Date:	Thu, 18 Jun 2015 11:41:07 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCHv2 8/8] zsmalloc: register a shrinker to trigger
 auto-compaction

Hi,

On (06/18/15 10:50), Minchan Kim wrote:
[..]
> > hm, what's the difference with the existing implementation?
> > The 'new one' aborts when (a) !zs_can_compact() and (b) !migrate_zspage().
> > It holds the class lock less time than current compaction.
> 
> At old, it unlocks periodically(ie, per-zspage migration) so other who
> want to allocate a zspage in the class can have a chance but your patch
> increases lock holding time until all of zspages in the class is done
> so other will be blocked until all of zspage migration in the class is
> done.

ah, I see.
it doesn't hold the lock `until all the pages are done`. it holds it
as long as zs_can_compact() returns > 0. hm, I'm not entirely sure that
this patch set has increased the locking time (in average).


> > 
> > > I will review remain parts tomorrow(I hope) but what I want to say
> > > before going sleep is:
> > > 
> > > I like the idea but still have a concern to lack of fragmented zspages
> > > during memory pressure because auto-compaction will prevent fragment
> > > most of time. Surely, using fragment space as buffer in heavy memory
> > > pressure is not intened design so it could be fragile but I'm afraid
> > > this feature might accelrate it and it ends up having a problem and
> > > change current behavior in zram as swap.
> > 
> > Well, it's nearly impossible to prove anything with the numbers obtained
> > during some particular case. I agree that fragmentation can be both
> > 'good' (depending on IO pattern) and 'bad'.
> 
> Yes, it's not easy and I believe a few artificial testing are not enough
> to prove no regression but we don't have any choice.
> Actually, I think this patchset does make sense. Although it might have
> a problem on situation heavy memory pressure by lacking of fragment space,


I tested exactly this scenario yesterday (and sent an email). We leave `no holes'
in classes only in ~1.35% of cases. so, no, this argument is not valid. we preserve
fragmentation.

	-ss

> I think we should go with this patchset and fix the problem with another way
> (e,g. memory pooling rather than relying on the luck of fragment).
> But I need something to take the risk. That's why I ask the number
> although it's not complete. It can cover a case at least, it is better than
> none. :)
> 
> > 
> > 
> > Auto-compaction of IDLE zram devices certainly makes sense, when system
> > is getting low on memory. zram devices are not always 'busy', serving
> > heavy IO. There may be N idle zram devices simply sitting and wasting
> > memory; or being 'moderately' busy; so compaction will not cause any
> > significant slow down there.
> > 
> > Auto-compaction of BUSY zram devices is less `desired', of course;
> > but not entirely terrible I think (zs_can_compact() can help here a
> > lot).
> 
> 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.
> 
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