linux-kernel - Re: [PATCH 2/3] mm: filemap: only do access activations on reads

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Date:	Mon, 4 Apr 2016 14:22:33 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andres Freund <andres@...razel.de>, Rik van Riel <riel@...hat.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	kernel-team@...com
Subject: Re: [PATCH 2/3] mm: filemap: only do access activations on reads

On Mon,  4 Apr 2016 13:13:37 -0400 Johannes Weiner <hannes@...xchg.org> wrote:

> Andres Freund observed that his database workload is struggling with
> the transaction journal creating pressure on frequently read pages.
> 
> Access patterns like transaction journals frequently write the same
> pages over and over, but in the majority of cases those pages are
> never read back. There are no caching benefits to be had for those
> pages, so activating them and having them put pressure on pages that
> do benefit from caching is a bad choice.

Read-after-write is a pretty common pattern: temporary files for
example.  What are the opportunities for regressions here?

Did you consider providing userspace with a way to hint "this file is
probably write-then-not-read"?