Re: [RFC][PATCH 03/10] zsmalloc: introduce zs_can_compact() function

[Sergey Senozhatsky]

On (06/04/15 12:15), Sergey Senozhatsky wrote:
> I'm still thinking how good it should be.
> 
> for automatic compaction we don't want to uselessly move objects between
> pages and I tend to think that it's better to compact less, than to waste
> more cpu cycless.
> 
> 
> on the other hand, this policy will miss cases like:
> 
> -- free objects in class: 5 (free-objs class capacity)
> -- page1: inuse 2
> -- page2: inuse 2
> -- page3: inuse 3
> -- page4: inuse 2
> 
> so total "insuse" is greater than free-objs class capacity. but, it's
> surely possible to compact this class. partial inuse summ <= free-objs class
> capacity (a partial summ is a ->inuse summ of any two of class pages:
> page1 + page2, page2 + page3, etc.).
> 
> otoh, these partial sums will badly affect performance. may be for automatic
> compaction (the one that happens w/o user interaction) we can do zs_can_compact()
> and for manual compaction (the one that has been triggered by a user) we can
> old "full-scan".
> 
> anyway, zs_can_compact() looks like something that we can optimize
> independently later.
> 

so what I'm thinking of right now, is:

-- first do "if we have enough free objects to free at least one page"
check. compact if true.

  -- if false, then we can do on a per-page basis
     "if page->inuse <= class free-objs capacity" then compact it,
     else select next almost_empty page.

     here would be helpful to have pages ordered by ->inuse. but this
     is far to expensive.


I have a patch that I will post later that introduces weak/partial
page ordering within fullness_list (really inexpensive: just one int
compare to add a page with a higher ->inuse to list head instead of
list tail).

	-ss
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