linux-kernel - Re: [RFC][PATCH] drivers: base: dynamic memory block creation

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Message-ID: <20130814211607.GB17423@variantweb.net>
Date:	Wed, 14 Aug 2013 16:16:07 -0500
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Dave Hansen <dave@...1.net>,
	Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
	Cody P Schafer <cody@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH] drivers: base: dynamic memory block creation

On Wed, Aug 14, 2013 at 01:35:46PM -0700, Greg Kroah-Hartman wrote:
> On Wed, Aug 14, 2013 at 01:05:33PM -0700, Dave Hansen wrote:
> > On 08/14/2013 12:43 PM, Greg Kroah-Hartman wrote:
> > > On Wed, Aug 14, 2013 at 02:31:45PM -0500, Seth Jennings wrote:
> > >> ppc64 has a normal memory block size of 256M (however sometimes as low
> > >> as 16M depending on the system LMB size), and (I think) x86 is 128M.  With
> > >> 1TB of RAM and a 256M block size, that's 4k memory blocks with 20 sysfs
> > >> entries per block that's around 80k items that need be created at boot
> > >> time in sysfs.  Some systems go up to 16TB where the issue is even more
> > >> severe.
> > > 
> > > The x86 developers are working with larger memory sizes and they haven't
> > > seen the problem in this area, for them it's in other places, as I
> > > referred to in my other email.
> > 
> > The SGI guys don't run normal distro kernels and don't turn on memory
> > hotplug, so they don't see this.  I do the same in my testing of
> > large-memory x86 systems to speed up my boots.  I'll go stick it back in
> > there and see if I can generate some numbers for a 1TB machine.
> > 
> > But, the problem on x86 is at _worst_ 1/8 of the problem on ppc64 since
> > the SECTION_SIZE is so 8x bigger by default.
> > 
> > Also, the cost of creating sections on ppc is *MUCH* higher than x86
> > when amortized across the number of pages that you're initializing.  A
> > section on ppc64 has to be created for each (2^24/2^16)=256 pages while
> > one on x86 is created for each (2^27/2^12)=32768 pages.
> > 
> > Thus, x86 folks with our small pages and large sections tend to be
> > focused on per-page costs.  The ppc folks with their small sections and
> > larger pages tend to be focused on the per-section costs.
> 
> Ah, thanks for the explaination, now it makes more sense why they are
> both optimizing in different places.

Yes, thanks Dave for explaining that for me :)

> 
> But a "cleanup" patch first, and then the "change the logic to go
> faster" would be better here, so that we can review what is really
> happening.

Will do.

> 
> thanks,
> 
> greg k-h
> 

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