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

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Date:	Fri, 25 Oct 2013 04:28:27 -0700 (PDT)
From:	David Lang <david@...g.hm>
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, 25 Oct 2013, 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.

If you go this direction, allow ratios larger than 100%, some people may be 
willing to have huge amounts of dirty data on large memory machines (if the load 
is extremely bursty, they don't have other needs for I/O, or they have a very 
fast storage system, as a few examples)

David Lang
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