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From Jared Biel <jared.b...@bolderthinking.com>
Subject Re: mmaped files and swap
Date Wed, 13 Mar 2013 22:14:25 GMT
See http://en.wikipedia.org/wiki/Swappiness


On 13 March 2013 19:56, Fredrik Stigbäck
<fredrik.l.stigback@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.duran@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 <edlinuxguru@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.stigback@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
>

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