linux-ext4 - Re: [PATCH 0/1] iomap regression for aio dio 4k writes

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Message-ID: <ZJO4OAYhJlXOBXMf@casper.infradead.org>
Date:   Thu, 22 Jun 2023 03:55:52 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Jeremy Bongio <bongiojp@...il.com>, Ted Tso <tytso@....edu>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Allison Henderson <allison.henderson@...cle.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH 0/1] iomap regression for aio dio 4k writes

On Thu, Jun 22, 2023 at 11:55:23AM +1000, Dave Chinner wrote:
> Ok, so having spent a bit more thought on this away from the office
> this morning, I think there is a generic way we can avoid deferring
> completions for pure overwrites.

OK, this is how we can, but should we?  The same amount of work
needs to be done, no matter whether we do it in interrupt context or
workqueue context.  Doing it in interrupt context has lower latency,
but maybe allows us to batch up the work and so get better bandwidth.
And we can't handle other interrupts while we're handling this one,
so from a whole-system perspective, I think we'd rather do the work in
the workqueue.

Latency is important for reads, but why is it important for writes?
There's such a thing as a dependent read, but writes are usually buffered
and we can wait as long as we like for a write to complete.