I confirmed the 20GB value as is described here [1]. 8k page can hold 409 member groups and each member group can hold 4 members, thus (2^32/(409 *4))*8192 = 20GB.
I also fixed whitespace issues in v3. This is ready-for-committer In my opinion. Regards, Sami [1] https://github.com/postgres/postgres/blob/HEAD/src/backend/access/transam/multixact.c#L131-L137
v3-0001-Doc-fix-of-aggressive-vacuum-threshold-for-multix.patch
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