Re: [Xen-users] 3.0.0-rc2: Xen: High amount of kernel "reserved"memory, about 33% in 256MB DOMU

[Konrad Rzeszutek Wilk]

On Mon, Jun 13, 2011 at 10:50:03PM +0200, Tobias Diedrich wrote:
> Hi,
> 
> another issue I'm seeing with 3.0-rc2 and Xen is that there is an
> unexpectedly high amount of kernel reserved memory.

> 
> I suspect that Linux allocates page table entries and corresponding
> data structures for the whole 6GB areas of the provided 'physical
> RAM map' even though it has rather big unusable holes in it.

Can you run it with 'memblock=debug debug loglevel=8 initcall_debug'?
It should tell you where it tries (and for much space) the pagetables.
> 
> [    0.000000] BIOS-provided physical RAM map:
> [    0.000000]  Xen: 0000000000000000 - 000000000009f000 (usable)
> [    0.000000]  Xen: 000000000009f000 - 0000000000100000 (reserved)
> [    0.000000]  Xen: 0000000000100000 - 0000000010000000 (usable)
> [    0.000000]  Xen: 0000000010000000 - 000000007fef0000 (unusable)
> [    0.000000]  Xen: 000000007fef0000 - 000000007fef3000 (ACPI NVS)
> [    0.000000]  Xen: 000000007fef3000 - 000000007ff00000 (ACPI data)
> [    0.000000]  Xen: 00000000fec00000 - 0000000100000000 (reserved)
> [    0.000000]  Xen: 0000000100000000 - 000000016fef0000 (usable)
> 
> 
> On DOMUs I can 'fix' this by adding 'memmap=0x800000$0x100000000' to
> the kernel command line, which changes 
> 
> [    0.000000] BIOS-provided physical RAM map:
> [    0.000000]  Xen: 0000000000000000 - 00000000000a0000 (usable)
> [    0.000000]  Xen: 00000000000a0000 - 0000000000100000 (reserved)
> [    0.000000]  Xen: 0000000000100000 - 0000000010000000 (usable)
> [    0.000000]  Xen: 0000000100000000 - 0000000100800000 (usable)
> [...]
> [    0.000000] Memory: 176356k/4202496k available (6096k kernel code, 3932608k absent, 93532k reserved, 4785k data, 572k init)
> 
> to
> 
> [    0.000000] BIOS-provided physical RAM map:
> [    0.000000]  Xen: 0000000000000000 - 00000000000a0000 (usable)
> [    0.000000]  Xen: 00000000000a0000 - 0000000000100000 (reserved)
> [    0.000000]  Xen: 0000000000100000 - 0000000010000000 (usable)
> [    0.000000]  Xen: 0000000100000000 - 0000000100800000 (usable)
> [...]
> [    0.000000] user-defined physical RAM map:
> [    0.000000]  user: 0000000000000000 - 00000000000a0000 (usable)
> [    0.000000]  user: 00000000000a0000 - 0000000000100000 (reserved)
> [    0.000000]  user: 0000000000100000 - 0000000010000000 (usable)
> [...]
> [    0.000000]  user: 0000000100000000 - 0000000100800000 (reserved)

What happens if you have 'maxmem' value equal to 'memory' in your guest config?

> [    0.000000] Memory: 244212k/262144k available (6096k kernel code, 448k absent, 17484k reserved, 4785k data, 572k init)
> 
> With 66MB of usable memory out of 256MB recovered and a reasonable
> 93% of memory usable for userspace instead of just 67%.
> 
> 
> I also see this on my 256MB DOM0:
> Memory: 146536k/6028224k available (6122k kernel code, 3932612k absent, 1949076k reserved, 4761k data, 576k init)
> Only 143MB out of 256MB allocated for DOM0 is usable for userspace.

> 
> Regards,
> 
> -- 
> Tobias						PGP: http://8ef7ddba.uguu.de
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