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  1. MariaDB Server
  2. MDEV-9873

Do not allocate trx id for read-only transactions

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      In MDEV-8684 we saw a large potion of CPU in the mutex implementation.

      The top calls on high CPU for mutex_spin_wait, _raw_spin_lock(kernel), ut_delay are all due to a single global lock trx_sys->mutex.

      All these go back to the mutex use in:

      • trx_start_low;
      • read_view_open_now; and
      • trx_commit_low.

      Every read only select query hits this global mutex 3 times.

      The first two are called from row_search_for_mysql and the trx_commit_low is the clean up of JOIN::optimize_inner within the same SELECT handler.

      trx_start_low needs the mutex for trx_sys_get_new_trx_id() and addition on trx_sys->ro_trx_list (or trx_sys->rw_trx_list) and trx_reserve_descriptor

      read_view_open_now(,_low) needs the lock for trx_find_descriptor and for inserting into trx_sys->view_list. This is called shortly via trx_assign_read_view after trx_start_if_not_started(ends up in trx_start_low) in the Phase 3 in row_search_for_mysql.

      trx_commit_low needs the mutex for removing from the above lists and also from trx->global_read_view.

      It might be possible to group trx_start_low and read_view_open_now update the same mutex.

      MySQL-5.7 has mitigated this with http://dev.mysql.com/worklog/task/?id=6047
      This removes the mutex in trx_start_low, trx_sys_get_new_trx_id and tx_commit_low for read only queries by not allocating a trx_id.

      It has fairly invasive patches like https://github.com/mysql/mysql-server/commit/ed460aae81f9897984157bfe7759075182efb2b7
      and the worklog task references previous commits that I'll need to port too.

      I'll attempt to do this porting.

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              Assignee:
              danblack Daniel Black
              Reporter:
              danblack Daniel Black
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