Date: Wed, 14 Aug 2013 13:35:46 -0700
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Dave Hansen <dave@sr71.net>
Cc: Seth Jennings <sjenning@linux.vnet.ibm.com>,
        Nathan Fontenot <nfont@linux.vnet.ibm.com>,
        Cody P Schafer <cody@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Lai Jiangshan <laijs@cn.fujitsu.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [RFC][PATCH] drivers: base: dynamic memory block creation
Message-ID: <20130814203546.GA6200@kroah.com>
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 <20130814194348.GB10469@kroah.com>
 <520BE30D.3070401@sr71.net>
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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.

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.

thanks,

greg k-h
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