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

[Rik van Riel <riel@redhat.com>]

On 04/26/2013 07:44 PM, Pierre-Loup A. Griffais wrote:
> I initially observed this between kernels 3.2 and 3.5: on 3.2, copying a
> 180M shared object on the same ext4 filesystem takes 0.6s. On 3.5, it
> takes between two and three minutes. It looks like a similar throughput
> regression happens on any machine running an i386 PAE kernel with high
> amounts of memory; the threshold seems to be 16G; passing mem=15G to the
> kernel commandline fixes it.

If you have that much memory in the system, you will
want to run a 64 bit kernel to avoid all kinds of
memory management corner cases.

> I bisected it to the following change:
>
> commit ab8fabd46f811d5153d8a0cd2fac9a0d41fb593d
> Author: Johannes Weiner <jweiner@redhat.com>
> Date:   Tue Jan 10 15:07:42 2012 -0800
>
>      mm: exclude reserved pages from dirtyable memory
>
> I realize running x86 kernels against high amounts of memory is not
> advised for various reasons, but I would assume that such a big
> regression in basic functionality to not be part of them. Is that
> accurate, or are these configurations expected to become unusable from
> 3.3 onwards?

Reverting that patch would probably break i686 PAE systems with
lots of memory at a different threshold.

With more than 8-12GB of memory, an i686 kernel is between a
rock and a hard place. Whether you move it closer to the rock,
or closer to the hard place, all you do is change the way in
which it breaks.

> Also CCing Sonny since it looks like he tried to fix an overflow issue
> related to the same change with commit c8b74c2f66049, but I'm still
> experiencing the problem with a kernel built from master.
>
> Thanks,
>   - Pierre-Loup


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