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From: Laurenz Albe <[email protected]>
To: vinay kumar <[email protected]>
Cc: [email protected]
Subject: Re: Understanding recovery conflict due to bufferpin
Date: Tue, 23 Mar 2021 22:42:01 +0100
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAMdzBE=2v6a61GWO+xHFbFX0rgAb1a+4fPxCK6Yi38x-wrDUSQ@mail.gmail.com>
References: <CAMdzBEkg+EytT1XcsykcsarpLVu+oQcDV4ekdf6CMjOv8JpvyA@mail.gmail.com>
	<[email protected]>
	<CAMdzBE=2v6a61GWO+xHFbFX0rgAb1a+4fPxCK6Yi38x-wrDUSQ@mail.gmail.com>

On Tue, 2021-03-23 at 22:48 +0530, vinay kumar wrote:
> My query is does the buffer pin pins block present in shared buffers(assuming the
>  block is already read into memory previously) or it's pinned to the block at the OS level?
> 
> Also have another query: 
> 
> Do we have any queue mechanism to attain buffer pins on a block?
> 
> To provide an example:
> 
> Let's say, I need to read tuples present in a block by multiple backends.
>  Shouldn't the backends wait in FIFO order to hold pins on the block?
> 
> To give you an example, if I have the few backends requesting access to a block
>  in the following order:
> 
> -> backend 1: reads the tuples from block
> -> backend 2: reads the tuples from block
> -> backend 3: reads the tuples from block
> -> WAL replay: waiting to modify block either due to replaying change or Vacuum operation.
> 
> Will the order of requests to access blocks be maintained in a cache or any other memory area? 
> 
> If possible to implement lazy cache invalidation (invalidating blocks in buffer cache
>  when no conflicts query is run), it would be great and helpful to lots of users who query
>  data from standby to avoid recovery conflict and don't have to re-run the entire query once
>  again consuming resources.

A pin is not a lock, it just protects a page from being swapped out of cache.
Several backends can pin the same page simultaneously.

The buffer pin is an internal PostgreSLQ concept and has nothing to do
with the operating system.

I recommend that you start by reading the appropriate README:
https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/storage/buffer/README
That explains the concepts better than I could.

The key sentence when it comes to replication conflicts is this:

  To physically remove a tuple or compact free space on a page, one
  must hold a pin and an exclusive lock, *and* observe while holding the
  exclusive lock that the buffer's shared reference count is one (ie,
  no other backend holds a pin).

Yours,
Laurenz Albe
-- 
Cybertec | https://www.cybertec-postgresql.com

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