Re: More OOM problems (sorry fro the mail bomb)

[Raymond Jennings <shentino@gmail.com>]

On Thu, Sep 29, 2016 at 2:20 PM, Vlastimil Babka <vbabka@suse.cz> wrote:
> On 09/29/2016 10:08 PM, Raymond Jennings wrote:
>>  Suggestion:
>> 
>>  1.  Make it a background process "kcompactd"
>>  2.  It is activated/woke up/semaphored awake any time a page is 
>> freed.
>>  3.  Once it is activated, it enters a loop:
>>  3.1.  Reset the semaphore.
>>  3.2.  Once a cycle, it takes the highest movable page
>>  3.3.  It then finds the lowest free page
>>  3.4.  Then, it migrates the highest used page to the lowest free 
>> space
>>  3.5.  maybe pace itself by sleeping for a teensy, then go back to 
>> step
>>  3.2
>>  3.6.  Do one page at a time to keep it neatly interruptible and 
>> keep it
>>  from blocking other stuff.  Since compaction is a housekeeping 
>> task, it
>>  should probably be eager to yield to other things.
>>  3.7.  Probably leave hugepages alone if detected since they are by
>>  definition fairly defragmented already.
>>  4.  Once all gaps are backfilled, go back to sleep and park back at
>>  step 2 waiting for the next wakeup.
>> 
>>  Would this be a good way to do it?
> 
> Yes, that's pretty much how it already works, except movable pages are
> taken from low pfn and free pages from high. Then there's ton of 
> subtle
> issues to tackle, mostly the balance between overhead and benefit.

Besides the kswapd hook, what would nudge kcompactd to run?  If its not 
proactively nudged after a page is freed how will it know that there's 
fragmentation that could be taken care of in advance before being 
shoved by kswapd?

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