Date: Mon, 3 Aug 2020 17:46:59 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: Stephen Frost <sfrost@snowman.net>,
	Dmitry Dolgov <9erthalion6@gmail.com>,
	David Steele <david@pgmasters.net>,
	Andres Freund <andres@anarazel.de>,
	Alvaro Herrera <alvherre@2ndquadrant.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: WIP: WAL prefetch (another approach)
Message-ID: <20200803154659.nzf5qzuozts5dujp@development>
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On Thu, Jul 02, 2020 at 03:09:29PM +1200, Thomas Munro wrote:
>On Sat, Jun 6, 2020 at 12:36 PM Stephen Frost <sfrost@snowman.net> wrote:
>> * Tomas Vondra (tomas.vondra@2ndquadrant.com) wrote:
>> > I wonder if we can collect some stats to measure how effective the
>> > prefetching actually is. Ultimately we want something like cache hit
>> > ratio, but we're only preloading into page cache, so we can't easily
>> > measure that. Perhaps we could measure I/O timings in redo, though?
>>
>> That would certainly be interesting, particularly as this optimization
>> seems likely to be useful on some platforms (eg, zfs, where the
>> filesystem block size is larger than ours..) and less on others
>> (traditional systems which have a smaller block size).
>
>I know one way to get information about cache hit ratios without the
>page cache fuzz factor: if you combine this patch with Andres's
>still-in-development AIO prototype and tell it to use direct IO, you
>get the undiluted truth about hits and misses by looking at the
>"prefetch" and "skip_hit" columns of the stats view.  I'm hoping to
>have a bit more to say about how this patch works as a client of that
>new magic soon, but I also don't want to make this dependent on that
>(it's mostly orthogonal, apart from the "how deep is the queue" part
>which will improve with better information).
>
>FYI I am still trying to reproduce and understand the problem Tomas
>reported; more soon.

Any luck trying to reproduce thigs? Should I try again and collect some
additional debug info?

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services