Re: Atomics for heap_parallelscan_nextpage()

From: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
To: Andres Freund <andres(at)anarazel(dot)de>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com>, Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>
Subject: Re: Atomics for heap_parallelscan_nextpage()
Date: 2017-08-16 17:19:29
Message-ID: 7e0a73a5-0df9-1859-b8ae-9acf122dc38d@iki.fi
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On 08/16/2017 08:10 PM, Andres Freund wrote:
>> Afaict shm_create/shm_toc_allocate don't actually guarantee that the end
>> of the toc's memory is suitably aligned. But I didn't yet have any
>> coffee, so ...
>
> Robert, I'm not quite sure what the intended behaviour of shm_toc is wrt
> alignment. I see that individual chunks are BUFFERALIGNed (both during
> estimation, and allocation). But I don't see how the size of the entire
> toc is aligned, which seems a requirement, given we allocate from the
> end.
> Seems like we'd just have to add alignment of the total size to
> shm_toc_estimate()?

Yeah, that's the gist of it.

The attached patch seems to fix this. I'm not too familiar with this DSM
stuff, but this seems right to me. Unless someone has a better idea
soon, I'll commit this to make the buildfarm happy.

- Heikki

Attachment Content-Type Size
fix-shm_toc-alignment-1.patch text/x-patch 1.5 KB

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