Re: kswapd craziness in 3.7

[Zlatko Calusic <zlatko.calusic@iskon.hr>]

On 10.12.2012 12:03, Mel Gorman wrote:
> There is a big difference between a direct reclaim/compaction for THP
> and kswapd doing the same work. Direct reclaim/compaction will try once,
> give up quickly and defer requests in the near future to avoid impacting
> the system heavily for THP. The same applies for khugepaged.
>
> kswapd is different. It can keep going until it meets its watermarks for
> a THP allocation are met. Two reasons why it might keep going for a long
> time are that compaction is being inefficient which we know it may be due
> to crap like this
>
> end_pfn = ALIGN(low_pfn + pageblock_nr_pages, pageblock_nr_pages);
>
> and the second reason is if the highest zone is relatively because
> compaction_suitable will keep saying that allocations are failing due to
> insufficient amounts of memory in the highest zone. It'll reclaim a little
> from this highest zone and then shrink_slab() potentially dumping a large
> amount of memory. This may be the case for Zlatko as with a 4G machine
> his ZONE_NORMAL could be small depending on how the 32-bit address space
> is used by his hardware.
>

The kernel is 64-bit, if it makes any difference (userspace, though is 
still 32-bit). There's no swap (swap support not even compiled in). The 
zones are as follows:

On node 0 totalpages: 1048019
   DMA zone: 64 pages used for memmap
   DMA zone: 6 pages reserved
   DMA zone: 3913 pages, LIFO batch:0
   DMA32 zone: 16320 pages used for memmap
   DMA32 zone: 831109 pages, LIFO batch:31
   Normal zone: 3072 pages used for memmap
   Normal zone: 193535 pages, LIFO batch:31

If I understand correctly, you think that because 193535 pages in 
ZONE_NORMAL is relatively small compared to 831109 pages of ZONE_DMA32 
the system has hard time balancing itself?

Is there any way I could force and test different memory layout? I'm 
slightly lost at all the memory models (if I have a choice at all), so 
if you have any suggestions, I'm all ears.

Maybe I could limit available memory and thus have only DMA32 zone, just 
to prove your theory? I remember doing tuning like that many years ago 
when I had more time to play with Linux MM, unfortunately didn't have 
much time lately, so I'm a bit rusty, but I'm willing to help testing 
and resolving this issue.

-- 
Zlatko

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