mergeFields depends on the fields from the various segments being
sorted for the fieldsSame comparison to work.
Of note, the 'fieldi > 1' guard skips the 0th field, which should
always be the '_id' field.
The optimization recently introduced in commit 530a3d24cf,
("scorch zap optimize merge by byte copying freq/norm/loc's") was to
byte-copy freq/norm/loc data directly during merging. But, it was
incorrect if the fields were different across segments.
This change now performs that byte-copying merging optimization only
when the fields are the same across segments, and if not, leverages
the old approach of deserializing & re-serializing the freq/norm/loc
information, which has the important step of remapping fieldID's.
See also: https://issues.couchbase.com/browse/MB-28781
pprof of bleve-blast was showing file merging was in syscall/write a
lot. The bufio.NewWriter() provides a default buffer size of 4K,
which is too small, and using bufio.NewWriterSize(1MB buffer size)
leads to syscall/write dropping out of the file merging flame graphs.
This change detects whether a deletion bitmap is empty, and treats
that as a nil bitmap, which allows further postings iterator codepaths
to avoid roaring bitmap operations (like, AndNot(docNums, drops)).
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).
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 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.
As part of this, zap.MergeToWriter() now returns more information --
enough so that callers can now create their own SegmentBase instances.
Also, the fieldsMap maintained and returned by zap.MergeToWriter() is
now a mapping from fieldName ==> fieldID+1 (instead of the previous
mapping from fieldName ==> fieldID). This makes it similar to how
fieldsMap are handled in other parts of zap to avoid "zero value"
issues.
The theory with this change is that the dicts and itrs should be
positionally in "lock-step" with paired entries.
And, since later code also uses the same array indexing to access the
drops and newDocNums, those also need to be positionally in pair-wise
lock-step, too.
During zap segment merging, a new zap PostingsIterator was allocated
for every field X segment X term.
This change optimizes by reusing a single PostingsIterator instance
per persistMergedRest() invocation.
And, also unused fields are removed from the PostingsIterator.
The optimization to byte-copy all the storedDocs for a given segment
during merging kicks in when the fields are the same across all
segments and when there are no deletions for that given segment. This
can happen, for example, during data loading or insert-only scenarios.
As part of this commit, the Segment.copyStoredDocs() method was added,
which uses a single Write() call to copy all the stored docs bytes of
a segment to a writer in one shot.
And, getDocStoredMetaAndCompressed() was refactored into a related
helper function, getDocStoredOffsets(), which provides the storedDocs
metadata (offsets & lengths) for a doc.
COMPATIBILITY NOTE: scorch zap version bumped in this commit.
The version bump is because mergeFields() now computes whether fields
are the same across segments and it relies on the previous commit
where fieldID's are assigned in field name sorted order (albeit with
_id field always having fieldID of 0).
Potential future commits might rely on this info that "fields are the
same across segments" for more optimizations, etc.
This is a stepping stone to allow easier future comparisons of field
maps and potential merge optimizations.
In bleve-blast tests on a 2015 macbook (50K wikipedia docs, 8
indexers, batch size 100, ssd), this does not seem to have a distinct
effect on indexing throughput.
This change turns zap.MergeToWriter() into a public func, so that it's
now directly callable from outside packages (such as from scorch's
top-level merger or persister). And, MergerToWriter() now takes input
of SegmentBases instead of Segments, so that it can now work on either
in-memory zap segments or file-based zap segments.
This is yet another stepping stone towards in-memory merging of zap
segments.
Instead of sorting docNum keys from a hashmap, this change instead
iterates from docNum 0 to N and uses an array instead of hashmap.
The array is also reused across outer loop iterations.
This optimizes for when there's a lot of structural similarity between
docs, where many/most docs have the same fields. i.e., beers,
breweries. If every doc has completely different fields, then this
change might produce worse behavior compared to the previous sparse
hashmap approach.
Instead of allocating a govarint.U64Base128Encoder in the inner loop,
allocate it just once on the outside, as it appears that it's just a
thin wrapper around binary.PutUvarint().
The zap SegmentBase struct is a refactoring of the zap Segment into
the subset of fields that are needed for read-only ops, without any
persistence related info. This allows us to use zap's optimized data
encoding as scorch's in-memory segments.
The zap Segment struct now embeds a zap SegmentBase struct, and layers
on persistence. Both the zap Segment and zap SegmentBase implement
scorch's Segment interface.
abhinavdangeti