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From: Michail Nikolaev <[email protected]>
To: Nathan Bossart <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Tom Lane <[email protected]>
Subject: Re: Replace known_assigned_xids_lck by memory barrier
Date: Tue, 15 Aug 2023 12:29:24 +0200
Message-ID: <CANtu0ohnGXW6wfM7eXts=m23A1g9D7s1X0ms7bV3-x5oQugX5g@mail.gmail.com> (raw)
In-Reply-To: <20230814153634.GB1395427@nathanxps13>
References: <CANtu0oh0si=jG5z_fLeFtmYcETssQ08kLEa8b6TQqDm_cinroA@mail.gmail.com>
	<20230814153634.GB1395427@nathanxps13>

Hello, Nathan.

> What sort of benefits do you see from this patch? It might be worthwhile
> in itself to remove spinlocks when possible, but IME it's much easier to
> justify such changes when there is a tangible benefit we can point to.

Oh, it is not an easy question :)

The answer, probably, looks like this:
1) performance benefits of spin lock acquire removing in
KnownAssignedXidsGetOldestXmin and KnownAssignedXidsSearch
2) it is closing 13-year-old tech depth

But in reality, it is not easy to measure performance improvement
consistently for this change.

> Are the assignments in question guaranteed to be atomic? IIUC we assume
> that aligned 4-byte loads/stores are atomic, so we should be okay as long
> as we aren't handling anything larger.

Yes, 4-bytes assignment are atomic, locking is used to ensure memory
write ordering in this place.

> This use of pg_write_barrier() looks correct to me, but don't we need
> corresponding read barriers wherever we obtain the pointers? FWIW I tend
> to review src/backend/storage/lmgr/README.barrier in its entirety whenever
> I deal with this stuff.

Oh, yeah, you're right! (1)
I'll prepare an updated version of the patch soon. I don't why I was
assuming pg_write_barrier is enough (⊙＿⊙')


[1]: https://github.com/postgres/postgres/blob/master/src/backend/storage/lmgr/README.barrier#L125

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