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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAPz6YkXqxNNvXcjRv1APbUoreLofSGAxM5XKz-F8fz_SCV0egw@mail.gmail.com>
Date:	Wed, 1 May 2013 21:37:27 -0700
From:	Sonny Rao <sonnyrao@...omium.org>
To:	"Pierre-Loup A. Griffais" <pgriffais@...vesoftware.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.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 Mon, Apr 29, 2013 at 3:08 PM, Pierre-Loup A. Griffais
<pgriffais@...vesoftware.com> wrote:
> 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.
>

Sorry just saw this (my stupid gmail filters for lkml) The slow-down
we ran into wasn't even on PAE -- it was *just* with highmem on a 2GB
system.  The non-zero amount (90MB? or so) of highmem was enough to
cause major problems due to that particular underflow.

I would say regardless of how much memory you have, if the system can
use a 64-bit kernel, then it almost certainly should.  I've seen some
very minor performance impacts on 64-bit capable Atom systems with
tiny L2 caches, but it's almost in the noise and not worth the pain.

> Thanks,
>  - Pierre-Loup
>
>>
>>                  Linus
>>
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/