linux-kernel - Re: [PATCH v2] zbud: allow up to PAGE

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Date:	Fri, 25 Sep 2015 17:47:13 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Vitaly Wool <vitalywool@...il.com>
Cc:	Seth Jennings <sjennings@...iantweb.net>,
	Dan Streetman <ddstreet@...e.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v2] zbud: allow up to PAGE_SIZE allocations

On Fri, Sep 25, 2015 at 10:17:54AM +0200, Vitaly Wool wrote:
> <snip>
> > I already said questions, opinion and concerns but anything is not clear
> > until now. Only clear thing I could hear is just "compaction stats are
> > better" which is not enough for me. Sorry.
> >
> > 1) https://lkml.org/lkml/2015/9/15/33
> > 2) https://lkml.org/lkml/2015/9/21/2
> 
> Could you please stop perverting the facts, I did answer to that:
> https://lkml.org/lkml/2015/9/21/753.
> 
> Apart from that, an opinion is not necessarily something I would
> answer. Concerns about zsmalloc are not in the scope of this patch's
> discussion. If you have any concerns regarding this particular patch,
> please let us know.

Yes, I don't want to interrupt zbud thing which is Seth should maintain
and I respect his decision but the reason I nacked is you said this patch
aims for supporing zbud into zsmalloc for determinism.
For that, at least, you should discuss with me and Sergey but I feel
you are ignoring our comments.

> 
> > Vitally, Please say what's the root cause of your problem and if it
> > is external fragmentation, what's the problem of my approach?
> >
> > 1) make non-LRU page migrate
> > 2) provide zsmalloc's migratpage
> 
> The problem with your approach is that in your world I need to prove
> my right to use zbud. This is a very strange speculation.

No. If you want to contribute something, you should prove why yours
is better. I already said my concerns and my approach. It's your turn
that you should explain why it's better.

> 
> > We should provide it for CMA as well as external fragmentation.
> > I think we could solve your issue with above approach and
> > it fundamentally makes zsmalloc/zbud happy in future.
> 
> I doubt that but I'll answer in this thread:
> https://lkml.org/lkml/2015/9/15/33 as zsmalloc deficiencies do not
> have direct relation to this particular patch.
> 
> > Also, please keep it in mind that zram has been in linux kernel for
> > memory efficiency for a long time and later zswap/zbud was born
> > for *determinism* at the cost of memory efficiency.
> 
> Yep, and determinism is more important to me than the memory
> efficiency. Dropping the compression ration from 3.2x to 1.8x is okay
> with me and stalls in UI are not.

Then, you could use zswap which have aimed for it with small changes
to prevent writeback.


> 
> ~vitaly

-- 
Kind regards,
Minchan Kim
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