>>Even though you only have two threads... how many different IndexReader
>>instances do you have?
One reader - kept open throughout.
>>I think the only way to make the current version
>>work correctly is to make whole method synchronized on this
>>object, i.e. declare it as public synchronized, and then
>>the synchronized(cache) occurrences can be removed.
Analysing the pros and cons of such a change...
1) Synchronisation costs for pre-cached content on the same reader (hopefully
the most common case) would be just as quick as it is now i.e. swapping a
synchronised(this) for synchronised(cache).
2) Synchronisation for uncached content would avoid reading multiple copies of
the same bitset (one would expect overall faster responses in this scenario)
3) However, there would be an additional synchronisation cost for threads using
unrelated readers - a thread using an old reader with pre-cached content would
be delayed (potentially significantly) by a thread caching new content on a new
reader.
This last one sounds a bit nasty?
Cheers
Mark
----- Original Message ----
From: Paul Elschot <[EMAIL PROTECTED]>
To: java-dev@lucene.apache.org
Sent: Monday, 18 February, 2008 3:23:01 PM
Subject: Re: Out of memory - CachingWrappperFilter and multiple threads
I think this code has been discussed before, but I can't find
the occasion on which that happened.
Actually, this code may do some things like creating two different
caches, and as you said calling filter.bits(reader) in parallel.
Creating different caches does not hurt because in the end
one only one will survive the assignment, but it's not nice as
different threads might synchronize on different caches
for the first cache.get() call, and I don't know whether
java guarantees that the cache.get() call will synchronize
on the same cache as the cache in the cache.get()
call in this case.
Doing the filter.bits(reader) call without synchronization
is probably on purpose: it's better not to hold any visible
lock while calling outside an object.
As it stands, I think the only way to make the current version
work correctly is to make whole method synchronized on this
object, i.e. declare it as public synchronized, and then
the synchronized(cache) occurrences can be removed.
It might be better to initialize the cache in the constructor,
and then synchronize on the cache while even while
calling filter.bits(reader). This is safe when the cache is private.
Regards,
Paul Elschot
Op Monday 18 February 2008 13:50:16 schreef mark harwood:
> I'm chasing down a bug in my application where multiple threads were
> readingand caching the same filter (same very common term, big index) and
> causedan Out of Memory exception when I would expect there to be plenty
> ofmemory to spare.
> There's a number of layers to this app to investigate (I was using
> theXMLQueryParser and the CachedFilter tag too) but
> CachingWrapperFilterunderpins all this stuff and I was led to this code in
> it...
>
> public BitSet bits(IndexReader reader) throws IOException {
> if (cache == null) {
> cache = new WeakHashMap();
> }
>
> synchronized (cache) { // check cache
> BitSet cached = (BitSet) cache.get(reader);
> if (cached != null) {
> return cached;
> }
> }
>
> final BitSet bits = filter.bits(reader);
>
> synchronized (cache) { // update cache
> cache.put(reader, bits);
> }
>
> return bits;
> }
>
>
> The first observation is - why the use of"final" for the variable "bits" ?
> Would there be anyside-effects to this?
>
> Perhaps more worryingly I can see that multiple threads asking for the same
> bitset simultaneously arelikely to unnecessarily read the same data from the
> same reader (butultimately only one bitset should end up cached). My app only
> had 2 simultaneous threads on the same reader so I don't see how that
> accounts for the large memory bloat I saw. In a high traffic environment
> though, I can see multiple requests for a popular term getting bottle-necked
> here creating the same bitset and causing an OOM error. It looks like this
> multiple-load scenario could/should be avoided with some careful
> synchronisation.
>
> Unfortunately I've been unable to reproduce my OOM problem outside of the
> live environment so can't fully pinpoint my particular issue or the solution
> just yet.
>
> Thoughts?
> Mark
>
>
>
>
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