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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: Thu, 2 Apr 2026 17:13:34 -0400
Message-ID: <CAAKRu_bbZFF9dMOOMopOMy6BgvCxEGeWmcrCvwffc6ibGE+ZdQ@mail.gmail.com> (raw)
In-Reply-To: <pj4kgtdrevvkfbmlri6p27belctxru7ytyprcb6v74c7zbh3l6@m4dcu2rljedv>
References: <f3xxfrkafjxpyqxywcxricxgyizjirfceychyxsgn7bwjp5eda@kwbduhy7tfmu>
	<CAAKRu_bfwBzg7=Zy88st6gBJf97Wkd3k=+m1ecApn=59SwmKSw@mail.gmail.com>
	<pj4kgtdrevvkfbmlri6p27belctxru7ytyprcb6v74c7zbh3l6@m4dcu2rljedv>

On Thu, Apr 2, 2026 at 11:47 AM Andres Freund <[email protected]> wrote:
>
> > On some level, relying on worker mode overhead feels fragile. If
> > worker overhead decreases—say, by moving to IO worker threads—we won't
> > be able to rely on this to keep the distance to an advantageous level.
>
> I don't see why lower overhead would prevent this from working?

needed_wait has to be true to increase the readahead distance and for
io_uring, when data was in the kernel buffer cache, needed_wait is
false, meaning the distance doesn't increase. Worker mode didn't have
this problem because of overhead. So needed_wait is true for workers.
But, now that we will have combine_distance, I guess we don't need to
rely on workers having overhead. So we are saying that
readahead_distance is completely irrelevant for copying from the
kernel buffer cache and only combine_distance matters for that now,
right?

> > Yea, I think running ahead far enough to get bigger IOs needs to
> > happen and can't be based on the consumer having to wait.
>
> What do you think about the updated patch to achieve that that I posted?

Will post separately.

- Melanie

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