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From Michael McCandless <luc...@mikemccandless.com>
Subject Re: heap memory issues when sorting by a string field
Date Tue, 08 Dec 2009 10:36:24 GMT
This is a rather disturbing implementation detail of WeakHashMap, that
it needs the one extra step (invoking one of its methods) for its weak
keys to be reclaimable.

Maybe on IndexReader.close(), Lucene should go and evict all entries
in the FieldCache associated with that reader.  Ie, step through the
sub-readers, and if they are truly closed as well (not shared w/ other
readers), evict.  I'll open an issue.

Even in TCK's code fragment, it's not until the final line is done
executing, that the cache key even loses all hard references, because
it's that line that assigns to sSearcher, replacing the strong
reference to the old searcher.  Inserting sSearcher = null prior to
that would drop the hard reference sooner, but because of this impl
detail of WeakHashMap, something would still have to touch it (eg, a
warmup query that hits the field cache) before it's reclaimable.

Mike

On Mon, Dec 7, 2009 at 7:38 PM, Tom Hill <solr-list@worldware.com> wrote:
> Hi -
>
> If I understand correctly, WeakHashMap does not free the memory for the
> value (cached data) when the key is nulled, or even when the key is garbage
> collected.
>
> It requires one more step: a method on WeakHashMap must be called to allow
> it to release its hard reference to the cached data. It appears that most
> methods in WeakHashMap end up calling expungeStaleEntries, which will clear
> the hard reference. But you have to call some method on the map, before the
> memory is eligible for garbage collection.
>
> So it requires four stages to free the cached data. Null the key; A GC to
> release the weak reference to the key; A call to some method on the map;
> Then the next GC cycle should free the value.
>
> So it seems possible that you could end up with double memory usage for a
> time. If you don't have a GC between the time that you close the old reader,
> and you start to load the field cache entry for the next reader, then the
> key may still be hanging around uncollected.
>
> At that point, it may run a GC when you allocate the new cache, but that's
> only the first GC. It can't free the cached data until after the next call
> to expungeStaleEntries, so for a while you have both caches around.
>
> This extra usage could cause things to move into tenured space. Could this
> be causing your problem?
>
> Workaround would be to cause some method to be called on the WeakHashMap.
> You don't want to call get(), since that will try to populate the cache.
> Maybe if you tried putting a small value to the cache, and doing a GC, and
> see if your memory drops then.
>
>
> Tom
>
>
>
> On Mon, Dec 7, 2009 at 1:48 PM, TCK <moonwatcher32329@gmail.com> wrote:
>
>> Thanks for the response. But I'm definitely calling close() on the old
>> reader and opening a new one (not using reopen). Also, to simplify the
>> analysis, I did my test with a single-threaded requester to eliminate any
>> concurrency issues.
>>
>> I'm doing:
>> sSearcher.getIndexReader().close();
>> sSearcher.close(); // this actually seems to be a no-op
>> IndexReader newIndexReader = IndexReader.open(newDirectory);
>> sSearcher = new IndexSearcher(newIndexReader);
>>
>> Btw, isn't it bad practice anyway to have an unbounded cache? Are there any
>> plans to replace the HashMaps used for the innerCaches with an actual
>> size-bounded cache with some eviction policy (perhaps EhCache or something)
>> ?
>>
>> Thanks again,
>> TCK
>>
>>
>>
>>
>> On Mon, Dec 7, 2009 at 4:37 PM, Erick Erickson <erickerickson@gmail.com
>> >wrote:
>>
>> > What this sounds like is that you're not really closing your
>> > readers even though you think you are. Sorting indeed uses up
>> > significant memory when it populates internal caches and keeps
>> > it around for later use (which is one of the reasons that warming
>> > queries matter). But if you really do close the reader, I'm pretty
>> > sure the memory should be GC-able.
>> >
>> > One thing that trips people up is IndexReader.reopen(). If it
>> > returns a reader different than the original, you *must* close the
>> > old one. If you don't, the old reader is still hanging around and
>> > memory won't be returne.... An example from the Javadocs...
>> >
>> >  IndexReader reader = ...
>> >  ...
>> >  IndexReader new = r.reopen();
>> >  if (new != reader) {
>> >   ...     // reader was reopened
>> >   reader.close();
>> >  }
>> >  reader = new;
>> >  ...
>> >
>> >
>> > If this is irrelevant, could you post your close/open
>> >
>> > code?
>> >
>> > HTH
>> >
>> > Erick
>> >
>> >
>> > On Mon, Dec 7, 2009 at 4:27 PM, TCK <moonwatcher32329@gmail.com> wrote:
>> >
>> > > Hi,
>> > > I'm having heap memory issues when I do lucene queries involving
>> sorting
>> > by
>> > > a string field. Such queries seem to load a lot of data in to the heap.
>> > > Moreover lucene seems to hold on to references to this data even after
>> > the
>> > > index reader has been closed and a full GC has been run. Some of the
>> > > consequences of this are that in my generational heap configuration a
>> lot
>> > > of
>> > > memory gets promoted to tenured space each time I close the old index
>> > > reader
>> > > and after opening and querying using a new one, and the tenured space
>> > > eventually gets fragmented causing a lot of promotion failures
>> resulting
>> > in
>> > > jvm hangs while the jvm does stop-the-world GCs.
>> > >
>> > > Does anyone know any workarounds to avoid these memory issues when
>> doing
>> > > such lucene queries?
>> > >
>> > > My profiling showed that even after a full GC lucene is holding on to a
>> > lot
>> > > of references to field value data notably via the
>> > > FieldCacheImpl/ExtendedFieldCacheImpl. I noticed that the WeakHashMap
>> > > readerCaches are using unbounded HashMaps as the innerCaches and I used
>> > > reflection to replace these innerCaches with dummy empty HashMaps, but
>> > > still
>> > > I'm seeing the same behavior. I wondered if anyone has gone through
>> these
>> > > same issues before and would offer any advice.
>> > >
>> > > Thanks a lot,
>> > > TCK
>> > >
>> >
>>
>

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