Strange calculation of free_memory

[Alexander Kobernik]

Hi everybody,

there is physical box with 32GB of RAM installed on it.

DomU domains aren't not launched yet. Here's output of 'xm list' command:

Name ID Mem VCPUs State Time(s)
Domain-0 0 512 8 r----- 107900.3

So, there's only Dom0 there. Here's output of 'free -m' command:

total used free shared buffers cached
Mem: 512 378 133 0 53 90
-/+ buffers/cache: 234 277
Swap: 2047 0 2047

Thus free/available memory (for DomUs) should be equal to something like:

32GB - 512MB = 32256MB = 33030144KB

However, according to Xen, there are about 31791MB of free memory only.

Here's output of 'xm info' command:

host : xxx
release : 2.6.18-92.1.6.el5xen
version : #1 SMP Wed Jun 25 14:13:10 EDT 2008
machine : x86_64
nr_cpus : 8
nr_nodes : 1
cores_per_socket : 4
threads_per_core : 1
cpu_mhz : 2660
hw_caps : bfebfbff:20100800:00000000:00000140:040ce3bd:00000000:00000001:00000000
virt_caps : hvm
total_memory : 32762
free_memory : 31791
node_to_cpu : node0:0-7
node_to_memory : node0:31791
xen_major : 3
xen_minor : 3
xen_extra : .0
xen_caps : xen-3.0-x86_64 xen-3.0-x86_32p hvm-3.0-x86_32 hvm-3.0-x86_32p hvm-3.0-x86_64
xen_scheduler : credit
xen_pagesize : 4096
platform_params : virt_start=0xffff800000000000
xen_changeset : unavailable
cc_compiler : gcc version 4.1.2 20071124 (Red Hat 4.1.2-42)
cc_compile_by : root
cc_compile_domain : xxx
cc_compile_date : Tue Sep 9 20:18:14 CEST 2008
xend_config_format : 4

Here's reduced output of 'xm top' command:

xentop - 12:51:59 Xen 3.3.0
1 domains: 1 running, 0 blocked, 0 paused, 0 crashed, 0 dying, 0 shutdown
Mem: 33549248k total, 995128k used, 32554120k free CPUs: 8 @ 2660MHz

NAME STATE CPU(sec) CPU(%) MEM(k) MEM(%) MAXMEM(k) MAXMEM(%)
Domain-0 -----r 107963 0.0 524288 1.6 no limit n/a


So, could someone explain who allocated/cached 465MB (32256MB - 31791MB) of RAM?

--
Alexander

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[Pasi Kärkkäinen]

On Fri, Nov 06, 2009 at 03:59:04PM +0300, Alexander Kobernik wrote:

>    Hi everybody,
>
>    there is physical box with 32GB of RAM installed on it.
>    DomU domains aren't not launched yet. Here's output of 'xm list' command:
>
>    Name ID Mem VCPUs State Time(s)
>    Domain-0 0 512 8 r----- 107900.3
>
>    So, there's only Dom0 there. Here's output of 'free -m' command:
>
>    total used free shared buffers cached
>    Mem: 512 378 133 0 53 90
>    -/+ buffers/cache: 234 277
>    Swap: 2047 0 2047
>
>    Thus free/available memory (for DomUs) should be equal to something like:
>
>    32GB - 512MB = 32256MB = 33030144KB
>
>    However, according to Xen, there are about 31791MB of free memory only.
>
>    Here's output of 'xm info' command:
>
>    host : xxx
>    release : 2.6.18-92.1.6.el5xen
>    version : #1 SMP Wed Jun 25 14:13:10 EDT 2008
>    machine : x86_64
>    nr_cpus : 8
>    nr_nodes : 1
>    cores_per_socket : 4
>    threads_per_core : 1
>    cpu_mhz : 2660
>    hw_caps :
>    bfebfbff:20100800:00000000:00000140:040ce3bd:00000000:00000001:00000000
>    virt_caps : hvm
>    total_memory : 32762
>    free_memory : 31791
>    node_to_cpu : node0:0-7
>    node_to_memory : node0:31791
>    xen_major : 3
>    xen_minor : 3
>    xen_extra : .0
>    xen_caps : xen-3.0-x86_64 xen-3.0-x86_32p hvm-3.0-x86_32 hvm-3.0-x86_32p
>    hvm-3.0-x86_64
>    xen_scheduler : credit
>    xen_pagesize : 4096
>    platform_params : virt_start=0xffff800000000000
>    xen_changeset : unavailable
>    cc_compiler : gcc version 4.1.2 20071124 (Red Hat 4.1.2-42)
>    cc_compile_by : root
>    cc_compile_domain : xxx
>    cc_compile_date : Tue Sep 9 20:18:14 CEST 2008
>    xend_config_format : 4
>
>    Here's reduced output of 'xm top' command:
>
>    xentop - 12:51:59 Xen 3.3.0
>    1 domains: 1 running, 0 blocked, 0 paused, 0 crashed, 0 dying, 0 shutdown
>    Mem: 33549248k total, 995128k used, 32554120k free CPUs: 8 @ 2660MHz
>
>    NAME STATE CPU(sec) CPU(%) MEM(k) MEM(%) MAXMEM(k) MAXMEM(%)
>    Domain-0 -----r 107963 0.0 524288 1.6 no limit n/a
>
>    So, could someone explain who allocated/cached 465MB (32256MB - 31791MB)
>    of RAM?
>

Xen hypervisor uses some memory, but not that much. The rest goes for
some DMA memory reservations etc.

-- Pasi


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