linux-kernel - Re: Disabling in-memory write cache for x86-64 in Linux II

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Date:	Mon, 4 Nov 2013 13:19:33 +0100
From:	Pavel Machek <pavel@....cz>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Ts'o <tytso@....edu>,
	"Artem S. Tashkinov" <t.artem@...os.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>,
	Maxim Patlasov <mpatlasov@...allels.com>
Subject: Re: Disabling in-memory write cache for x86-64 in Linux II

Hi!

> > Yes, but then the temp-file is long-lived enough that it *will* hit
> > the disk anyway. So it's only the "create temporary file and pretty
> > much immediately delete it" case that changes behavior (ie compiler
> > assembly files etc).
> > 
> > If the temp-file is for something like burning an ISO image, the
> > burning part is slow enough that the temp-file will hit the disk
> > regardless of when we start writing it.
> 
> The temp-file IO avoidance is an optimization not a guarantee. If a
> user want to avoid IO seriously, he will probably use tmpfs and
> disable swap.

No, sorry, they can't. Assuming ISO image fits in tmpfs would be
cruel.

> So if we have to do some trade-offs in the optimization, I agree that
> we should optimize more towards the "large copies to USB stick" use case.
> 
> The alternative solution, per-bdi dirty thresholds, could eliminate
> the need to do such trade-offs. So it's worth looking at the two
> solutions side by side.

Yes, please.
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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