Details
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Bug
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Status: Closed
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Major - P3
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Resolution: Fixed
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None
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None
Description
siddhartha.mahajan@mongodb.com found out while investigating WT-8707 that when a page is a candidate for forced eviction because it can be split in-memory, we fail to mark it to be evicted on release.
__evict_force_check returns true if a page needs to be force evicted, false otherwise. When returning true, it also sets the page's read_gen to WT_READGEN_OLDEST to mark the page to be forcibly evicted. If a page is a candidate for an in-memory split, it is considered for forced eviction. But, __evict_force_check ends up not marking the page to be evicted soon (i.e. read gen to the oldest) and hence the page doesn't get a chance to be evicted when it gets released.
/* If we can do an in-memory split, do it. */
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if (__wt_leaf_page_can_split(session, page))
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return (true); <<<<<<<<<<< Return true but fail to mark the page for forced eviction.
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if (footprint < btree->maxmempage)
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return (false);
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/* Bump the oldest ID, we're about to do some visibility checks. */
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WT_IGNORE_RET(__wt_txn_update_oldest(session, 0));
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/*
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* Allow some leeway if the transaction ID isn't moving forward since it is unlikely eviction
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* will be able to evict the page. Don't keep skipping the page indefinitely or large records
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* can lead to extremely large memory footprints.
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*/
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if (!__wt_page_evict_retry(session, page))
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return (false);
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/* Trigger eviction on the next page release. */
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__wt_page_evict_soon(session, ref); <<<<<<< Mark page for forced eviction on next release.
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/* If eviction cannot succeed, don't try. */
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return (__wt_page_can_evict(session, ref, NULL));
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Sid says fixing __evict_force_check helps with the regression that motivated WT-8707.
Change suggested:
Mark the page to be evicted soon in __evict_force_check when an in-memory split can succeed.
Testing:
WT and MongoDB performance tests (independent of impact due to changes from WT-8707).
Attachments
Issue Links
- is caused by
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WT-8707 Mongod reader yielding with WT::commit_transaction instead of rollback pins WT pages such that pages cannot be split/evicted by writer threads
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- Closed
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