Horton

Proposal: Limitations of palloc inside checkpointer

Ekaterina Sokolova <e.sokolova@postgrespro.ru>

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From: Ekaterina Sokolova <e.sokolova@postgrespro.ru>
To: pgsql-hackers@lists.postgresql.org
Date: 2025-02-25T19:44:33Z
Lists: pgsql-hackers 
Hi, hackers!

Historically, the checkpointer process use palloc() into 
AbsorbSyncRequests() function. Therefore, the checkpointer does not 
expect to receive a request larger than 1 GB.

We encountered a case where the database went into recovery state, after 
applying all wal, the checkpointer process generated an "invalid memory 
alloc request size" error and entered a loop. But it is quite acceptable 
for the recovery state to receive such a large allocation request.

A simple solution to this problem is to use palloc_extended() instead of 
palloc(). But is it safe to allow the checkpointer to allocate so much 
memory at once?

I have proposal to update this memory allocation but I need your ideas 
and advices on how to do it in appropriate way. As an idea, we can 
replace the array with a list of arrays to allocate memory in chunks. As 
a bad idea, we can process a temporary array without locking.

I would be glad to hear your ideas and suggestions about this topic. 
Have a nice day!

-- 
Ekaterina Sokolova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

Commits

  1. Fix checkpointer shared memory allocation

  2. Limit checkpointer requests queue size

  3. Process sync requests incrementally in AbsorbSyncRequests