Re: Dmesg: Use a PAE enabled kernel

Re: Dmesg: Use a PAE enabled kernel

[Andrew Walrond]

The dump looks like this:

BIOS-provided physical RAM map:
   BIOS-e820: 0000000000000000 - 00000000000a0000 (usable)
   BIOS-e820: 00000000000f0000 - 0000000000100000 (reserved)
   BIOS-e820: 0000000000100000 - 00000000bfffa000 (usable)
   BIOS-e820: 00000000bfffa000 - 00000000bffff000 (ACPI data)
   BIOS-e820: 00000000bffff000 - 00000000c0000000 (ACPI NVS)
   BIOS-e820: 00000000fec00000 - 00000000fec01000 (reserved)
   BIOS-e820: 00000000fee00000 - 00000000fee01000 (reserved)
   BIOS-e820: 00000000ffff0000 - 0000000100000000 (reserved)
   BIOS-e820: 0000000100000000 - 0000000140000000 (usable)
Warning only 4GB will be used.
Use a PAE enabled kernel.
3200MB HIGHMEM available.
896MB LOWMEM available.

So you are saying that not all the 4Gb of ram will get mapped/used
(specifically, everything not marked 'usable') ?

Can you quantify the performance degredation of a PAE enabled kernel?

Andrew Walrond

Re: Dmesg: Use a PAE enabled kernel

[H. Peter Anvin]

Followup to:  <3E6381B9.4090708@walrond.org>
By author:    Andrew Walrond <andrew@walrond.org>
In newsgroup: linux.dev.kernel
>
> The dump looks like this:
> 
> BIOS-provided physical RAM map:
>   BIOS-e820: 0000000000000000 - 00000000000a0000 (usable)
>   BIOS-e820: 00000000000f0000 - 0000000000100000 (reserved)
>   BIOS-e820: 0000000000100000 - 00000000bfffa000 (usable)
>   BIOS-e820: 00000000bfffa000 - 00000000bffff000 (ACPI data)
>   BIOS-e820: 00000000bffff000 - 00000000c0000000 (ACPI NVS)
>   BIOS-e820: 00000000fec00000 - 00000000fec01000 (reserved)
>   BIOS-e820: 00000000fee00000 - 00000000fee01000 (reserved)
>   BIOS-e820: 00000000ffff0000 - 0000000100000000 (reserved)
>   BIOS-e820: 0000000100000000 - 0000000140000000 (usable)  
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	       This memory lives above the 4GB mark.

> Warning only 4GB will be used.
> Use a PAE enabled kernel.
> 3200MB HIGHMEM available.
> 896MB LOWMEM available.
> 
> So you are saying that not all the 4Gb of ram will get mapped/used 
> (specifically, everything not marked 'usable') ?

Basically, your motherboard only allows 3 GB below the 4 GB boundary;
the rest ends up above.

	-hpa
-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
Architectures needed: cris ia64 m68k mips64 ppc ppc64 s390 s390x sh v850 x86-64

Re: Dmesg: Use a PAE enabled kernel

[Alan Cox]

On Mon, 2003-03-03 at 16:49, Andrew Walrond wrote:
> That accounts for the missing Gb then. I think the warning needs to be 
> more like "WARNING WARNING (WILL ROBINSON)! 1GB of very expensive ram 
> won't be used unless you enable PAE!!!"

That would require we could detect rambus memory 8)

	printk("Warning %dGb of %sram won't be used.\n", lost>>10,
rambus()?"expensive ");

But yes more seriously it would be a lot more useful and more
meaningful to tell people how much ram is not being used

> Just how slow is PAE then ?

It varies, it can cost you 5-10%. The bigger hit is that most 
I/O devices can't reach that memory directly from PCI space
so you get bounce buffer costs and more paging/vm management
overhead.

Re: Dmesg: Use a PAE enabled kernel

[Alan Cox]

On Mon, 2003-03-03 at 16:24, Andrew Walrond wrote:
>   BIOS-e820: 0000000100000000 - 0000000140000000 (usable)

This chunk wont get used

Re: Dmesg: Use a PAE enabled kernel

[Martin J. Bligh]

