XEN服务器dom0内存可以在启动时限制，以便能够将更多内存保留给domU使用。
但是系统服务也可能随着负载增加而逐渐内存不足并并且存在内存碎片
以下XEN服务器dom0存在sys极高，负载远超过分配给dom0的CPU数量
top - 09:05:16 up 1014 days, 17:17, 2 users, load average: 28.10, 25.35, 26.06
Tasks: 483 total, 11 running, 466 sleeping, 0 stopped, 6 zombie
Cpu0 : 21.9%us, 73.8%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 4.4%si, 0.0%st
Cpu1 : 39.2%us, 56.7%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 4.1%si, 0.0%st
Cpu2 : 51.0%us, 42.8%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 6.3%si, 0.0%st
Cpu3 : 38.9%us, 58.1%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 3.0%si, 0.0%st
Cpu4 : 40.1%us, 59.6%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.3%si, 0.0%st
Cpu5 : 45.1%us, 50.5%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 4.4%si, 0.0%st
Cpu6 : 32.8%us, 60.4%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 6.8%si, 0.0%st
Cpu7 : 35.9%us, 60.8%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 3.3%si, 0.0%st
Mem: 16384000k total, 15644412k used, 739588k free, 185284k buffers
Swap: 0k total, 0k used, 0k free, 7101404k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
27209 root 20 0 2111m 315m 8788 S 114.0 2.0 59564:40 AAAAA
...
3871 root 20 0 1002m 88m 5952 S 39.1 0.6 233:17.71 BBBB
...
6637 root 20 0 9804 2084 240 R 33.2 0.0 159892:47 xenstored
27183 root 20 0 2110m 1.1g 10m S 25.2 6.9 5955:24 CCCC
检查服务发现除了xenstored在运行外，很多系统进程堆栈，显示处于sys_nanosleep状态
#cat /proc/27209/stack
[<ffffffff81078c7e>] hrtimer_nanosleep+0x6f/0xdf
[<ffffffff81078d47>] sys_nanosleep+0x59/0x6f
[<ffffffff81012d72>] system_call_fastpath+0x16/0x1b
[<ffffffffffffffff>] 0xffffffffffffffff
#cat /proc/3871/stack
[<ffffffff81078c7e>] hrtimer_nanosleep+0x6f/0xdf
[<ffffffff81078d47>] sys_nanosleep+0x59/0x6f
[<ffffffff81012d72>] system_call_fastpath+0x16/0x1b
[<ffffffffffffffff>] 0xffffffffffffffff
#cat /proc/6637/stack
[<ffffffff81052efd>] __cond_resched+0x2a/0x35
[<ffffffff8111ec1c>] do_select+0x3df/0x54f
[<ffffffff8111ef03>] core_sys_select+0x177/0x212
[<ffffffff8111f03a>] sys_select+0x9c/0xc4
[<ffffffff81012d72>] system_call_fastpath+0x16/0x1b
[<ffffffffffffffff>] 0xffffffffffffffff
#cat /proc/27183/stack
[<ffffffff81078c7e>] hrtimer_nanosleep+0x6f/0xdf
[<ffffffff81078d47>] sys_nanosleep+0x59/0x6f
[<ffffffff81012d72>] system_call_fastpath+0x16/0x1b
[<ffffffffffffffff>] 0xffffffffffffffff
检查当前dom0配置内存
#grep MemTotal /proc/meminfo
MemTotal: 16384000 kB
物理服务器实际内存可以从dmidecode查看
这台服务器我尝试了
drop_cache方法：
echo 3 > /proc/sys/vm/drop_caches
似乎负载满满降低了下来。按照我的想法，可能需要通过扩容dom0来解决 （不过目前还没有测试）
sudo xl mem-set 0 18000m