[RFC][PATCH] drivers: base: dynamic memory block creation

[Michael Ellerman]

On Wed, Aug 14, 2013 at 04:52:53PM -0500, Seth Jennings wrote:
> On Wed, Aug 14, 2013 at 02:37:26PM -0700, Yinghai Lu wrote:
> > On Wed, Aug 14, 2013 at 1:35 PM, Greg Kroah-Hartman
> > <gregkh at linuxfoundation.org> 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.
> > 
> > I had one local patch that sent before, it will probe block size for
> > generic x86_64.
> > set it to 2G looks more reasonable for system with 1T+ ram.
> 
> If I am understanding you correctly, you are suggesting we make the block size
> a boot time tunable.  It can't be a runtime tunable since the memory blocks are
> currently created a boot time.
> 
> On ppc64, we can't just just choose a memory block size since it must align
> with the underlying LMB (logical memory block) size, set in the hardware ahead
> of time.

As long as the Linux block size is a multiple of the LMB size it should
be possible. You'd just have to plug/unplug multiple LMBs at once.

It would be possible to construct an LMB layout that defeats that, eg.
every 2nd LMB not present, but I don't think that's actually a concern
in practice.

cheers