> BIOS-provided physical RAM map:
>   BIOS-e820: 0000000000000000 - 00000000000a0000 (usable)
>   BIOS-e820: 00000000000f0000 - 0000000000100000 (reserved)
>   BIOS-e820: 0000000000100000 - 00000000bfffa000 (usable)
>   BIOS-e820: 00000000bfffa000 - 00000000bffff000 (ACPI data)
>   BIOS-e820: 00000000bffff000 - 00000000c0000000 (ACPI NVS)
>   BIOS-e820: 00000000fec00000 - 00000000fec01000 (reserved)
>   BIOS-e820: 00000000fee00000 - 00000000fee01000 (reserved)
>   BIOS-e820: 00000000ffff0000 - 0000000100000000 (reserved)
>   BIOS-e820: 0000000100000000 - 0000000140000000 (usable)
> Warning only 4GB will be used.
> Use a PAE enabled kernel.
> 3200MB HIGHMEM available.
> 896MB LOWMEM available.
> 
> So you are saying that not all the 4Gb of ram will get mapped/used (specifically, everything not marked 'usable') ?
> 
> Can you quantify the performance degredation of a PAE enabled kernel?

Depends on your workload ... there's only one way to be sure, benchmark it.
If you're doing heavy page replacment / setup / teardown, it'll be more
expensive. As Alan has pointed out, you're loosing a quarter of your RAM ;-)

M.

Re: Dmesg: Use a PAE enabled kernel

[Alan Cox]

On Mon, 2003-03-03 at 15:23, Andrew Walrond wrote:
> During bootup I see
> 
>    Warning only 4GB will be used.
>    Use a PAE enabled kernel.
> 
> But I only have 4Gb installed, so is this message wrong?

Yes. Its confusing 4Gb of memory with 4Gb of address space.
4Gb of RAM comes out as
[RAM][ISA][RAM....][PCI] {4GbLimit} [RAM]

Re: Dmesg: Use a PAE enabled kernel

[Andrew Walrond]

Alan Cox wrote:
> On Mon, 2003-03-03 at 16:24, Andrew Walrond wrote:
> 
>>  BIOS-e820: 0000000100000000 - 0000000140000000 (usable)
> 
> 
> This chunk wont get used
> 
> 

Ouch

That accounts for the missing Gb then. I think the warning needs to be 
more like "WARNING WARNING (WILL ROBINSON)! 1GB of very expensive ram 
won't be used unless you enable PAE!!!"

Just how slow is PAE then ?

Andrew

Re: Dmesg: Use a PAE enabled kernel

[Andrew Walrond]

Hmmm

free suggests I only have about 3Gb...

hal3 root # free -m
         total       used       free     shared    buffers     cached
Mem:     3029       1843       1186          0          0       1696

I assume this means the hole is bigger than 256Mb? :(

Re: Dmesg: Use a PAE enabled kernel

[Andrew Walrond]

The dump looks like this:

BIOS-provided physical RAM map:
  BIOS-e820: 0000000000000000 - 00000000000a0000 (usable)
  BIOS-e820: 00000000000f0000 - 0000000000100000 (reserved)
  BIOS-e820: 0000000000100000 - 00000000bfffa000 (usable)
  BIOS-e820: 00000000bfffa000 - 00000000bffff000 (ACPI data)
  BIOS-e820: 00000000bffff000 - 00000000c0000000 (ACPI NVS)
  BIOS-e820: 00000000fec00000 - 00000000fec01000 (reserved)
  BIOS-e820: 00000000fee00000 - 00000000fee01000 (reserved)
  BIOS-e820: 00000000ffff0000 - 0000000100000000 (reserved)
  BIOS-e820: 0000000100000000 - 0000000140000000 (usable)
Warning only 4GB will be used.
Use a PAE enabled kernel.
3200MB HIGHMEM available.
896MB LOWMEM available.

So you are saying that not all the 4Gb of ram will get mapped/used 
(specifically, everything not marked 'usable') ?

Can you quantify the performance degredation of a PAE enabled kernel?

Andrew Walrond

Re: Dmesg: Use a PAE enabled kernel

[Martin J. Bligh]

> During bootup I see
> 
>    Warning only 4GB will be used.
>    Use a PAE enabled kernel.
> 
> But I only have 4Gb installed, so is this message wrong?

Publish the dump of the e820 map you get at the start of boot.
You'll probably find that the 4Gb of RAM is spread over a range of
physical addresses > 4Gb in size (for a start, there's a PCI window
from 3.75 to 4 Gb). If there's no other bigger holes, probably it's
faster if you just let it waste that 256Mb.

M.

Dmesg: Use a PAE enabled kernel

[Andrew Walrond]

During bootup I see

   Warning only 4GB will be used.
   Use a PAE enabled kernel.

But I only have 4Gb installed, so is this message wrong?

Andrew Walrond