-
Type:
Task
-
Resolution: Fixed
-
Priority:
Major - P3
-
Affects Version/s: None
-
Component/s: None
-
Labels:
-
Storage Engines
-
3
-
2024-02-06 tapioooooooooooooca
WT-11850 implemented a solution for pre-fetching threads to skip certain visibility checks that are not required. It attempts to use the WT_SESSION_PREFETCH flag to check if a pre-fetching session is trying to read from a checkpoint file, however this is actually incorrect as sessions specifically created for pre-fetch threads are never configured with this flag.
*
* The visibility information is not referenced on the page so we need to ensure that
* the value is globally visible at the point in time where we read the page into cache.
* Pages from checkpoint-related files that have been pushed onto the pre-fetch queue
* will be comprised of data that is globally visible, and so the reader thread which
* attempts to read the page into cache can skip the visible all check.
*/
if (!btree->huffman_value &&
!(WT_READING_CHECKPOINT(session) && F_ISSET(session, WT_SESSION_PREFETCH)) &&
This is where the pre-fetch threads are configured - it will never be configured with WT_SESSION_PREFETCH enabled.
session_flags = WT_THREAD_CAN_WAIT | WT_THREAD_PANIC_FAIL;
WT_RET(__wt_thread_group_create(session, &conn->prefetch_threads, "prefetch-server", 8, 8,
session_flags, __wt_prefetch_thread_chk, __wt_prefetch_thread_run, NULL));
Rather than checking the WT_SESSION_PREFETCH flag, we should either introduce a new flag in the session structure and check that, or implement some other kind of mechanism.