Pruning
-------
Key: LUCENE-851
URL: https://issues.apache.org/jira/browse/LUCENE-851
Project: Lucene - Java
Issue Type: New Feature
Components: Index, Search
Reporter: Marvin Humphrey
Priority: Minor
Greets,
A thread on java-dev a couple of months ago drew my attention to a technique
used by Nutch for cutting down the number of hits that have to be processed:
if you have an algorithm for ordering documents by importance, and you sort
them so that the lowest document numbers have the highest rank, then most of
your high-scoring hits are going to occur early on in the hit-collection
process. Say you're looking for the top 100 matches -- the odds are pretty
good that after you've found 1000 hits, you've gotten most of the good stuff.
It may not be necessary to score the other e.g. 5,000,000 hits.
To pull this off in Nutch, they run the index through a post process whereby
documents are re-ordered by page score using the IndexSorter class.
Unfortunately, post-processing does not live happily with incremental indexing.
However, if we ensure that document numbers are ordered according to our
criteria within each segment, that's almost as good.
Say we're looking for 100 hits, as before; what we do is collect a maximum of
1000 hits per segment. If we are dealing with an index made up of 25 segments,
that's 25,000 hits max we'll have to process fully -- the rest we can skip
over. That's not as quick as only processing 1000 hits then stopping in a
fully optimized index, but it's a lot better than churning through all
5,000,000 hits.
A lot of those hits from the smallest segments will be garbage; we'll get most
of our good hits from a few large segments most of the time. But that's fine
-- the cost to process any one segment is small.
Writing a low-level scoring loop which implements pruning per segment is
straightforward. KinoSearch's version (in C) is below.
To control the amount of pruning, we need a high-level Searcher.setPruneFactor
API, which sets a multiplier; the number of hits-per-segment which must be
processed is determined by multiplying the number of hits you need by
pruneFactor. Here's code from KS for deriving hits-per-seg:
# process prune_factor if supplied
my $seg_starts;
my $hits_per_seg = 2**31 - 1;
if ( defined $self->{prune_factor} and defined $args{num_wanted} ) {
my $prune_count = $self->{prune_factor} * $args{num_wanted};
if ( $prune_count < $hits_per_seg ) { # don't exceed I32_MAX
$hits_per_seg = $prune_count;
$seg_starts = $reader->get_seg_starts;
}
}
What I have not yet written is the index-time mechanism for sorting documents.
In Nutch, they use the norms from a known indexed, non-tokenized field
("site"). However, in Lucene and KS, we can't count on any existing fields.
Document boost isn't stored directly, either. The obvious answer is to start
storing it, which would suffice for Nutch-like uses. However, it may make
sense to to avoid coupling document ordering to boost in order to influence
pruning without affecting scores.
The sort ordering information needs a permanent home in the index, since it
will be needed whenever segment merging occurs. The fixed-width per-document
storage in Lucene's .fdx file seems like a good place. If we use one float per
document, we can simply put it before or after the 64-bit file pointer and seek
into the file after multiplying the doc num by 12 rather than 8.
During indexing, we'd keep the ordering info in an array; after all documents
for a segment have been added, we create an array of sorted document numbers.
When flushing the postings, their document numbers get remapped using the
sorted array. Then we rewrite the .fdx file (and also the .tvx file), moving
the file pointers (and ordering info) to remapped locations. The fact that the
.fdt file is now "out of order" is isn't a problem -- optimizing sequential
access to that file isn't important.
This issue is closely tied to LUCENE-843, "Improve how IndexWriter uses RAM to
buffer added documents", and LUCENE-847, "Factor merge policy out of
IndexWriter". Michael McCandless, Steven Parks, Ning Li, anybody else...
comments? Suggestions?
Marvin Humphrey
Rectangular Research
http://www.rectangular.com/
--------------------------------------------------------------------
void
Scorer_collect(Scorer *self, HitCollector *hc, u32_t start, u32_t end,
u32_t hits_per_seg, VArray *seg_starts)
{
u32_t seg_num = 0;
u32_t doc_num_thresh = 0;
u32_t hits_this_seg = 0;
u32_t hits_thresh = hits_per_seg;
/* get to first doc */
if ( !Scorer_Skip_To(self, start) )
return;
/* execute scoring loop */
do {
u32_t doc_num = Scorer_Doc(self);
Tally *tally;
if (hits_this_seg >= hits_thresh || doc_num >= doc_num_thresh) {
if (doc_num >= end) {
/* bail out of loop if we've hit the user-spec'd end */
return;
}
else if (seg_starts == NULL || seg_starts->size == 0) {
/* must supply seg_starts to enable pruning */
hits_thresh = U32_MAX;
doc_num_thresh = end;
}
else if (seg_num == seg_starts->size) {
/* we've finished the last segment */
return;
}
else {
/* get start of upcoming segment */
Int *this_start = (Int*)VA_Fetch(seg_starts, seg_num);
Int *next_start = (Int*)VA_Fetch(seg_starts, seg_num + 1);
u32_t this_seg_start = this_start->value;
seg_num++;
/* skip docs as appropriate if we're pruning */
if (doc_num < this_seg_start) {
if ( Scorer_Skip_To(self, this_seg_start) )
doc_num = Scorer_Doc(self);
else
return;
}
/* set the last doc_num we'll score this upcoming segment */
doc_num_thresh = next_start == NULL
? end /* last segment */
: next_start->value;
}
/* start over counting hits for the new segment */
hits_this_seg = 0;
}
/* this doc is in range, so collect it */
tally = Scorer_Tally(self);
hc->collect(hc, doc_num, tally->score);
hits_this_seg++;
} while (Scorer_Next(self));
}
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
---------------------------------------------------------------------
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
