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@xxxxxxxxxxxxxxxxxxx> 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. Seth -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>