When the memory requirements for running OnRC are not met before the estimation phase the estimator will run nevertheless. The process will only be cancelled at the beginning of the compaction phase. The entries in the log file reflect this:
TarMK GC #1: canceling compaction because available memory level 306.4 MB (306395472 bytes) is too low...
TarMK GC #1: estimation started
TarMK GC #1: estimation completed in 343.5 ms (343 ms). Estimation skipped because of missing gc journal data (expected on first run)
TarMK GC #1: running full compaction
TarMK GC #1: compaction started ...
TarMK GC #1: unable to estimate number of nodes for compaction, missing gc history.
TarMK GC #1: compaction cancelled: Not enough memory.
TarMK GC #1: cleaning up after failed compaction
However they can easily be (mis-)read as compaction being re-triggered after having been cancelled and then being cancelled again.