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Limit checkpointer requests queue size

If the number of sync requests is big enough, the palloc() call inAbsorbSyncRequests() will attempt to allocate more than 1 GB of memory,resulting in failure. This can lead to an infinite loop in the checkpointerprocess, as it repeatedly fails to absorb the pending requests.This commit limits the checkpointer requests queue size to 10M items. Inaddition to preventing the palloc() failure, this change helps to avoid longqueue processing time.Also, this commit is for backpathing only. The master branch receivesa more invasive yet comprehensive fix for this problem.Discussion:https://postgr.es/m/db4534f83a22a29ab5ee2566ad86ca92%40postgrespro.ruBackpatch-through: 13

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src/backend/postmaster
- checkpointer.c

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`‎src/backend/postmaster/checkpointer.c‎`

Lines changed: 4 additions & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -138,6 +138,9 @@ static CheckpointerShmemStruct *CheckpointerShmem;`
`138`	`138`	`/* interval for calling AbsorbSyncRequests in CheckpointWriteDelay */`
`139`	`139`	`#defineWRITES_PER_ABSORB1000`
`140`	`140`
	`141`	`+/* Max number of requests the checkpointer request queue can hold */`
	`142`	`+#defineMAX_CHECKPOINT_REQUESTS 10000000`
	`143`	`+`
`141`	`144`	`/*`
`142`	`145`	`* GUC parameters`
`143`	`146`	`*/`
`@@ -903,7 +906,7 @@ CheckpointerShmemInit(void)`
`903`	`906`	`*/`
`904`	`907`	`MemSet(CheckpointerShmem,0,size);`
`905`	`908`	`SpinLockInit(&CheckpointerShmem->ckpt_lck);`
`906`		`-CheckpointerShmem->max_requests=NBuffers;`
	`909`	`+CheckpointerShmem->max_requests=Min(NBuffers,MAX_CHECKPOINT_REQUESTS);`
`907`	`910`	`ConditionVariableInit(&CheckpointerShmem->start_cv);`
`908`	`911`	`ConditionVariableInit(&CheckpointerShmem->done_cv);`
`909`	`912`	`}`

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Commitf0cdc2a

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`‎src/backend/postmaster/checkpointer.c‎`

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