Hi,

I don't understand the balloon driver in Xen.

I'm running Xen 3:
xm info
host                   : <snip>
release                : 5.11
version                : snv_111b
machine                : i86pc
nr_cpus                : 2
nr_nodes               : 1
sockets_per_node       : 1
cores_per_socket       : 2
threads_per_core       : 1
cpu_mhz                : 3158
hw_caps                : 
bfebfbff:20100800:00000000:00000140:0408e3fd:00000000:00000001
total_memory           : 8063
free_memory            : 2867
xen_major              : 3
xen_minor              : 1
xen_extra              : .4-xvm
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          : Wed Apr 08 14:27:49 2009 -0700 15914:cd2d74aa8832
cc_compiler            : gcc version 3.4.3 (csl-sol210-3_4-20050802)
cc_compile_by          : xen-discuss
cc_compile_domain      : opensolaris.org
cc_compile_date        : Wed Apr  8 14:28:25 PDT 2009
xend_config_format     : 4



# uname -a
SunOS uklabxen02 5.11 snv_111b i86pc i386 i86xpv Solaris

I'm getting a lot of these:

-bash: fork: Resource temporarily unavailable

On a machine with about 8Gb memory:

prtconf
System Configuration:  Sun Microsystems  i86pc
Memory size: 7807 Megabytes

The domUs I have are:

xm list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0   903     2     r----- 2352956.9
lv1_de2                                      2   512     1     -b----  71825.3
lv1_dev                                     17   512     1     ------  73425.4
lv1_e22                                      5   512     1     -b----  71620.2
lv1_e22dr                                        512     1               122.3
lv1_e2e                                          512     1               137.5
lv1_e2edr                                   12   512     1     -b----  65642.1
lv1_sit                                          512     1             48536.7
sv1_de2                                      6   512     1     -b----  24920.9
sv1_dev                                          256     1             26022.0
sv1_e22                                      8   512     1     ------  25070.9

vmstat doesn't look happy at all, the sr value is very high:

# vmstat 3
 kthr      memory            page            disk          faults      cpu
 r b w   swap  free  re  mf pi po fr de sr lf lf lf lf   in   sy   cs us sy id
 0 0 59 1769324 73716 501 521 1 741 1867 124768 3989041 0 0 0 0 886 1508 4823 0 
19 80
 0 0 64 1809604 65896 421 536 0 952 2496 90960 9040216 0 0 0 0 828 1582 4985 0 
41 59
 0 0 64 1809532 66044 593 648 0 897 2363 66312 9349749 0 0 0 0 679 1130 2845 0 
42 58

I looked at this:

# mdb -k
Loading modules: [ unix genunix specfs dtrace mac cpu.generic xpv_uppc xpv_psm 
scsi_vhci zfs sd sockfs ip hook neti sctp arp usba uhci fctl md lofs audiosup 
fcip fcp random crypto logindmux ptm ufs nsmb sppp nfs ipc ]
> ::memstat
Page Summary                Pages                MB  %Tot
------------     ----------------  ----------------  ----
Kernel                     162288               633    8%
ZFS File Data                6281                24    0%
Anon                         1405                 5    0%
Exec and libs                  35                 0    0%
Page cache                     37                 0    0%
Free (cachelist)             9355                36    0%
Free (freelist)             49718               194    2%
Balloon                   1767211              6903   89%

Total                     1996330              7798
>

Which seems to tell me that the Balloon driver is taking up loads of space and 
possibly crowding out the dom0? 
I've been trying to understand the balloon driver but can't really find 
comprehensive doc on it. What I think I want to do is to shrink the amount of 
memory allocated to Balloon in the above mdb/memstat output. But as I said, I 
don't really understand the Balloon memory bit so I could be mistaken.

I did try to shrink the balloon size by stopping/undefining a VM but it had no 
impact on the balloon size reported by mdb::memstat, or on the forking errors I 
get. 

Any advice greatly appreciated. 

regards
Kristin.
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