This change produces less garbage by switching from a map[uint16]'s to
array's for the fieldLens and docMap, and then reusing those arrays
across multiple processDocument() calls.
NOTE: this is a scorch zap file format change / bump to version 4.
In this optimization, the uint64 val stored in the vellum FST (term
dictionary) now may either be a uint64 postingsOffset (same as before
this change) or a uint64 encoding of the docNum + norm (in the case
where a term appears in just a single doc).
Do not re-account for certain referenced data in the zap structures.
New estimates:
ESTIMATE BENCHMEM
TermQuery 11396 12437
MatchQuery 12244 12951
DisjunctionQuery (Term queries) 20644 20709
Before this change, writeRoaringWithLen() would leverage a reused
bytes.Buffer (#A) and invoke the roaring.WriteTo() API.
But, it turns out the roaring.WriteTo() API has a suboptimal
implementation, in that underneath-the-hood it converts the roaring
bitmap to a byte buffer (using roaring.ToBytes()), and then calls
Write(). But, that Write() turns out to be an additional memcpy into
the provided bytes.Buffer (#A).
By directly invoking roaring.ToBytes(), this change to
writeRoaringWithLen() avoids the extra memory allocation and memcpy.
This change leverages the ability for the chunkedIntCoder.Add() method
to accept multiple input param values (via the '...' param signature),
meaning there are fewer Add() invocations.
This callback if registered with context will invoke the api to estimate
the memory needed to execute a search query. The callback defined at
the client side will be responsible for determining whether to
continue with the search or abort based on the threshold settings.
This API (unexported) will estimate the amount of memory needed to execute
a search query over an index before the collector begins data collection.
Sample estimates for certain queries:
{Size: 10, BenchmarkUpsidedownSearchOverhead}
ESTIMATE BENCHMEM
TermQuery 4616 4796
MatchQuery 5210 5405
DisjunctionQuery (Match queries) 7700 8447
DisjunctionQuery (Term queries) 6514 6591
ConjunctionQuery (Match queries) 7524 8175
Nested disjunction query (disjunction of disjunctions) 10306 10708
…
This change adds a zap PostingsIterator.nextBytes() method, which is
similar to Next(), but instead of returning a Posting instance,
nextBytes() returns the encoded freq/norm and location byte slices.
The zap merge code then provides those byte slices directly to the
intCoder's via a new method, intCoder.AddBytes(), thereby avoiding
having to encode many uvarint's.
Due to the usage rules of iterators, mem.PostingsIterator.Next() can
reuse its returned Postings instance.
Also, there's a micro optimization in persistDocValues() for one fewer
access to the docTermMap in the inner-loop.
