Another question about *_buffer_size
Changed values to

join_buffer_size = 2M
sort_buffer_size = 2M
read_buffer_size = 2M


Now server creates many on-disk tmp tables with zero-length. Then, re-run script and it gives:
@SMP=258, @SMP_RATE=3096

TUNING-PRIMER script gives advise to increase tmp_table_size/max_heap_table_size. Will set tmp_table_size=96M, max_heap_table_size=96M and see the reaction.