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From Ravikumar Govindarajan <ravikumar.govindara...@gmail.com>
Subject Re: Help on MMap of SSTables
Date Thu, 06 Dec 2012 06:48:12 GMT
Thanks Aaron,

I found the implementation in CLibrary.trySkipCache() method which uses
fadvise DONTNEED flag after going through
https://issues.apache.org/jira/browse/CASSANDRA-1470

I also came across the link mentioned in JIRA
http://blog.mikemccandless.com/2010/06/lucene-and-fadvisemadvise.html?showComment=1303235497682#c2572106601600642254

which says 2.6.29 version above has implemented madvise SEQUENTIAL in a
better manner.

So for memory mapped files, compaction can do a madvise SEQUENTIAL instead
of current DONTNEED flag after detecting appropriate OS versions. Will this
help?

--
Ravi

On Thu, Dec 6, 2012 at 8:19 AM, aaron morton <aaron@thelastpickle.com>wrote:

> Background http://en.wikipedia.org/wiki/Memory-mapped_file
>
> Is it going to load only relevant pages per SSTable on read or is it going
> to load an entire SSTable on first access?
>
> It will load what is requested, and maybe some additional data taking into
> account the amount of memory available for caches.
>
> Say suppose compaction kicks in. Will it then evict hot MMapped pages for
> read and substitute it with a lot of pages involving full SSTables?
>
> Some file access in cassandra, such as compaction, hints to the OS that
> the reads should not be cached. Technically is uses posix_fadvise if you
> want to look it up.
>
> Cheers
>
>
> -----------------
> Aaron Morton
> Freelance Cassandra Developer
> New Zealand
>
> @aaronmorton
> http://www.thelastpickle.com
>
> On 5/12/2012, at 11:04 PM, Ravikumar Govindarajan <
> ravikumar.govindarajan@gmail.com> wrote:
>
> Thanks Aaron,
>
> I am not quite clear on how MMap loads SSTables other than the fact that
> it kicks in only during a first-time access
>
> Is it going to load only relevant pages per SSTable on read or is it going
> to load an entire SSTable on first access?
>
> Say suppose compaction kicks in. Will it then evict hot MMapped pages for
> read and substitute it with a lot of pages involving full SSTables?
>
> --
> Ravi
>
> On Wed, Dec 5, 2012 at 1:22 AM, aaron morton <aaron@thelastpickle.com>wrote:
>
>> Will MMapping data files be detrimental for reads, in this case?
>>
>> No.
>>
>> In general, when should we opt for MMap data files and what are the
>> factors that need special attention when enabling the same?
>>
>> mmapping is the default, so I would say use it until you have a reason
>> not to.
>>
>> mmapping will map the entire file, but pages of data are read into memory
>> on demand and purged when space is needed.
>>
>> Cheers
>>
>>    -----------------
>> Aaron Morton
>> Freelance Cassandra Developer
>> New Zealand
>>
>> @aaronmorton
>> http://www.thelastpickle.com
>>
>> On 4/12/2012, at 11:59 PM, Ravikumar Govindarajan <
>> ravikumar.govindarajan@gmail.com> wrote:
>>
>> Our current SSTable sizes are far greater than RAM. {150 Gigs of data,
>> 32GB RAM}. Currently we run with mlockall and mmap_index_only options and
>> don't experience swapping at all.
>>
>> We use wide rows and size-tiered-compaction, so a given key will
>> definitely be spread across multiple sstables. Will MMapping data files be
>> detrimental for reads, in this case?
>>
>> In general, when should we opt for MMap data files and what are the
>> factors that need special attention when enabling the same?
>>
>> --
>> Ravi
>>
>>
>>
>
>

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