Re: [RFC][PATCHv2 8/8] zsmalloc: register a shrinker to trigger auto-compaction

From: Sergey Senozhatsky
Date: Wed Jun 17 2015 - 23:01:21 EST

On (06/18/15 11:41), Sergey Senozhatsky wrote:
> > My concern is not a compacion overhead but higher memory footprint
> > consumed by zram in reserved memory.
> > It might hang system if zram used up reserved memory of system with
> > ALLOC_NO_WATERMARKS. With auto-compaction, userspace has a higher chance
> > to use more memory with uncompressible pages or file-backed pages
> > so zram-swap can use more reserved memory. We need to evaluate it, I think.
> >

a couple of _not really related_ ideas that I want to voice.

(a) I'm thinking of extending zramX/compact attr. right now it's WO,
and I think it makes sense to make it RW:
->write will trigger compaction
->read will return estimated number of bytes
"zs_can_compact() * pages per zspage * page_size" that can be freed.
so user-space will have at least minimal idea whether compaction is
reasonable. but sure, this is racy and in general case things may
change between `cat compact` and `echo 1 > compact`.

(b) adding a knob (yeah, like we don't have enough knobs already :-))
that will allow 'enable/disable auto compaction'.

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