how to set bulk_insert_buffer_size in mysql?
When you ran

SET GLOBAL bulk_insert_buffer_size =1024*1024*256;


what you did was set the insert buffer to 256M for incoming DB Connections going forward.

To set it for your current DB session, run this in that session:

SET bulk_insert_buffer_size =1024*1024*256;


Then, you can run 

SHOW VARIABLES LIKE '%bulk%';


and see a different value.

In a post made back on April 18, 2013 (see Steps 2 and 3), I suggested setting the bulk_insert_buffer_size to 256M within a current session.

If you want to keep that value when you restart mysql, just add this to my.cnf

[mysqld]
bulk_insert_buffer_size = 256M


Give it a Try !!!