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

From: Pierre-Loup A. Griffais
Date: Mon Apr 29 2013 - 18:16:46 EST

On 04/29/2013 03:03 PM, Linus Torvalds wrote:
On Mon, Apr 29, 2013 at 2:53 PM, Pierre-Loup A. Griffais
<pgriffais@xxxxxxxxxxxxxxxxx> 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.

- Pierre-Loup


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