Re: Implementing caches that may be cleared if heap space shrinks away

From:
"Matt Humphrey" <matth@iviz.com>
Newsgroups:
comp.lang.java.programmer
Date:
Thu, 26 Mar 2009 09:33:37 -0400
Message-ID:
<Yb-dnXIH1vyuGVbU4p2dnAA@giganews.com>
"Tassilo Horn" <tassilo@member.fsf.org> wrote in message
news:87k56cftj9.fsf@thinkpad.tsdh.de...

Hi all,

I have a client that queries a server via RMI and uses some caches for
the results to speed up things. Unfortunatily, these caches can grow
till heap space is completely filled up and I get an exception.

So now my cache classes don't use Map<S,T> anymore but
SoftReference<Map<S,T>>. But with that approach I always have to check
if the soft reference has already been cleared before accessing the
referenced Map, i.e. in each of my cache classes I have something like
this:

--8<---------------cut here---------------start------------->8---
   private void initializeCache() {
words = new SoftReference<HashMap<String, WordInfo>>(
new HashMap<String, WordInfo>());
}

WordInfo getWordInfo(String word) throws RemoteException, JGWNLException {
if (words.get() == null) {
initializeCache();
} else if (words.get().containsKey(word)) {
return words.get().get(word);
}
WordInfo wi = jgwnl.lookupWord(word);
for (SynsetInfo si : wi.getSynsetInfos()) {
synsetCache.updateSynsetInfo(si.getUid(), si);
}
words.get().put(word, wi);
return wi;
}
--8<---------------cut here---------------end--------------->8---

And there may be the possibility that after the check the soft ref is
cleared before I access it...


I don't think this is true. From the javadocs on SoftReference---
"As long as the referent of a soft reference is strongly reachable, that is,
is actually in use, the soft reference will not be cleared."
At the time you retrieve the soft reference it is either null (already
cleared) or it refers to the object. If it refers to the object it is now
strongly reachable via the local variable and will not be cleared. This
guarantee is atomic so there's no need to double check. Just retrieve the
reference and if it's not present reload it. It will remain as long as you
have a reference to it.

Although I've not implemented caches this way I'm puzzled as to why the
entire map is under softreference rather than the individual entries,
although this is likely just a characteristic of your particular design. It
seems that if you cease to use the cache it will be discarded in its
entirety. Alternatively, with a Map <S,SoftReference<T>> the data contents
will be released but the keys will remain, which could cause a different
problem.

One poor-man's alternative would be to poll Runtime.freeMemory() in
another Thread and clear the caches when it falls beyond a certain
limit.

The other alternative I've thought about is deriving the cache classes
directly from SoftReference and overriding the clear() method. But its
docs state that the garbage collector won't call clear() but delete the
referent directly. So that's no solution, too.

Basically, what I want is an interface Clearable which defines a clear()
method that's called automatically if heap space shrinks away, but I
couldn't find anything like that.


That is similar to what I've done (appropriate for my system). I use direct
LRU / LFU with a scavenger thread so that each entry has a last-used-at
timestamp or frequency. The scavenger invokes "cleanup" which scans the
table and drops items not being used.

Matt Humphrey http://www.iviz.com/

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