Re: [PATCH v2] mm: Warn about costly page allocation

[David Rientjes <rientjes@google.com>]

On Wed, 11 Jul 2012, Minchan Kim wrote:

> > Should we consider enabling CONFIG_COMPACTION in defconfig?  If not, would 
> 
> I hope so but Mel didn't like it because some users want to have a smallest
> kernel if they don't care of high-order allocation.
> 

CONFIG_COMPACTION adds 0.1% to my kernel image using x86_64 defconfig, 
that's the only reason we don't enable it by default?

> > it be possible with a different extfrag_threshold (and more aggressive 
> > when things like THP are enabled)?
> 
> Anyway, we should enable compaction for it although the system doesn't 
> care about high-order allocation and it ends up make bloting kernel unnecessary.
> 

The problem with this approach (and the appended patch) is that we can't 
define a system that "doesn't care about high-order allocations."  Even if 
you discount thp, an admin has no way of knowing how many high-order 
allocations his or her kernel will be doing and it will change between 
kernel versions.  Almost 50% of slab caches on my desktop machine running 
with slub have a default order greater than 0.

So I don't believe that adding this warning will be helpful and will 
simply lead to confusion.

> I tend to agree Andrew and your concern but I don't have a good idea but
> alert vague warning message. Anyway, we need *alert* this fact which removed
> lumpy reclaim for being able to disabling CONFIG_COMPACTION.

Can we ignore the fact that lumpy reclaim was removed and look at 
individual issues as they arise and address them by fixing the VM or by 
making a case for enabling CONFIG_COMPACTION by default?

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