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

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Message-ID: <20131025091555.GA30895@kipc2.localdomain>
Date:	Fri, 25 Oct 2013 11:15:55 +0200
From:	Karl Kiniger <karl.kiniger@....ge.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Artem S. Tashkinov" <t.artem@...os.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Disabling in-memory write cache for x86-64 in Linux II

On Fri 131025, Linus Torvalds wrote:
> On Fri, Oct 25, 2013 at 9:30 AM, Artem S. Tashkinov <t.artem@...os.com> wrote:
> >
> > My feeling is that vm.dirty_ratio/vm.dirty_background_ratio should _not_ be
> > percentage based, 'cause for PCs/servers with a lot of memory (say 64GB or
> > more) this value becomes unrealistic (13GB) and I've already had some
> > unpleasant effects due to it.
> 
> Right. The percentage notion really goes back to the days when we
> typically had 8-64 *megabytes* of memory So if you had a 8MB machine
> you wouldn't want to have more than one megabyte of dirty data, but if
> you were "Mr Moneybags" and could afford 64MB, you might want to have
> up to 8MB dirty!!
> 
> Things have changed.
> 
> So I would suggest we change the defaults. Or pwehaps make the rule be
> that "the ratio numbers are 'ratio of memory up to 1GB'", to make the
> semantics similar across 32-bit HIGHMEM machines and 64-bit machines.
> 
> The modern way of expressing the dirty limits are to give the actual
> absolute byte amounts, but we default to the legacy ratio mode..
> 
>                 Linus

Is it currently possible to somehow set above values per block device?

I want default behaviour for almost everything but  DVD drives in DVD+RW
packet writing mode may easily take several minutes in case of a sync.

Karl


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