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  1. MariaDB Server
  2. MDEV-15104

Remove trx_sys_t::rw_trx_ids and trx_sys_t::serialisation_list

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      Take snapshot of registered read-write transaction identifiers directly
      from rw_trx_hash. It immediately saves one trx_sys.mutex lock, reduces
      size of another critical section protected by this mutex, and makes
      further optimisations like removing trx_sys_t::serialisation_list
      possible.

      Downside of this approach is bigger overhead for view opening, because
      iterating LF_HASH is more expensive compared to taking snapshot of an
      array. However for low concurrency overhead difference is negligible,
      while for high concurrency mutex is much bigger evil.

      Currently we still take trx_sys.mutex to serialise ReadView creation.
      This is required to keep serialisation_list ordered by trx->no as well
      as not to let purge thread to create more recent snapshot while another
      thread gets suspended during creation of older snapshot. This will
      become completely mutex free along with serialisation_list removal.

      Compared to previous implementation removing element from rw_trx_hash
      and serialisation_list is not atomic. We disregard all possible bad
      consequences (if there're any) since it will be solved along with
      serialisation_list removal.

      serialisation_list was supposed to instantly give minimum registered
      transaction serialisation number. However maintaining and accessing
      this list requires global mutex protection.

      Since we already take MVCC snapshot by iterating trx_sys_t::rw_trx_hash,
      it is cheap to integrate minimum registered transaction lookup into this
      iteration.

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              • Assignee:
                svoj Sergey Vojtovich
                Reporter:
                svoj Sergey Vojtovich
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                • Created:
                  Updated:
                  Resolved: