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From Daniel Doubleday <>
Subject Alternative Row Cache Implementation
Date Thu, 30 Jun 2011 16:44:01 GMT
Hi all - or rather devs

we have been working on an alternative implementation to the existing row cache(s)

We have 2 main goals:

- Decrease memory -> get more rows in the cache without suffering a huge performance penalty
- Reduce gc pressure

This sounds a lot like we should be using the new serializing cache in 0.8. 
Unfortunately our workload consists of loads of updates which would invalidate the cache all
the time.

The second unfortunate thing is that the idea we came up with doesn't fit the new cache provider

It looks like this:

Like the serializing cache we basically only cache the serialized byte buffer. we don't serialize
the bloom filter and try to do some other minor compression tricks (var ints etc not done
yet). The main difference is that we don't deserialize but use the normal sstable iterators
and filters as in the regular uncached case.

So the read path looks like this:

return filter.collectCollatedColumns(memtable iter, cached row iter)

The write path is not affected. It does not update the cache

During flush we merge all memtable updates with the cached rows.

These are early test results:

- Depending on row width and value size the serialized cache takes between 30% - 50% of memory
compared with cached CF. This might be optimized further
- Read times increase by 5 - 10%

We haven't tested the effects on gc but hope that we will see improvements there because we
only cache a fraction of objects (in terms of numbers) in old gen heap which should make gc
cheaper. Of course there's also the option to use native mem like serializing cache does.

We believe that this approach is quite promising but as I said it is not compatible with the
current cache api.

So my question is: does that sound interesting enough to open a jira or has that idea already
been considered and rejected for some reason?

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