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 -- 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>