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From: Melanie Plageman <[email protected]>
To: Andres Freund <[email protected]>
Cc: [email protected]
Cc: Thomas Munro <[email protected]>
Cc: Peter Geoghegan <[email protected]>
Cc: Tomas Vondra <[email protected]>
Cc: Nazir Bilal Yavuz <[email protected]>
Subject: Re: AIO / read stream heuristics adjustments for index prefetching
Date: Wed, 1 Apr 2026 10:52:03 -0400
Message-ID: <CAAKRu_aG2zvObX6gQ3Kw=MPydbphZ-qKeMGwZDEFe55FFiXYWw@mail.gmail.com> (raw)
In-Reply-To: <f3xxfrkafjxpyqxywcxricxgyizjirfceychyxsgn7bwjp5eda@kwbduhy7tfmu>
References: <f3xxfrkafjxpyqxywcxricxgyizjirfceychyxsgn7bwjp5eda@kwbduhy7tfmu>

On Tue, Mar 31, 2026 at 12:02 PM Andres Freund <[email protected]> wrote:
>
> 0008: WIP: read stream: Split decision about look ahead for AIO and combining
>
>     Until now read stream has used a single look-ahead distance to control
>     lookahead for both IO combining and read-ahead. That's sub-optimal, as we
>     want to do IO combining even when we don't need to do any readahead, as
>     avoiding the syscall overhead is important to reduce CPU overhead when
>     data is in the kernel page cache.
>
>     This is a prototype for what it could look like to split those
>     decisions. Thereby fixing the regression mentioned in 0006.

I wonder if we need to keep the combine_limit member in the read
stream. Could we just use io_combine_limit without ramping up and
down? This is mainly for code complexity reasons. Perhaps to allow
fast path reentry, we could use distance_decay_holdoff == 0 and
ios_in_progress == 0 instead of combine_distance == 0.

- Melanie

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