linux-kernel - Re: [RFC][PATCH 03/10] zsmalloc: introduce zs_can

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Date:	Thu, 4 Jun 2015 12:42:30 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 03/10] zsmalloc: introduce zs_can_compact() function

On (06/04/15 12:30), Minchan Kim wrote:
> > -- free objects in class: 5 (free-objs class capacity)
> > -- page1: inuse 2
> > -- page2: inuse 2
> > -- page3: inuse 3
> > -- page4: inuse 2
> 
> What scenario do you have a cocern?
> Could you describe this example more clear?

you mean "how is this even possible"?

well, for example,

make -jX
make clean

can introduce a significant fragmentation. no new objects, just random
objs removal. assuming that we keep some of the objects, allocated during
compilation.

e.g.

...

page1
  allocate baz.so
  allocate foo.o
page2
  allocate bar.o
  allocate foo.so
...
pageN



now `make clean`

page1:
  allocated baz.so
  empty

page2
  empty
  allocated foo.so

...

pageN

in the worst case, every page can turn out to be ALMOST_EMPTY.

	-ss
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