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From Jay Kreps <jay.kr...@gmail.com>
Subject Re: soft references for object caching in the key-value storage engine
Date Thu, 12 Sep 2013 03:22:45 GMT
Sriram, yes, I think you raise the best criticism of this approach. In the
current design the caches are per task-store combination. This is arguably
a nightmare to tune, and in my experience people never do this kind of
thing right, but you do at least have the ability to say X% of memory for
store A, Y% for store B. Arguably at the store level LRU across should be
fine (better even), but between tasks this could be an issue.

Martin, both you and Sriram raise the possibility of GC latency but that is
actually kind of a minor issue for a stream processing system (certainly in
comparison to a real-time request-response service).

Overall I think both these issues would tend to be minor because this is
just the object cache. LevelDB still has a block cache.

In either case I threw this out there more speculatively to see if anyone
knew of any critical drawbacks.

-Jay




On Tue, Sep 10, 2013 at 11:54 AM, Martin Scholl <m@funkpopes.org> wrote:

> 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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