Re: [patch 5/9] x86: Cure per CPU madness on UP

[Guenter Roeck]

On 3/19/24 09:21, Thomas Gleixner wrote:
> On Mon, Mar 18 2024 at 20:13, Arnd Bergmann wrote:
> > FWIW, I did some experiments a few weeks ago on 32-bit ARM,
> > using a fairly minimal kernel in a virtual machine, and
> > checking the runtime memory consumption rather than compile-time.
> > In a kvm guest with 32MiB RAM, I saw a difference of multiple
> > megabytes in memory usage:
> >
> > Linux testvm 6.8.0-rc4-00410-gc02197fc9076-dirty #1 SMP PREEMPT armv7l
> > root@testvm:~# free
> >             	total   used    free  	shared  buff/cache   available
> > Mem:       	26932   14956   1732   	    52       12800   	11976
> > Swap:      	16360    3632   12728
> >
> > Linux testvm 6.8.0-rc4-00410-gc02197fc9076-dirty #2 PREEMPT armv7l
> > root@testvm:~# free
> >             	total    used  	free  	shared  buff/cache   available
> > Mem:       	26932   13744  	5648        32       10092   	13188
> > Swap:      	16360    3880  	12480
> >
> > There is a little difference between runs, but this does seem
> > significant enough to keep it. The SMP build was with
> > CONFIG_NR_CPUS=2 (the smallest supported compile-time number),
> > but running on a single-CPU qemu instance.
>
> With a SMP=y, NR_CPUS=1 build on x86 64bit I get:
>
>                 total        used        free      shared  buff/cache   available
> Mem:        32882056      498068    32590580        4884      128884    32383988
> Swap:         998396           0      998396
>
> Same config just SMP=n:
>
>                 total        used        free      shared  buff/cache   available
> Mem:        32885804      461704    32635284        4876      119480    32424100
> Swap:         998396           0      998396
>
> So the delta for available is ~40 MiB.
>
> But if I look at it with init=/bin/sh on the command line then the delta
> is significantly different:
>
> With a SMP=y, NR_CPUS=1 build on x86 64bit I get:
>
>                 total        used        free      shared  buff/cache   available
> Mem:        32883680      324120    32822728         216       10864    32559560
> Swap:              0           0           0
>
> Same config just SMP=n:
>
>                 total        used        free      shared  buff/cache   available
> Mem:        32885804      326876    32821972         216       11100    32558928
> Swap:              0           0           0
>
> Delta available = 632 KiB
>
> I haven't had the time to stare at that in detail, but comparing
> /proc/meminfo for the full boot case above does not immediately give me
> a hint. It's confusing at best...

That makes me wonder if the number is affected by the total memory size.
How about a system with 1GB of memory or less ?

Guenter