Commits on Jun 30, 2021

1. Introduce dsl_dir_diduse_transfer_space(). …

   Most of dsl_dir_diduse_space() and dsl_dir_transfer_space() CPU time
   is a dd_lock overhead and time spent in dmu_buf_will_dirty(). Calling
   them one after another is a waste of time and even more contention.
   Doing that twice for each rewritten block within dbuf_write_done()
   via dsl_dataset_block_kill() and dsl_dataset_block_born() created one
   of the biggest CPU overheads in case of small blocks rewrite.
   
   dsl_dir_diduse_transfer_space() combines functionality of these two
   functions for cases where it is needed, but without double overhead,
   practically for the cost of dsl_dir_diduse_space() or even cheaper.
   
   While there, optimize dsl_dir_phys() calls in dsl_dir_diduse_space()
   and dsl_dir_transfer_space().  It seems Clang detects some aliasing
   there, repeating dd->dd_dbuf->db_data dereference multiple times,
   increasing dd_lock scope and contention.
   
   Signed-off-by: Alexander Motin <mav@FreeBSD.org>
   Sponsored-By: iXsystems, Inc.

   

   amotin committed Jun 30, 2021
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