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Old 01-09-2010, 11:15 AM
lnx
 
Default xen-only-3GB-ram

Hello
I would like to share my experience with ram issue on gentoo (1.12.13)
I am running 64bit xen fedora kernel 2.6.31-6
(http://wiki.xensource.com/xenwiki/XenParavirtOps)
Why? - because previous 2.6.31-r7 from
http://code.google.com/p/gentoo-xen-kernel/downloads/list was unstable and
also only 3GB ram were visible.

There are 4GB ram installed on server. With standard kernel (2.6.30-r5), I
am able to see it all. Because of this, I disqualify grub bug.
With xen kernel, I saw only 3GB.

xm dmesg:
(XEN) Xen-e820 RAM map:
(XEN) 0000000000000000 - 000000000009f000 (usable)
(XEN) 000000000009f000 - 00000000000a0000 (reserved)
(XEN) 00000000000f0000 - 0000000000100000 (reserved)
(XEN) 0000000000100000 - 00000000bbeb0000 (usable) <-----------
3GB
(XEN) 00000000bbeb0000 - 00000000bbee3000 (ACPI NVS)
(XEN) 00000000bbee3000 - 00000000bbef0000 (ACPI data)
(XEN) 00000000bbef0000 - 00000000bbf00000 (reserved)
(XEN) 00000000bc000000 - 00000000c0000000 (reserved)
(XEN) 00000000f0000000 - 00000000f4000000 (reserved)
(XEN) 00000000fec00000 - 0000000100000000 (reserved)
(XEN) 0000000100000000 - 0000000140000000 (usable) <--------- 1
GB
(XEN) System RAM: 4030MB (4127036kB)

I do not know how this is possible: 140000000 = 5120MB and only 4030MB are
installed.

In standard dmesg output (just "dmesg" in dom0, not "xm dmesg") I saw only
3GB address space.
I double checked kernel configuration and compared with standard kernel
config, however no dirrefence found (except xen options).


lnx2 linux-2.6.31-6-xen # grep -i mem .config | grep -v "#"
CONFIG_SHMEM=y
CONFIG_XEN_MAX_DOMAIN_MEMORY=32
CONFIG_ARCH_SPARSEMEM_DEFAULT=y
CONFIG_ARCH_SPARSEMEM_ENABLE=y
CONFIG_ARCH_SELECT_MEMORY_MODEL=y
CONFIG_SELECT_MEMORY_MODEL=y
CONFIG_SPARSEMEM_MANUAL=y
CONFIG_SPARSEMEM=y
CONFIG_HAVE_MEMORY_PRESENT=y
CONFIG_SPARSEMEM_EXTREME=y
CONFIG_SPARSEMEM_VMEMMAP_ENABLE=y
CONFIG_SPARSEMEM_VMEMMAP=y
CONFIG_X86_BOOTPARAM_MEMORY_CORRUPTION_CHECK=y
CONFIG_ARCH_ENABLE_MEMORY_HOTPLUG=y
CONFIG_INPUT_FF_MEMLESS=y
CONFIG_DEVKMEM=y
CONFIG_FIX_EARLYCON_MEM=y
CONFIG_FIRMWARE_MEMMAP=y
CONFIG_DEBUG_MEMORY_INIT=y
CONFIG_HAVE_ARCH_KMEMCHECK=y
CONFIG_ASYNC_MEMCPY=y
CONFIG_HAS_IOMEM=y


lnx2 linux-2.6.31-6-xen # grep -i ram .config | grep -v "#"
CONFIG_X86_TRAMPOLINE=y
CONFIG_SCHED_OMIT_FRAME_POINTER=y
CONFIG_X86_BOOTPARAM_MEMORY_CORRUPTION_CHECK=y
CONFIG_BLK_DEV_RAM=y
CONFIG_BLK_DEV_RAM_COUNT=16
CONFIG_BLK_DEV_RAM_SIZE=16384
CONFIG_NVRAM=y
CONFIG_FRAMEBUFFER_CONSOLE=y
CONFIG_FRAME_WARN=2048
CONFIG_ARCH_WANT_FRAME_POINTERS=y
CONFIG_FRAME_POINTER=y
lnx2 linux-2.6.31-6-xen #


Another symptom:

xm top:
=======
dom0 = 4GB

xm list:
========
dom0= 3GB

Once VPS was started, dom0 ram size was decreased. So only 3GB were
available for dom0 + VPS



As a workaround I added dom0_mem=512M to kernel line in grub.conf

title XEN-6-fedora
root (hd0,0)
kernel /boot/xen.gz dom0_mem=512M
module /boot/vmlinuz-2.6.31-6-xen-dom0 root=/dev/md0
acpi_enforce_resources=lax


xm top:
=======
dom0 = 512MB

xm list:
========
dom0= 512MB

Now, I can see only 512MB in dom0, however rest of memory (3,5GB) is
usable for VPS

I was searching on google over 2 months, but noone report this issue (some
users had turned off PAE on 32bit, some had wrongly configured BIOS).
So I hope that this mail will help to someone who have the same problem,
or that someone will find reason of this strangeness.


Thanks and regards

samox86
lnx@lnx.sk
 

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