MDEV-34515: Contention between purge and workload
In a Sysbench oltp_update_index workload that involves 1 table, a serious contention between the workload and the purge of history was observed. This was the worst when the table contained only 1 record. This turned out to be fixed by setting innodb_purge_batch_size=128, which corresponds to the number of usable persistent rollback segments. When we go above that, there would be contention between row_purge_poss_sec() and the workload, typically on the clustered index page latch, sometimes also on a secondary index page latch. It might be that with smaller batches, trx_sys.history_size() will end up pausing all concurrent transaction start/commit frequently enough so that purge will be able to make some progress, so that there would be less contention on the index page latches between purge and SQL execution. In commit aa719b50 (part of MDEV-32050) the interpretation of the parameter innodb_purge_batch_size was slightly changed. It would correspond to the maximum desired size of the purge_sys.pages cache. Before that change, the parameter was referring to a number of undo log pages, but the accounting might have been inaccurate. To avoid a regression, we will reduce the default value to innodb_purge_batch_size=127, which will also be compatible with innodb_undo_tablespaces>1 (which will disable rollback segment 0). Additionally, some logic in the purge and MVCC checks is simplified. The purge tasks will make use of purge_sys.pages when accessing undo log pages to find out if a secondary index record can be removed. If an undo page needs to be looked up in buf_pool.page_hash, we will merely buffer-fix it. This is correct, because the undo pages are append-only in nature. Holding purge_sys.latch or purge_sys.end_latch or the fact that the current thread is executing as a part of an in-progress purge batch will prevent the contents of the undo page from being freed and subsequently reused. The buffer-fix will prevent the page from being evicted form the buffer pool. Thanks to this logic, we can refer to the undo log record directly in the buffer pool page and avoid copying the record. buf_pool_t::page_fix(): Look up and buffer-fix a page. This is useful for accessing undo log pages, which are append-only by nature. There will be no need to deal with change buffer or ROW_FORMAT=COMPRESSED in that case. purge_sys_t::view_guard::view_guard(): Allow the type of guard to be acquired: end_latch, latch, or no latch (in case we are a purge thread). purge_sys_t::view_guard::get(): Read-only accessor to purge_sys.pages. purge_sys_t::get_page(): Invoke buf_pool_t::page_fix(). row_vers_old_has_index_entry(): Replaced with row_purge_is_unsafe() and row_undo_mod_sec_unsafe(). trx_undo_get_undo_rec(): Merged to trx_undo_prev_version_build(). row_purge_poss_sec(): Add the parameter mtr and remove redundant or unused parameters sec_pcur, sec_mtr, is_tree. We will use the caller's mtr object but release any acquired page latches before returning. btr_cur_get_page(), page_cur_get_page(): Do not invoke page_align(). row_purge_remove_sec_if_poss_leaf(): Return the value of PAGE_MAX_TRX_ID to be checked against the page in row_purge_remove_sec_if_poss_tree(). If the secondary index page was not changed meanwhile, it will be unnecessary to invoke row_purge_poss_sec() again. trx_undo_prev_version_build(): Access any undo log pages using the caller's mini-transaction object. row_purge_vc_matches_cluster(): Moved to the only compilation unit that needs it. Reviewed by: Debarun Banerjee
Showing
This diff is collapsed.
This diff is collapsed.
This diff is collapsed.
Please register or sign in to comment