presto-main/src/main/java/io/prestosql/operator/OperatorContext.java/OperatorContext/decrementSpilledReservation
class OperatorContext:        private long decrementSpilledReservation(long reservedBytes, long bytesBeingFreed)
        {
            checkArgument(bytesBeingFreed >= 0);
            checkArgument(bytesBeingFreed <= reservedBytes, "tried to free %s spilled bytes from %s bytes reserved", bytesBeingFreed, reservedBytes);
            return reservedBytes - bytesBeingFreed;
        }