From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | Andres Freund <andres(at)2ndquadrant(dot)com> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Greg Stark <stark(at)mit(dot)edu>, Peter Geoghegan <pg(at)heroku(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Misaligned BufferDescriptors causing major performance problems on AMD |
Date: | 2014-02-05 17:36:42 |
Message-ID: | CA+TgmoZ0WUCh2x0WzVP-svWe4JKLaEPV0PUU81EO_4uYmK=6bA@mail.gmail.com |
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Lists: | pgsql-hackers |
On Wed, Feb 5, 2014 at 11:37 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> On 2014-02-05 11:23:29 -0500, Tom Lane wrote:
>> Andres Freund <andres(at)2ndquadrant(dot)com> writes:
>> > And I think somebody already thought about it (c.f. ALIGNOF_BUFFER), it
>> > just wasn't updated in the last 10 years.
>>
>> No, ALIGNOF_BUFFER is there because we read something that said that I/O
>> transfers between userspace and kernel disk cache would be faster with
>> aligned buffers. There's been no particular thought given to alignment
>> of other data structures, AFAIR.
>
> But it's not aligned anymore on at last half a decade's hardware, and
> it's what we already align *all* bigger ShmemAlloc() values with. And
> BufferDescriptors surely counts as larger in its entirety.
>
>> It may well be that your proposal is spot on. But I'd like to see some
>> data-structure-by-data-structure measurements, rather than assuming that
>> alignment must be a good thing.
>
> I am fine with just aligning BufferDescriptors properly. That has
> clearly shown massive improvements.
I thought your previous idea of increasing BUFFERALIGN to 64 bytes had
a lot to recommend it. But that doesn't mean it doesn't need testing.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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