Re: CheckpointLock needed in CreateCheckPoint()? - Mailing list pgsql-hackers

On Thu, Jan 7, 2021 at 11:32 AM Amul Sul <sulamul@gmail.com> wrote:
> Snip from CreateCheckPoint():
> --
> /*
>  * Acquire CheckpointLock to ensure only one checkpoint happens at a time.
>  * (This is just pro forma, since in the present system structure there is
>  * only one process that is allowed to issue checkpoints at any given
>  * time.)
>  */
> LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);
> --
>
> As per this comment, it seems to be that we don't really need this LW lock. We
> could have something else instead if we are afraid of having multiple
> checkpoints at any given time which isn't possible, btw.

Looks like CheckpointLock() can also be called in standalone backends
(RequestCheckpoint->CreateCheckPoint) along with the checkpointer
process. Having said that, I'm not quite sure whether it can get
called at the same time from a backend(may be with checkpoint;
command) and the checkpointer process.

    /*
     * If in a standalone backend, just do it ourselves.
     */
    if (!IsPostmasterEnvironment)
    {
        /*
         * There's no point in doing slow checkpoints in a standalone backend,
         * because there's no other backends the checkpoint could disrupt.
         */
        CreateCheckPoint(flags | CHECKPOINT_IMMEDIATE);

See the below comment for IsPostmasterEnvironment:

/*
 * IsPostmasterEnvironment is true in a postmaster process and any postmaster
 * child process; it is false in a standalone process (bootstrap or
 * standalone backend).

> This CheckpointLock is acquired almost at the start of CreateCheckPoint() and
> released at the end. The in-between code can take a while to be complete. All
> that time interrupt will be on hold which doesn't seem to be a great idea but
> that wasn't problematic until now.  I am having trouble due to this interrupt
> hold for a long since I was trying to add code changes where the checkpointer
> process is suppose to respond to the system read-write state change request as
> soon as possible[1]. That cannot be done if the interrupt is disabled
> since read-write state change uses the global barrier mechanism to convey system
> state changes to other running processes.
>
> So my question is, can we completely get rid of the CheckpointLock need in
> CreateCheckPoint()?
>
> Thoughts/Comments/Suggestions?

I don't think we can completely get rid of CheckpointLock in
CreateCheckPoint given the fact that it can get called from multiple
processes.

How about serving that ASRO barrier request alone for checkpointer
process in ProcessInterrupts even though the CheckpointLock is held by
the checkpointer process? And also while the checkpointing is
happening, may be we should call CHECK_FOR_INTERRUPTS to see if the
ASRO barrier has arrived. This may sound bit hacky, but just a
thought. I'm thinking that in ProcessInterrupts we can get to know the
checkpointer process type via MyAuxProcType, and whether the lock is
held or not using CheckpointLock, and then we can handle the ASRO
global barrier request.

I may be missing something.

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com