Re: kswapd skips compaction if reclaim order drops to zero?

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

Hi Minchan,
On Thu, Aug 15, 2013 at 11:10:04PM +0900, Minchan Kim wrote:
>On Thu, Aug 15, 2013 at 02:56:27PM +0100, Mel Gorman wrote:
>> On Thu, Aug 15, 2013 at 10:41:39PM +0900, Minchan Kim wrote:
>> > Hey Mel,
>> > 
>> > On Thu, Aug 15, 2013 at 11:47:27AM +0100, Mel Gorman wrote:
>> > > On Thu, Aug 15, 2013 at 06:02:53PM +0800, Hillf Danton wrote:
>> > > > If the allocation order is not high, direct compaction does nothing.
>> > > > Can we skip compaction here if order drops to zero?
>> > > > 
>> > > 
>> > > If the allocation order is not high then
>> > > 
>> > > pgdat_needs_compaction == (order > 0) == false == no calling compact_pdatt
>> > > 
>> > > In the case where order is reset to 0 due to fragmentation then it does
>> > > call compact_pgdat but it does no work due to the cc->order check in
>> > > __compact_pgdat.
>> > > 
>> > 
>> > I am looking at mmotm-2013-08-07-16-55 but couldn't find cc->order
>> > check right before compact_zone in __comact_pgdat.
>> > Could you pinpoint code piece?
>> > 
>> 
>> Thanks, I screwed up as that check happens too late. However, it still
>> ends up not mattering because it does this
>> 
>> compact_pgdat
>>   -> __compact_pgdat
>>     -> compact_zone
>>       -> compaction_suitable
>> 
>> For order == 0, compaction_suitable will return either COMPACT_SKIPPED
>> (if the watermarks are not met) and COMPACT_PARTIAL otherwise. Either
>> way, compaction doesn't run.
>
>In compaction_suitable, it could pass first zone_watermark_ok
>but failed second zone_watermark_ok while fragindex is -1000
>so compaction could run.
>

I'm not sure why you said that second zone_watermark_ok failed while 
fragindex is -1000, actually they are the same check against order 0.

First:
watermark = low_wmark_pages(zone) + (2UL << order);
zone_watermark_ok(zone, 0, watermark, 0, 0);

Second:
fragindex == -1000 && zone_watermark_ok(zone, order, watermark, 0, 0)

Both COMPACT_SKIPPED and COMPACT_PARTIAL will fail compaction.

Regards,
Wanpeng Li 

>And we shouldn't depend on such coincidence.
>
>
>> 
>> -- 
>> Mel Gorman
>> SUSE Labs
>> 
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>-- 
>Kind regards,
>Minchan Kim
>
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