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

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Date:	Mon, 29 Apr 2013 15:08:50 -0700
From:	"Pierre-Loup A. Griffais" <pgriffais@...vesoftware.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	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 03:03 PM, Linus Torvalds wrote:
> On Mon, Apr 29, 2013 at 2:53 PM, Pierre-Loup A. Griffais
> <pgriffais@...vesoftware.com> wrote:
>>
>> Other than this particular concern, what's the high-level take-away? Is PAE
>> support in the Linux kernel a false promise than distros should not be
>> shipping by default, if at all? Should it be removed from the kernel
>> entirely if these configurations are knowingly broken by commits like this?
>
> PAE is "make it barely work". The whole concept is fundamentally
> flawed, and anybody who runs a 32-bit kernel with 16GB or RAM doesn't
> even understand *how* flawed and stupid that is.
>
> Don't do it. Upgrade to 64-bit, or live with the fact that IO
> performance will suck. The fact that it happened to work better under
> your particular load with one particular IO size is entirely just
> "random noise".
>
> Yeah, the difference between "we can cache it" and "we have to do IO"
> is huge. With a 32-bit kernel, we do IO much earlier now, just to
> avoid some really nasty situations. That makes you go from the "can
> sit in the cache" to the "do lots of IO" situation. Tough.
>
> 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.

All of this came from me trying to reproduce slowdowns reported by other 
people; I personally run a 64-bit kernel and understand how bad of an 
idea it is to attempt to run 32-bit kernels with PAE enabled on modern 
machines. However, my goal is to avoid ending up with a variety of 
end-users that don't necessarily understand this getting bitten by it 
and breaking their systems by upgrading their kernels. I will indeed 
bring this up with distributors and point out than shipping PAE kernels 
by default is not a good idea given these problems and your stance on 
the matter.

Thanks,
  - Pierre-Loup

>
>                  Linus
>

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