incubator-cassandra-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Fredrik Stigbäck <fredrik.l.stigb...@sitevision.se>
Subject Re: mmaped files and swap
Date Wed, 13 Mar 2013 19:56:12 GMT
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

Mime
View raw message