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We use live words rather than live blocks to determine the size of the heap for determining memory retention. Most of the time these two metrics align, but they can come apart in normal usage when using the nonmoving collector. The nonmoving collector leads to a lot of partially occupied blocks. So, using live words is more accurate. They can also come apart when the heap is suffering from high levels fragmentation caused by small pinned objects, but in this case, the block size is the more accurate metric. Since this case is best avoided anyway. It is ok to accept the trade-off that we might try (and probably) fail to return more memory in this case. See also the Note [Statistics for retaining memory] Resolves #23397
