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.
+ Account for all the overhead incurred from the data structures
within mem.Segment and zap.Segment.
- SizeOfMap = 8
- SizeOfPointer = 8
- SizeOfSlice = 24
- SizeOfString = 16
+ Include overhead from certain new fields as well.
This change preallocates more of the backing arrays for Locfields,
Locstarts, Locends, Locpos, Locaaraypos sub-slices of a scorch mem
segment.
On small bleve-blast tests (50K wiki docs) on a dev macbook, scorch
indexing throughput seems to improve from 15MB/sec to 20MB/sec after
the recent series of preallocation changes.
The scorch mem segment build phase uses the append() idiom to populate
various slices that are keyed by postings list id's. These slices
include...
* Postings
* PostingsLocs
* Freqs
* Norms
* Locfields
* Locstarts
* Locends
* Locpos
* Locarraypos
This change introduces an initialization step that preallocates those
slices up-front, by assigning postings list id's to terms up-front.
This change also has an additional effect of simplifying the
processDocument() logic to no longer have to worry about a first-time
initialization case, removing some duplicate'ish code.
The first time through, startNumFields should be 0, where there ought
to be more optimization assuming later docs have similar fields as the
first doc.
-VisitableDocValueFields API for persisted DV field list
-making dv configs overridable at field level
-enabling on the fly/runtime un inverting of doc values
-few UT updates
previous approach used SetEventCallback method which allowed
you to change the callback, unfotunately that also included
times after the goroutines were started and potentially firing
the callback.
checking lock on this would be too expensive, so instead we go
for an approach that allows callbacks to be registered by name
during process init(), then upon opening up an index a string
config key 'eventCallbackName' is used to look up the
appropriate callback function. also, since this string config
name is serializable, it fits into the existing bleve index
metadata without any new issues.
zap command-line tool added to main bleve command-line tool
this required physical relocation due to the vendoring used
only on the bleve command-line tool (unforseen limitation)
a new scorch command-line tool has also been introduced
and for the same reasons it is physically store under
the top-level bleve command-line tool as well
New APIs:
+ RollbackPoints()
- Retrieves the available list of rollback points: epoch+meta.
- The application will need to check with the meta to decide
on the rollback point.
+ Rollback()
- API requires a rollback point identified by the first API.
- Atomically & Durably rolls back the index to specified point,
provided the specified rollback point is still available.
+ Unit test: TestIndexRollback
- Writes a batch.
- Sets the rollback point.
- Writes second batch.
- Rollback to previously decided point.
- Ensure that data is as is before the second batch.
inserted to the list of dv enabled fields list -
DocValueFields in mem segment.
Moved back to the original type `DocValueFields map[uint16]bool`
for easy look up to check whether the fieldID is
configured for dv storage.
Observed problem:
Persisted index state (in root bolt) would contain index snapshots which
pointed to index files that did not exist.
Debugging this uncovered two main problems:
1. At the end of persisting a snapshot, the persister creates a new index
snapshot with the SAME epoch as the current root, only it replaces in-memory
segments with the new disk based ones. This is problematic because reference
counting an index segment triggers "eligible for deletion". And eligible for
deletion is keyed by epoch. So having two separate instances going by the same
epoch is problematic. Specifically, one of them gets to 0 before the other,
and we wrongly conclude it's eligible for deletion, when in fact the "other"
instance with same epoch is actually still in use.
To address this problem, we have modified the behavior of the persister. Now,
upon completion of persistence, ONLY if new files were actually created do we
proceed to introduce a new snapshot. AND, this new snapshot now gets it's own
brand new epoch. BOTH of these are important because since the persister now
also introduces a new epoch, it will see this epoch again in the future AND be
expected to persist it. That is OK (mostly harmless), but we cannot allow it
to form a loop. Checking that new files were actually introduced is what
short-circuits the potential loop. The new epoch introduced by the persister,
if seen again will not have any new segments that actually need persisting to
disk, and the cycle is stopped.
2. The implementation of NumSnapshotsToKeep, and related code to deleted old
snapshots from the root bolt also contains problems. Specifically, the
determination of which snapshots to keep vs delete did not consider which ones
were actually persisted. So, lets say you had set NumSnapshotsToKeep to 3, if
the introducer gets 3 snapshots ahead of the persister, what can happen is that
the three snapshots we choose to keep are all in memory. We now wrongly delete
all of the snapshots from the root bolt. But it gets worse, in this instant of
time, we now have files on disk that nothing in the root bolt points to, so we
also go ahead and delete those files. Those files were still being referenced
by the in-memory snapshots. But, now even if they get persisted to disk, they
simply have references to non-existent files. Opening up one of these indexes
results in lost data (often everything).
To address this problem, we made large change to the way this section of code
operates. First, we now start with a list of all epochs actually persisted in
the root bolt. Second, we set aside NumSnapshotsToKeep of these snapshots to
keep. Third, anything else in the eligibleForRemoval list will be deleted. I
suspect this code is slower and less elegant, but I think it is more correct.
Also, previously NumSnapshotsToKeep defaulted to 0, I have now defaulted it to
1, which feels like saner out-of-the-box behavior (though it's debatable if the
original intent was perhaps instead for "extra" snapshots to keep, but with the
variable named as it is, 1 makes more sense to me)
Other minor changes included in this change:
- Location of 'nextSnapshotEpoch', 'eligibleForRemoval', and
'ineligibleForRemoval' members of Scorch struct were moved into the
paragraph with 'rootLock' to clarify that you must hold the lock to access it.
- TestBatchRaceBug260 was updated to properly Close() the index, which leads to
occasional test failures.
+ Track memory usage at a segment level
+ Add a new scorch API: MemoryUsed()
- Aggregate the memory consumption across
segments when API is invoked.
+ TODO:
- Revisit the second iteration if it can be gotten
rid off, and the size accounted for during the first
run while building an in-mem segment.
- Accounting for pointer and slice overhead.
+ Adding new entries to the stats struct of scorch.
+ These stats are atomically incremented upon every segment
introduction, and upon successful persistence.
docValues are persisted along with the index,
in a columnar fashion per field with variable
sized chunking for quick look up.
-naive chunk level caching is added per field
-data part inside a chunk is snappy compressed
-metaHeader inside the chunk index the dv values
inside the uncompressed data part
-all the fields are docValue persisted in this iteration
Added an option to the kvconfig JSON, called "unsafe_batch" (bool).
Default is false, so Batch() calls are synchronously persisted by
default. Advanced users may want to unsafe, asynchronous persistence
to tradeoff performance (mutations are queryable sooner) over safety.
{
"index_type": "scorch",
"kvconfig": { "unsafe_batch": true }
}
This change replaces the previous kvstore=="moss" workaround.
With more pprof focusing (zooming in on a particular func), there were
still some memory allocations showing up with docNumberToBytes() in
micro benchmarks of bleve-query. On a dev macbook, on an index of 50K
wikipedia docs, using search of relatively common "text:date"...
400 qps - upsidedown/moss
680 qps - scorch before
775 qps - scorch after
