >> > >> 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 > >> RAM. > >> > >> 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 > >> <fredrik.l.stigb...@sitevision.se> wrote: > >>> > >>> 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 > >>> > >>> > >>> > >>> > >> > > > > > > -- > Fredrik Larsson Stigbäck > SiteVision AB Vasagatan 10, 107 10 Örebro > 019-17 30 30 >
