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

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.

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