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https://issues.apache.org/jira/browse/LUCENE-505?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12664280#action_12664280
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Michael McCandless commented on LUCENE-505:
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bq. In my opinion the problem with large indexes is more, that each
SegmentReader has a cache of the last used norms.
I believe when MultiReader.norms is called (as Doug & Yonik said above), the
underlying SegmentReaders do not in fact cache the norms (this is not readily
obvious until you scrutinize the code). Ie, it's only MultiReader that caches
the full array.
But I agree there would be good benefits (not creating fakeNorms) to moving
away from byte[] for norms. I think an iterator only API might be fine (giving
us more freedom on the impl.), though I would worry about performance impact.
Or we could make a new method to replace norms() that returns null when the
field has no norms, and then Scorers that use this API would handle the null
correctly. We could fix all core/contribs to use the new API...
Also note that with LUCENE-1483, we are moving to searching each segment at a
time, so MultiReader.norms should not normally be called, unless it doesn't
expose its underlying readers.
> MultiReader.norm() takes up too much memory: norms byte[] should be made into
> an Object
> ---------------------------------------------------------------------------------------
>
> Key: LUCENE-505
> URL: https://issues.apache.org/jira/browse/LUCENE-505
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Index
> Affects Versions: 2.0.0
> Environment: Patch is against Lucene 1.9 trunk (as of Mar 1 06)
> Reporter: Steven Tamm
> Priority: Minor
> Attachments: LazyNorms.patch, NormFactors.patch, NormFactors.patch,
> NormFactors20.patch
>
>
> MultiReader.norms() is very inefficient: it has to construct a byte array
> that's as long as all the documents in every segment. This doubles the
> memory requirement for scoring MultiReaders vs. Segment Readers. Although
> this is cached, it's still a baseline of memory that is unnecessary.
> The problem is that the Normalization Factors are passed around as a byte[].
> If it were instead replaced with an Object, you could perform a whole host of
> optimizations
> a. When reading, you wouldn't have to construct a "fakeNorms" array of all
> 1.0fs. You could instead return a singleton object that would just return
> 1.0f.
> b. MultiReader could use an object that could delegate to NormFactors of the
> subreaders
> c. You could write an implementation that could use mmap to access the norm
> factors. Or if the index isn't long lived, you could use an implementation
> that reads directly from the disk.
> The patch provided here replaces the use of byte[] with a new abstract class
> called NormFactors.
> NormFactors has two methods on it
> public abstract byte getByte(int doc) throws IOException; // Returns the
> byte[doc]
> public float getFactor(int doc) throws IOException; // Calls
> Similarity.decodeNorm(getByte(doc))
> There are four implementations of this abstract class
> 1. NormFactors.EmptyNormFactors - This replaces the fakeNorms with a
> singleton that only returns 1.0
> 2. NormFactors.ByteNormFactors - Converts a byte[] to a NormFactors for
> backwards compatibility in constructors.
> 3. MultiNormFactors - Multiplexes the NormFactors in MultiReader to prevent
> the need to construct the gigantic norms array.
> 4. SegmentReader.Norm - Same class, but now extends NormFactors to provide
> the same access.
> In addition, Many of the Query and Scorer classes were changes to pass around
> NormFactors instead of byte[], and to call getFactor() instead of using the
> byte[]. I have kept around IndexReader.norms(String) for backwards
> compatibiltiy, but marked it as deprecated. I believe that the use of
> ByteNormFactors in IndexReader.getNormFactors() will keep backward
> compatibility with other IndexReader implementations, but I don't know how to
> test that.
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