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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: memory utilization
Date Sun, 13 Mar 2011 01:24:17 GMT
Probably relevant: we only use mmap'd I/O for single-row reads. When
we are paging through entire files like we do for compaction or AES we
do buffered i/o to avoid the complexity of having to manage multiple
mmap segments (Java limits us to 2GB per segment).

On Sat, Mar 12, 2011 at 7:06 PM, Peter Schuller
<peter.schuller@infidyne.com> wrote:
>>> Nothing happens, because it _doesn't have to be resident_.
>>>
>>
>> Hm, but why in my case top show RSS 10g, when max HEAP_SIZE is 6G?
>
> The point is that it is a result of how the kernel manages memory and
> how it is reported in top. It is not reflective of actual memory
> "use", the way users normally use the term.
>
> If you take a file that is 1 gig in file and "cat" it, it will end up
> in page cache. But that does not get accounted to any particular
> process. If on the other hand you mmap() the file in a process and
> stream through it, it will be accounted as part of the resident set of
> the process. But it is not indicative that the process is "using" that
> memory in the usual sense of the word.
>
> Since this keeps coming up I decided to put up the little test I have
> that can be used to demonstrate the effect:
>
>   https://github.com/scode/alloctest
>
> You can run that and observe the effects in top.
>
> That said, there's *something* fishy going on in that whether pages
> get counted towards the process may be depending on something else.
> E.g., I have a node with ~ 1 TB virtual/sstable sizes that is actively
> doing AES (so should be doing mmap():ed i/o), yet I have exactly 10
> gig (max heap size) RSS instead of > 10 gb. I haven't investigated
> properly. Maybe just depending on mmap flags.
>
> --
> / Peter Schuller
>



-- 
Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of DataStax, the source for professional Cassandra support
http://www.datastax.com

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