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From Kent Fitch <kent.fi...@gmail.com>
Subject UnInvertedField performance on faceted fields containing many unique terms
Date Mon, 15 Jun 2009 23:43:55 GMT
Hi,

This may be of interest to other users of SOLR's UnInvertedField who
have a very large number of unique terms in faceted fields.

Our setup is :

- about 34M lucene documents of bibliographic and full text content
- index currently 115GB, will at least double over next 6 months
- moving to support real-time-ish updates (maybe 5 min delay)

We facet on 8 fields, 6 of which are "normal" with small numbers of
distinct values.  But 2 faceted fields, creator and subject, are huge,
with 18M and 9M terms respectively.  (Whether we should be faceting on
such a huge number of values, and at the same time attempting to
provide real time-ish updates is another question!  Whether facets
derived from all of the hundreds of thousands of results regardless of
match quality which typically happens in a large full text index is
yet another question!).  The app is visible here:
http://sbdsproto.nla.gov.au/

On a server with 2xquad core AMD 2382 processors and 64GB memory, java
1.6.0_13-b03, 64 bit run with "-Xmx15192M -Xms6000M -verbose:gc", with
the index on Intel X25M SSD, on start-up the elapsed time to create
the 8 facets is 306 seconds (best time).  Following an index reopen,
the time to recreate them in 318 seconds (best time).

[We have made an independent experimental change to create the facets
with 3 async threads, that is, in parallel, and also to decouple them
from the underlying index, so our facets lag the index changes by the
time to recreate the facets.  With our setup, the 3 threads reduced
facet creation elapsed time from about 450 secs to around 320 secs,
but this will depend a lot on IO capabilities of the device containing
the index, amount of file system caching, load, etc]

Anyway, we noticed that huge amounts of garbage were being collected
during facet generation of the creator and subject fields, and tracked
it down to this decision in UnInvertedField univert():

      if (termNum >= maxTermCounts.length) {
        // resize, but conserve memory by not doubling
        // resize at end??? we waste a maximum of 16K (average of 8K)
        int[] newMaxTermCounts = new int[maxTermCounts.length+4096];
        System.arraycopy(maxTermCounts, 0, newMaxTermCounts, 0, termNum);
        maxTermCounts = newMaxTermCounts;
      }

So, we tried the obvious thing:

- allocate 10K terms initially, rather than 1K
- extend by doubling the current size, rather than adding a fixed 4K
- free unused space at the end (but only if unused space is
"significant") by reallocating the array to the exact required size

And also:

- created a static HashMap lookup keyed on field name which remembers
the previous allocated size for maxTermCounts for that field, and
initially allocates that size + 1000 entries

The second change is a minor optimisation, but the first change, by
eliminating thousands of array reallocations and copies, greatly
improved load times, down from 306 to 124 seconds on the initial load
and from 318 to 134 seconds on reloads after index updates.  About
60-70 secs is still spend in GC, but it is a significant improvement.

Unless you have very large numbers of facet values, this change won't
have any positive benefit.

Regards,

Kent Fitch

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