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

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Message-ID: <520BE30D.3070401@sr71.net>
Date:	Wed, 14 Aug 2013 13:05:33 -0700
From:	Dave Hansen <dave@...1.net>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	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 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.
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