[LU-12535] lov_page_index surprisingly slow

• Type: Improvement
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.13.0
• Assignee: Patrick Farrell (Inactive)
• Reporter: Patrick Farrell (Inactive)

Description

When doing readahead, we see an amazing amount of time (~5%) just looking up the page index from the lov layer.

 a
-   84.42%     0.34%  ior              [kernel.kallsyms]     [k] ll_readpage                a
   - 84.08% ll_readpage                                                                     a
      - 82.68% ll_io_read_page                                                              a
         + 68.53% ll_readahead.isra.27                                                      a
         - 14.14% cl_io_submit_rw                                                           a
            - 13.40% lov_io_submit                                                          a
               - 8.24% lov_page_index                                                       a
                    5.56% cl_page_at  

This never changes once a page is created, so we can just store it at the top layer.