linux-kernel - RE: [PATCHv3 5/6] zswap: add to mm/

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Date:	Wed, 6 Feb 2013 15:47:29 -0800 (PST)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Konrad Wilk <konrad.wilk@...cle.com>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>,
	Jenifer Hopper <jhopper@...ibm.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: RE: [PATCHv3 5/6] zswap: add to mm/

> From: Seth Jennings [mailto:sjenning@...ux.vnet.ibm.com]
> Subject: Re: [PATCHv3 5/6] zswap: add to mm/
> 
> On 01/29/2013 12:27 AM, Minchan Kim wrote:
> > First feeling is it's simple and nice approach.
> > Although we have some problems to decide policy, it could solve by later patch
> > so I hope we make basic infrasture more solid by lots of comment.
> 
> Thanks very much for the review!
> >
> > Another question.
> >
> > What's the benefit of using mempool for zsmalloc?
> > As you know, zsmalloc doesn't use mempool as default.
> > I guess you see some benefit. if so, zram could be changed.
> > If we can change zsmalloc's default scheme to use mempool,
> > all of customer of zsmalloc could be enhanced, too.
> 
> In the case of zswap, through experimentation, I found that adding a
> mempool behind the zsmalloc pool added some elasticity to the pool.
> Fewer stores failed if we kept a small reserve of pages around instead
> of having to go back to the buddy allocator who, under memory
> pressure, is more likely to reject our request.
> 
> I don't see this situation being applicable to all zsmalloc users
> however.  I don't think we want incorporate it directly into zsmalloc
> for now.  The ability to register custom page alloc/free functions at
> pool creation time allows users to do something special, like back
> with a mempool, if they want to do that.

(sorry, still catching up on backlog after being gone last week)

IIUC, by using mempool, you are essentially setting aside a
special cache of pageframes that only zswap can use (or other
users of mempool, I don't know what other subsystems use it).
So one would expect that fewer stores would fail if more
pageframes are available to zswap, the same as if you had
increased zswap_max_pool_percent by some small fraction.

But by setting those pageframes aside, you are keeping them from
general use, which may be a use with a higher priority as determined
by the mm system.

This seems wrong to me.  Should every subsystem hide a bunch of
pageframes away in case it might need them?

Or am I missing something?

Dan
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