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

[Yinghai Lu]

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.

Also can we add block_size in that /sys directly so could generate
less entries ?

Thanks

Yinghai

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