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I noticed this while debugging a fix of MDEV-26445 by running the test innodb.undo_truncate:
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10.5 699de65d5e1cda0b8c6a994f8513e97804b1a7ad with some changes
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#9 0x000055af0cd10286 in buf_flush_LRU_list_batch (n=<synthetic pointer>, max=32) at /mariadb/10.5m/storage/innobase/buf/buf0flu.cc:1332
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#10 buf_do_LRU_batch (max=32) at /mariadb/10.5m/storage/innobase/buf/buf0flu.cc:1380
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#11 buf_flush_LRU (max_n=32) at /mariadb/10.5m/storage/innobase/buf/buf0flu.cc:1704
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#12 0x000055af0cd1357d in buf_LRU_get_free_block (have_mutex=have_mutex@entry=false) at /mariadb/10.5m/storage/innobase/buf/buf0lru.cc:499
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#13 0x000055af0cd13d4f in buf_page_init_for_read (unzip=false, zip_size=0, page_id={m_id = 326417514774}, mode=132) at /mariadb/10.5m/storage/innobase/buf/buf0rea.cc:110
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#14 buf_read_page_low (err=0x7f825810545c, space=0x7f81b01354f0, sync=false, mode=132, page_id={m_id = 326417514774}, zip_size=0, unzip=false) at /mariadb/10.5m/storage/innobase/buf/buf0rea.cc:309
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#15 0x000055af0cd14c16 in buf_read_page_background (space=space@entry=0x7f81b01354f0, page_id={m_id = 326417514774}, zip_size=<optimized out>) at /mariadb/10.5m/storage/innobase/buf/buf0rea.cc:506
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#16 0x000055af0ccf02c0 in btr_cur_prefetch_siblings (block=0x7f8258d7eac0, index=<optimized out>) at /mariadb/10.5m/storage/innobase/include/buf0types.h:127
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While this is executing, another thread is waiting as well:
#3 0x000055af0cd1343e in buf_LRU_get_free_block (have_mutex=have_mutex@entry=false) at /mariadb/10.5m/storage/innobase/buf/buf0lru.cc:458
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#4 0x000055af0cd13d4f in buf_page_init_for_read (unzip=false, zip_size=0, page_id={m_id = 322122547570}, mode=132) at /mariadb/10.5m/storage/innobase/buf/buf0rea.cc:110
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#5 buf_read_page_low (err=0x7f825815045c, space=0x7f81b005d490, sync=false, mode=132, page_id={m_id = 322122547570}, zip_size=0, unzip=false) at /mariadb/10.5m/storage/innobase/buf/buf0rea.cc:309
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#6 0x000055af0cd14c16 in buf_read_page_background (space=space@entry=0x7f81b005d490, page_id={m_id = 322122547570}, zip_size=<optimized out>) at /mariadb/10.5m/storage/innobase/buf/buf0rea.cc:506
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#7 0x000055af0ccf02c0 in btr_cur_prefetch_siblings (block=0x7f8258d703c0, index=<optimized out>) at /mariadb/10.5m/storage/innobase/include/buf0types.h:127
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This prefetch could be detrimental to performance when we have a small buffer pool.
Possible fix: Allow buf_page_init_for_read() to instantly fail if a buffer pool block is not immediately available when initiating a background read.
The performance of any fix would of course have to be evaluaed carefully on different types of storage (ranging from HDD to NVMe).
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