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

From: Pierre-Loup A. Griffais
Date: Mon Apr 29 2013 - 21:29:43 EST

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

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.

- Pierre-Loup
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