Re: [patch 1/2] mm, zone: track number of pages in free area by migratetype

[David Rientjes]

On Thu, 17 Nov 2016, Vlastimil Babka wrote:

> > The total number of free pages is still tracked, however, to not make
> > zone_watermark_ok() more expensive.  Reading /proc/pagetypeinfo, however,
> > is faster.
> 
> Yeah I've already seen a case with /proc/pagetypeinfo causing soft
> lockups due to high number of iterations...
> 

Thanks for taking a look at the patchset!

Wow, I haven't seen /proc/pagetypeinfo soft lockups yet, I thought this 
was a relatively minor point :)  But it looks like we need some 
improvement in this behavior independent of memory compaction anyway.

> > This patch introduces no functional change and increases the amount of
> > per-zone metadata at worst by 48 bytes per memory zone (when CONFIG_CMA
> > and CONFIG_MEMORY_ISOLATION are enabled).
> 
> Isn't it 48 bytes per zone and order?
> 

Yes, sorry, I'll fix that in v2.  I think less than half a kilobyte for 
each memory zone is satisfactory for extra tracking, compaction 
improvements, and optimized /proc/pagetypeinfo, though.

> > Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> 
> I'd be for this if there are no performance regressions. It affects hot
> paths and increases cache footprint. I think at least some allocator
> intensive microbenchmark should be used.
> 

I can easily implement a test to stress movable page allocations from 
fallback MIGRATE_UNMOVABLE pageblocks and freeing back to the same 
pageblocks.  I assume we're not interested in memory offline benchmarks.

What do you think about the logic presented in patch 2/2?  Are you 
comfortable with a hard-coded ratio such as 1/64th of free memory or would 
you prefer to look at the zone's watermark with the number of free pages 
from MIGRATE_MOVABLE pageblocks rather than NR_FREE_PAGES?  I was split 
between the two options.