linux-kernel - Re: IO regression after ab8fabd46f on x86 kernels with high memory

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Date:	Mon, 29 Apr 2013 18:06:19 -0700
From:	"Pierre-Loup A. Griffais" <pgriffais@...vesoftware.com>
To:	Rik van Riel <riel@...hat.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	<sonnyrao@...omium.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: IO regression after ab8fabd46f on x86 kernels with high memory

On 04/29/2013 05:48 PM, Rik van Riel wrote:
> On 04/29/2013 06:03 PM, Linus Torvalds wrote:
>
>> Seriously, you can compile yourself a 64-bit kernel and continue to
>> use your 32-bit user-land. And you can complain to whatever distro you
>> used that it didn't do that in the first place. But we're not going to
>> bother with trying to tune PAE for some particular load. It's just not
>> worth it to anybody.
>
> I can think of one way to "tune PAE" that will help
> avoid the breakage, and at the same time draw the
> attention of users.
>
> Limit the memory that a 32 bit PAE kernel uses, to
> something small enough where the user will not
> encounter random breakage.  Maybe 8 or 12GB?
>
> It could also print out a friendly message, to
> inform the user they should upgrade to a 64 bit
> kernel to enjoy the use of all of their memory.
>
> It is a bit of a heavy stick, but I suspect that
> it would clue in all of the affected users.
>
> If you have no objection to this, I'll whip up a
> patch.
>

That would be pretty useful, especially if I can then convince 
distributors to apply it and roll it out ASAP. I haven't personally 
observed any problems with mem=15G whereas mem=16G exhibits the IO issue 
upfront and more than that exhibits the OOM-killer / low memory 
starvation issue that existed before Johannes change.

Thanks,
  - Pierre-Loup
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