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From: Dimitrios Apostolou <jimis@gmx.net>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: [PING] [PATCH v2] parallel pg_restore: avoid disk seeks when jumping short distance forward
Date: Fri, 13 Jun 2025 01:00:26 +0200 (CEST)
Message-ID: <5ed51587-5a24-4d79-e36e-927620d0e3a2@gmx.net> (raw)
In-Reply-To: <aEioDuePEBRnfJYk@nathan>
References: <2edb7a57-b225-3b23-a680-62ba90658fec@gmx.net>
	<a7344d8f-927c-1f97-6cc0-ac630deb98c4@gmx.net>
	<c0e531f9-accc-3a93-e9e6-2cec1471f9e9@gmx.net>
	<1282067.1744651649@sss.pgh.pa.us>
	<c6bb93c1-4042-743c-096c-27c5e7c73347@gmx.net>
	<f7a23708-c336-c189-f155-4b3197bf6ea0@gmx.net>
	<aEioDuePEBRnfJYk@nathan>

On Tue, 10 Jun 2025, Nathan Bossart wrote:

> So, fseeko() starts winning around 4096 bytes.  On macOS, the differences
> aren't quite as dramatic, but 4096 bytes is the break-even point there,
> too.  I imagine there's a buffer around that size somewhere...
>
> This doesn't fully explain the results you are seeing, but it does seem to
> validate the idea.  I'm curious if you see further improvement with even
> lower thresholds (e.g., 8KB, 16KB, 32KB).

By the way, I might have set the threshold to 1MB in my program, but 
lowering it won't show a difference in my test case, since the lseek()s I 
was noticing before the patch were mostly 8-16KB forward. Not sure what is 
the defining factor for that. Maybe the compression algorithm, or how wide 
the table is?


Dimitris






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