As per docs [1] (see maintenance_work_mem), the maximum amount of memory used by the Vacuum command must be no more than maintenance_work_mem. However, during the review/discussion of the "parallel vacuum" patch [2], we observed that it is not true. Basically, if there is a gin index defined on a table, then the vacuum on that table can consume up to 2 * maintenance_work_mem memory space. The vacuum can use maintenance_work_mem memory space to keep track of dead tuples and another maintenance_work_mem memory space to move tuples from pending pages into regular GIN structure (see ginInsertCleanup). The behavior related to Gin index consuming extra maintenance_work_mem memory is introduced by commit e2c79e14d998cd31f860854bc9210b37b457bb01. It is not clear to me if this is acceptable behavior and if so, shouldn't we document it?
We wanted to decide how a parallel vacuum should use memory? Can each worker consume maintenance_work_mem to clean up the gin Index or all workers should use no more than maintenance_work_mem? We were thinking of later but before we decide what is the right behavior for parallel vacuum, I thought it is better to once discuss if the current memory usage model is right.