"Changes merging in MVCC to apply newer version to older instead of older to
newer.
Before (v0 = oldest):
(((v3 + v2) + v1) + v0)
After:
(v0 + (v1 + (v2 + v3)))
or:
(((v0 + v1) + v2) + v3)
There are several reasons to do this:
1) When continuity merging will change semantics to support eviction
from older versions, it will be easier to implement apply() if we
can assume that we merge newer to older instead of older to
newer, since newer version may have entries falling into a
continuous interval in older, but not the other way around. If we
didn't revert the order, apply() would have to keep track of
lower bound of a continuous interval in the right-hand side
argument (older version) as it is applied and update continuity
flags in the left hand side by scanning all entries overlapping
with it. If order is reversed, merging only needs to deal with
the current entry. Also, if we were to keep the old order, we
cannot simply move entries from the left hand side as we merge
because we need to keep track of the lower bound of a continuous
interval, and we need to provide monotonic exception
guarantees. So merging would be both more complicated and slower.
2) With large partitions older versions are typically larger than
newer versions, and since merging is O(N_right*(1 + log(N_left))),
it's better to merge newer into older.
This fixes latency spikes seen in perf_cache_eviction.
Fixes #2715."
* tag 'tgrabiec/reverse-order-of-mvcc-version-merging-v1' of github.com:scylladb/seastar-dev:
mvcc: Reverse order of version merging
anchorless_list: Introduce last()
mvcc: Implement partition_entry::upgrade() using squashed()
mvcc: Extract version merging functions
mutation_partition: Add rows_entry::set_dummy()
position_in_partition: Introduce after_key()
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