linux-kernel - Re: [PATCH] arm64: Increase the max granular size

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Message-ID: <20151110001949.GA13894@js1304-P5Q-DELUXE>
Date:	Tue, 10 Nov 2015 09:19:50 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Robert Richter <rric@...nel.org>,
	Will Deacon <will.deacon@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Robert Richter <rrichter@...ium.com>,
	Tirumalesh Chalamarla <tchalamarla@...ium.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: Increase the max granular size

On Mon, Nov 09, 2015 at 06:36:09PM +0000, Catalin Marinas wrote:
> On Mon, Nov 09, 2015 at 04:41:58PM +0900, Joonsoo Kim wrote:
> > 2015-11-05 21:17 GMT+09:00 Catalin Marinas <catalin.marinas@....com>:
> > > On Thu, Nov 05, 2015 at 08:45:08PM +0900, Joonsoo Kim wrote:
> > >> If it isn't possible, is there another way to reduce memory waste due to
> > >> increase of dma alignment requirement in arm64?
> > >
> > > I first need to see how significant the impact is (especially for
> > > embedded/mobiles platforms).
> > 
> > I don't have any ARM64 device. What I have just one report
> > about slab usage from our developer.
> > 
> > The report shows slab usage just after android boot is done
> > in ARM64.
> > 
> > Total slab usage: 90 MB
> > kmalloc usage: 25 MB
> > kmalloc (<=64) usage: 7 MB
> > 
> > This would be measured without slab_nomerge so there is
> > a possibility that some usages on kmem_cache is merged
> > into usage of kmalloc (<=64).
> > 
> > Anyway, if ARM64 increase L1_CACHE_BYTES to 128, roughly
> > 7 MB would be wasted. I don't know how this picture is varied
> > in runtime, but, even boot time overhead, 7 MB looks large to me.
> 
> 7MB is considerable but I guess it wouldn't be all wasted with
> L1_CACHE_BYTES == 128, maybe half or slightly over. It would be good to
> know the other kmalloc caches, maybe up to 256.
> 
> I don't have an Android filesystem but I just tried to boot Arch
> (aarch64). Immediately after boot and slab_nomerge, with 128 L1 I get:
> 
> kmalloc-128:	6624
> kmalloc-256:	1488
> 
> With L1 64, I get:
> 
> kmalloc-64:	5760
> kmalloc-128:	1152
> kmalloc-192:	1155
> kmalloc-256:	 320
> 
> So that's about 1.2MB vs 0.8MB. The ratio is 3:2, though I'm not sure it
> will stay the same as the slab usage increases.
> 
> It would be good to get more numbers, we could add a Kconfig option just
> for specific builds while keeping the default to 128.

Okay.

Thanks.
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