Lift the restriction of minimum 8-byte keys for the compact index #746
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When the size of the raw bytes is less than 8, then a fallback method is used that is likely slower. When the size of the raw bytes is 8 or higher, then an additional integer comparison is made, but this has very little impact on performance, which I verified using micro-benchmarks. The upside to making the function safe for any input raw bytes is that the API and test generators become simpler, because there is one fewer constraint to satisfy: the minimum size of 8 bytes for serialised keys.
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Now that `topBits64` is fully safe, remove the 8-byte key constraint when using the compact index. Instead, the config option for the index type includes a hint not to use the compact index if their keys are too small, because that will lead to bad performance.
A small bug related to the merge schedule and union levels was unearted after
recent changes to key generators for the state machine tests.
```
❯ cabal run lsm-tree-test -- --quickcheck-replay="(SMGen 13447116701882578385 16048829213438376903,35)" -p '$NF=="propLockstep_RealImpl_MockFS_IO"'
lsm-tree
Test.Database.LSMTree.StateMachine
propLockstep_RealImpl_MockFS_IO: FAIL (28.54s)
*** Failed! Assertion failed (after 1 test and 196 shrinks):
...
1 out of 1 tests failed (28.54s)
```
The bug was that the last level was always removing `Delete` entries even if
there was union level. This is now fixed.
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jorisdral
requested review from
dcoutts,
mheinzel,
recursion-ninja and
wenkokke
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We make
topBits64safe for raw bytes of size less than 8. When the size of the raw bytes is less than 8, then a fallback method is used that is likely slower. When the size of the raw bytes is 8 or higher, then anadditional integer comparison is made, but this has very little impact on
performance, which I verified using micro-benchmarks. The upside to making the
function safe for any input raw bytes is that the API and test generators become
simpler, because there is one fewer constraint to satisfy: the minimum size of 8
bytes for serialised keys.
Now that
topBits64is fully safe, remove the 8-byte key constraint when usingthe compact index. Instead, the config option for the index type includes a hint
not to use the compact index if keys are too small, because that will lead
to bad performance.