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

From: David Lang
Date: Fri Oct 25 2013 - 07:28:35 EST

On Fri, 25 Oct 2013, Linus Torvalds wrote:

On Fri, Oct 25, 2013 at 9:30 AM, Artem S. Tashkinov <t.artem@xxxxxxxxx> 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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