linux-kernel - Re: [PATCH] mm: make fault_around

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Date:	Tue, 17 May 2016 15:34:23 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Vinayak Menon <vinmenon@...eaurora.org>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, dan.j.williams@...el.com,
	mgorman@...e.de, vbabka@...e.cz, kirill.shutemov@...ux.intel.com,
	dave.hansen@...ux.intel.com, hughd@...gle.com
Subject: Re: [PATCH] mm: make fault_around_bytes configurable

On Mon, May 16, 2016 at 11:56:32PM +0900, Minchan Kim wrote:
> On Mon, May 16, 2016 at 05:29:00PM +0300, Kirill A. Shutemov wrote:
> > > Kirill,
> > > You wanted to test non-HW access bit system and I did.
> > > What's your opinion?
> > 
> > Sorry, for late response.
> > 
> > My patch is incomlete: we need to find a way to not mark pte as old if we
> > handle page fault for the address the pte represents.
> 
> I'm sure you can handle it but my point is there wouldn't be a big gain
> although you can handle it in non-HW access bit system. Okay, let's be
> more clear because I don't have every non-HW access bit architecture.
> At least, current mobile workload in ARM which I have wouldn't be huge
> benefit.
> I will say one more.
> I tested the workload on quad-core system and core speed is not so slow
> compared to recent other mobile phone SoC. Even when I tested the benchmark
> without pte_mkold, the benefit is within noise because storage is really
> slow so major fault is dominant factor. So, I decide test storage from eMMC
> to eSATA. And then finally, I manage to see the a little beneift with
> fault_around without pte_mkold.
> 
> However, let's consider side-effect aspect from fault_around.
> 
> 1. Increase slab shrinking compard to old
> 2. high level vmpressure compared to old
> 
> With considering that regressions on my system, it's really not worth to
> try at the moment.
> That's why I wanted to disable fault_around as default in non-HW access
> bit system.

Feel free to post such patch. I guess it's reasonable.

> > Once this will be done, the number of page faults shouldn't be higher with
> > fault-around enabled even on machines without hardware accessed bit. This
> > will address performance regression with the patch on such machines.
> 
> Although you solves that, I guess the benefit would be marginal in
> some architectures but we should solve above side-effects.
> 
> > 
> > I'll try to find time to update the patch soon.
> 
> I hope you can solve above those regressions as well.

The patch is posted. Please test.

-- 
 Kirill A. Shutemov