mysql - innodb_sort_buffer_size can not change dynamically -

I tried to change "innodb_sort_buffer_size" to improve the InnoDB index creation performance.

When I issue the following question (with root user), I have an error "Error 1238 (HAA 2000): variable 'innodb_sort_buffer_size' is a read only variable."

  mysql> Set global innoadab_sort_bufar_sys = 1024 * 1024 * 5; Error 1238 (HY 1000): Convertible 'innodb_sort_buffer_size' is a read only variable   
 As far as I see the document, this variable can change dynamically with SET Global.  
  
 So my question is as follows.  
 
 
 Has this variable really been read only?   
 
 Is there another way to improve InnoDB Index creation performance?     
 
 
 This global variable, but it is not dynamic - it means That it can not be set to runtime