head_only mode cna be run with_lookup - but there is no L0 index created in this case.
So the L0 index wasn't returning a potition list and the L0 cache wasn't being checked.
Code now checks every position in the L0 cache, when a lookup is attempted in head_only mode.
Switch from magic hash to md5 - to hopefully remove the need for some
of the artificial jumps required to get expected fall positive ratios.
Also split the hash into two 16-bit integers. We assume that SegmentID
(from the perspective of AAE merkle/tictac trees) will always be at
least 16 bits. the idea is that hashes should be used in blooms and
indexes such that some advantage can be gained from just knowing the
segmentID - in particular when folding over all the keys in a bucket.
Performance testing has been difficult so far - I think due to “cloud”
mysteries.
Remove the ets index in pmem and use a binary index instead. This may
be slower, but avoids the bulk upload to ets, and means that matches
know of position (so only skiplists with a match need be tried).
Also stops the discrepancy between snapshots and non-snapshots - as
previously the snapshots were always slowed by not having access to the
ETS table.
Move to using the DJ Bernstein Magic Hash consistently, and trying to
make sure we only hash once for each operation (as the hash is more
expensive than phash2).
The improved lookup time for missing keys should allow for the L0 index
to be removed, and hence speed up the completion time for push_mem
operations.
It is expected there will be a second stage of creating a tinybloom as
part of the SFT creation process, and then adding that tinybloom to the
manifest. This will then reduce the message passing required for a GET
not in the cache or higher levels
Hope is that this will cause less garbage collection, and also will be
slightly faster.
Note that snapshots don't now get an index - they get the special index
'snap'. However, the SkipLists have bloom protection, and most
snapshots are iterators not fetchers.
Move legacy CDB code used only in unit tests into test area. Fix column
width in pmem and comment out the unused case statement (in healthy
tests) from the penciller test code
There was some unpredictable performance in tests, that was related to
the amount of time it took the sft gen_server to accept a cast whihc
passed the levelzero_cache.
The response time looked to be broadly proportional to the size of the
cache - so it appeared to be an issue with passing the large object to
the process queue.
To avoid this, the penciller now instructs the SFT gen_server to
callback to the server for each tree in the cache in turn as it is
building the list from the cache. Each of these requests should be
reltaively short, and the processing in-between should space out the
requests so the Pencille ris not blocked from answering queries when
pompting a L0 write.
Plugged the ne wpencille rmemory into the Penciller, and took advantage
of the increased speed to simplify the callbacks involved.
The outcome is much simpler code
Not yet integrated, but there is now unit-tested module for the new way
of managing L0 memory cache in the Penciller.
This mechansim is considerably more efficient than previous efforts and
should allow for further simplification of the code.
