当我尝试启动虚拟机（分配了 1GB）时，尽管系统上有足够的内存（16GB），但我遇到了内存分配问题：
error: Failed to start domain exampleVM
error: internal error: qemu unexpectedly closed the monitor: 2023-09-10T11:49:20.853205Z qemu-system-x86_64: cannot set up guest memory 'pc.ram': Cannot allocate memory
我有三个虚拟机正在运行，总共分配了 6GB：
那时我还没有分配交换空间（现在已经添加了交换空间以使系统正常运行），但为什么这是问题所在呢？系统难道不应该交换内存中的页面来为虚拟机的内存腾出空间吗？
请解释一下这个问题，因为这对我来说相当神秘。谷歌只给出了不充分的答案和可能。
附言
% free -g
total used free shared buff/cache available
Mem: 15 6 2 5 7 3
Swap: 7 3 4
% ps_mem.py
Private + Shared = RAM used Program
44.0 KiB + 82.0 KiB = 126.0 KiB dnsmasq
120.0 KiB + 79.0 KiB = 199.0 KiB xrdp
292.0 KiB + 81.0 KiB = 373.0 KiB cron
280.0 KiB + 115.0 KiB = 395.0 KiB agetty (2)
540.0 KiB + 100.5 KiB = 640.5 KiB auditd
600.0 KiB + 58.5 KiB = 658.5 KiB mdadm
868.0 KiB + 150.5 KiB = 1.0 MiB xrdp-sesman
996.0 KiB + 157.5 KiB = 1.1 MiB exim4
200.0 KiB + 1.0 MiB = 1.2 MiB ha_logd (2)
984.0 KiB + 333.5 KiB = 1.3 MiB systemd-timesyncd
1.2 MiB + 368.0 KiB = 1.5 MiB systemd-logind
1.0 MiB + 564.0 KiB = 1.5 MiB upowerd
1.6 MiB + 154.0 KiB = 1.7 MiB dbus-daemon
1.8 MiB + 154.0 KiB = 1.9 MiB smartd
1.5 MiB + 462.0 KiB = 1.9 MiB unattended-upgr
2.0 MiB + 94.5 KiB = 2.1 MiB systemd-udevd
1.6 MiB + 583.5 KiB = 2.2 MiB virtlogd
1.7 MiB + 559.5 KiB = 2.2 MiB polkitd
1.9 MiB + 314.0 KiB = 2.2 MiB systemd-journald
2.2 MiB + 126.5 KiB = 2.3 MiB rsyslogd
1.5 MiB + 854.5 KiB = 2.3 MiB ssh
1.4 MiB + 933.5 KiB = 2.4 MiB (sd-pam)
3.3 MiB + 617.5 KiB = 3.9 MiB udisksd
4.4 MiB + 1.2 MiB = 5.6 MiB tuned
4.2 MiB + 1.5 MiB = 5.7 MiB bash (2)
2.6 MiB + 3.5 MiB = 6.2 MiB sshd (3)
4.0 MiB + 3.2 MiB = 7.2 MiB systemd (2)
9.4 MiB + 1.2 MiB = 10.6 MiB libvirtd
13.6 MiB + 1.1 MiB = 14.7 MiB fail2ban-server
17.5 MiB + 300.5 KiB = 17.8 MiB zabbix_agent2
12.4 MiB + 40.5 MiB = 52.9 MiB heartbeat (6)
7.4 GiB + 6.2 MiB = 7.5 GiB qemu-system-x86_64 (5)
---------------------------------
7.6 GiB
=================================
% cat /proc/meminfo
MemTotal: 16298980 kB
MemFree: 2471024 kB
MemAvailable: 5498208 kB
Buffers: 2161672 kB
Cached: 3251996 kB
SwapCached: 10692 kB
Active: 8700828 kB
Inactive: 4551496 kB
Active(anon): 7763376 kB
Inactive(anon): 2282256 kB
Active(file): 937452 kB
Inactive(file): 2269240 kB
Unevictable: 59280 kB
Mlocked: 59280 kB
SwapTotal: 8388604 kB
SwapFree: 2057932 kB
Dirty: 0 kB
Writeback: 0 kB
AnonPages: 7888728 kB
Mapped: 83356 kB
Shmem: 2164944 kB
Slab: 236904 kB
SReclaimable: 144204 kB
SUnreclaim: 92700 kB
KernelStack: 4448 kB
PageTables: 23336 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 16538092 kB
Committed_AS: 19768952 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 0 kB
VmallocChunk: 0 kB
Percpu: 11904 kB
HardwareCorrupted: 0 kB
AnonHugePages: 7198720 kB
ShmemHugePages: 0 kB
ShmemPmdMapped: 0 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
Hugetlb: 0 kB
DirectMap4k: 3027424 kB
DirectMap2M: 13621248 kB