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From: Peter Eisentraut <[email protected]>
To: Bertrand Drouvot <[email protected]>
Cc: Heikki Linnakangas <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: PGPROC alignment (was Re: pgsql: Separate RecoveryConflictReasons from procsignals)
Date: Mon, 16 Mar 2026 11:42:35 +0100
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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On 24.02.26 12:28, Bertrand Drouvot wrote:
>> You can/should use C11 standard alignas(), so you don't need to worry about
>> whether it's supported or not.
> 
> Oh right, I did not notice 300c8f53247 and following like e7075a3405c, d4c0f91f7d5
> and 97e04c74bed.
> 
> PFA, 0001 doing so for PGPROC and PgAioUringContext. As those are typedef,
> the patch puts alignas within the struct.
> 
> For PGPROC at the start of the struct, I think that placing it on the first member
> is the right location because it ensures the whole struct is aligned to PG_CACHE_LINE_SIZE
> without adding padding before this member. For example if I set it on backendType,
> then it adds 100 bytes of padding and the struct is obviously still a multiple of
> PG_CACHE_LINE_SIZE but is now 1024 bytes (instead of 896).
> 
> For PgAioUringContext at completion_lock (like suggested by Andres in [1]), which
> is also the start of the struct.
> 
> I checked and the padding for those are exactly the same after the changes.

I have committed the 0001 patch.

> 0002, is also making use of alignas in ItemPointerData, but this one is more
> tricky so I'm not sure that's worth it (given the fact that we still need to
> keep pg_attribute_aligned() as explained by Peter in [2]).

This doesn't seem worth it to me.  Let's skip it.

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