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From Zhibin Mai <>
Subject Re: how to estimate how much memory is required to support the large index search
Date Tue, 18 Nov 2008 01:00:59 GMT

I figured it out that most of heap was consumed by the Term cache. In our case, the index
has 233 millions of terms and 6.4 millions of them were loaded into the cache when we did
the search. I roughly did a calculation that each term will need how much memory, it is about
16 bytes for Term Object + 32 bytes for TermInfo Object + 24 bytes for String Object for term
text + 2 * length(Char[]) for term text.

In our case, the average length of term text is 25 characters, that means each term needs
at least 122 bytes. The cache for 6.4 millions of terms needs 6.4 * 122 = 780MB. Plus 200MB
for caching norm, the RAM for cache is larger than 980MB. We work around the cache issue for
Terms by setting index divisor of the IndexReader to a higher value. Actually, the performance
of search is good even using index divisor as 4.



From: Aleksander M. Stensby <>
Sent: Monday, November 17, 2008 2:31:04 AM
Subject: Re: how to estimate how much memory is required to support the large index search

One major factor that may result in heap space problems is if you are doing any form of sorting
when searching. Do you have any form of default sort in your application? Also, the type of
field used for sorting is important with regard to memory consumption.

This issue has been discussed before on the list. (You can search the archive for sorting
and memory consumption.)

- Aleksander

On Sun, 16 Nov 2008 14:36:36 +0100, Zhibin Mai <> wrote:

> Hello,
> I
> am a beginner on using lucene. We developed an application to
> create and search index using lucene 2.3.1. We would like to know how
> to estimate how much memory is required to support
> the index search given an index.
> Recently,
> the size of the index has reached to about 200GB with 197M of documents
> and 223M of terms. Our application starts having intermittent
> "OutOfMemoryError: Java heap space" when we use
> it to search the index. We use JProfiler to get the following memory allocation when
we do one keyword search:
> char[]                                                        332MB
> org.apache.lucene.index.TermInfo            194MB
> java.lang.String                                        146MB
> org.apache.lucene.index.Term                99,823KB
> org.apache.lucene.index.Term                24,956KB
> org.apache.lucene.index.TermInfo[]        24,956KB
> byte[]                                                    188MB
> long[]                                                    49,912KB
> The memory allocation for the first 6 types of objects does not change when we change
the search criteria. Could you please give me some advice what major factors will affect the
memory allocation
> and how those factors will affect the memory usage precisely on search? Is it possible
to reduce the memory usage on search?
> Thank you,
> Zhibin

--Aleksander M. Stensby
Senior software developer
Integrasco A/S

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