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From Martin Scholl...@funkpopes.org>
Subject Re: soft references for object caching in the key-value storage engine
Date Tue, 10 Sep 2013 18:54:01 GMT
I'm by no means a JVM expert and I am by no means able to give any final
judgement on this, but I can say I remember various problems people ran
into when using SoftReferences as well as WeakReferences.

What a quick search yielded:

"Soft references contribute to memory pressure but throughput collectors
clear them all at once when memory fills up while CMS gradually clears
them, so while you do get this memory sensitive gradual eviction of soft
reference data, you also get increased unpredictability of your garbage
collectors and that's not really what you want with CMS."
-- http://www.javaperformancetuning.com/news/newtips136.shtml

This is a nice argument that would defeat the purpose you make up here
though I cannot tell if only CMS shows this behavior.
This having said, [1] seems to imply that SoftReferences, like
WeakReferences,are GC'd LRU'ish.

My humble suggestion is to rather extend LevelDB to allow expunge data by
time in constant time.


Hope it helps,
Martin

[1]
http://stackoverflow.com/questions/299659/what-is-the-difference-between-a-soft-reference-and-a-weak-reference-in-java


On Tue, Sep 10, 2013 at 5:50 PM, Jay Kreps <jay.kreps@gmail.com> wrote:

> One idea I had was to use soft references for object cache in key-value
> store. Currently we use an LRU hashmap, but the drawback of this is that it
> needs to be carefully sized based on heap size and the number of
> partitions. It is a little hard to know when to add memory to the object
> cache vs the block cache. Plus, since the size is based both on the objects
> in it, but also the overhead per object this is pretty impossible to
> calculate the worst case memory usage of N objects to make this work
> properly with a given heap size.
>
> Another option would be to use soft references:
> http://docs.oracle.com/javase/7/docs/api/java/lang/ref/SoftReference.html
>
> Soft references will let you use all available heap space as a cache that
> gets gc'd only when strong These are usually frowned upon for caches due to
> the unpredictability of the discard--basically the garbage collector has
> some heuristic by which it chooses what to discard (
>
> http://jeremymanson.blogspot.com/2009/07/how-hotspot-decides-to-clear_07.html
> )
> but it is based on a heuristic of how much actual free memory to maintain.
> This makes soft references a little dicey for latency sensitive services.
>
> But for Samza the caching is really about optimizing throughput not
> reducing the latency of a particular lookup. So using the rest of the free
> memory in the heap for caching is actually attractive. It is true that the
> garbage collector might occasionally destroy our cache but that is actually
> okay and possibly worth getting orders of magnitude extra cache space.
>
> This does seem like the kind of thing that would have odd corner cases.
> Anyone have practical experience with these who can tell me why this is a
> bad idea?
>
> -Jay
>

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