前次修改crashkernel设置将可用内存提拔了至少160M,也提到可用内存与硬件内存还存在较大的差距，本文就来简朴分析另有哪些硬件内存被预先分配了。
1.重要的内存数据
1）硬件内存1024M，1048576kB
# dmidecode -t 6|moreHandle 0x0085, DMI type 6, 12 bytesMemory Module Information Socket Designation: RAM socket #0 Bank Connections: None Current Speed: Unknown Type: EDO DIMM Installed Size: 1024 MB (Single-bank Connection) Enabled Size: 1024 MB (Single-bank Connection) Error Status: OK2）可用内存959796 kB，937M
# more /proc/meminfo |grep MemTotalMemTotal: 959796 kB# free -m total used free shared buff/cache availableMem: 937 400 83 9 453 357Swap: 2099 232 18673）预分配内存123852kB，121M
# dmesg |grep -i mem|more[ 0.000000] Memory: 261120K/1048052K available (12293K kernel code, 5878K rwdata, 8380K rodata, 2552K init, 14256K bss, 123852K reserved, 0K cma-reserved)2.哪些资源预分配了内存:e820
在操纵体系开始管理内存之前，起首要获取物理内存的信息，好比共有多少物理地点是可用的? 有哪些物理地点是被ACPI（Advanced Configuration and Power Interface）数据使用? 这些信息从何而来？e820就是x86架构（包罗x86_64）上的操纵体系引导步调提供物理内存信息的功能。当哀求BIOS停止号15H，而且置操纵码AX=E820H的时间，BIOS就会向调用者陈诉可用的物理地点区间等信息，e820由此得名。
什么是E820?
E820是基于X86的盘算机体系的BIOS将内存映射陈诉给操纵体系或引导加载步调的便利工具。通过将AX寄存器设置为十六进制值E820，可以通过INT15H调用对其举行访问，陈诉哪些内存地点范围可用，哪些保存给BIOS使用。"BIOS-e820"通常是启动Linux内核陈诉的第一件事，可以通过dmesg命令看到它。
Usable：体现已经被映射到物理内存的物理地点，即可用地点段
Reserved：体现这些区间是没有被映射到任何地方，不能看成RAM来使用，但是Kernel可决定将该区间映射到其他地方，好比PCI装备。通过读取/proc/iomem可以查察物理地点空间的映射环境,可以知道这些reserved的空间，是怎样进一步分配给差别的装备来使用的。
ACPI data：体现映射到用来存放ACPI数据的RAM空间，操纵体系应该将ACPI Table读入到这个区间内。
ACPI NVS：体现映射到用来存放ACPI数据的非易失性存储空间，操纵体系不能使用。
Unusable：体现检测到发生错误的物理内存。
Usable# dmesg |grep -i mem|grep usable[ 0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009ebff] usable[ 0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000003fedffff] usable[ 0.000000] BIOS-e820: [mem 0x000000003ff00000-0x000000003fffffff] usable[ 0.000000] e820: update [mem 0x00000000-0x00000fff] usable ==> reserved[ 0.000000] e820: remove [mem 0x000a0000-0x000fffff] usablereserved# dmesg |grep -i mem|grep reserved[ 0.000000] BIOS-e820: [mem 0x000000000009ec00-0x000000000009ffff] reserved[ 0.000000] BIOS-e820: [mem 0x00000000000dc000-0x00000000000fffff] reserved[ 0.000000] BIOS-e820: [mem 0x00000000f0000000-0x00000000f7ffffff] reserved[ 0.000000] BIOS-e820: [mem 0x00000000fec00000-0x00000000fec0ffff] reserved[ 0.000000] BIOS-e820: [mem 0x00000000fee00000-0x00000000fee00fff] reserved[ 0.000000] BIOS-e820: [mem 0x00000000fffe0000-0x00000000ffffffff] reserved[ 0.000000] e820: update [mem 0x00000000-0x00000fff] usable ==> reserved[ 0.000000] Memory: 261120K/1048052K available (12293K kernel code, 5878K rwdata, 8380K rodata, 2552K init, 14256K bss, 123852K reserved, 0K cma-reserved)[ 0.020659] PCI: MMCONFIG at [mem 0xf0000000-0xf7ffffff] reserved in E820[ 0.168259] system 00:04: [mem 0xfed00000-0xfed003ff] has been reserved[ 0.169441] system 00:05: [mem 0xf0000000-0xf7ffffff] has been reserved[ 0.169442] system 00:05: [mem 0xfe800000-0xfe9fffff] has been reservedACPI data&&ACPI NVS
# dmesg |grep -i mem|grep -i acpi[ 0.000000] BIOS-e820: [mem 0x000000003fee0000-0x000000003fefefff] ACPI data[ 0.000000] BIOS-e820: [mem 0x000000003feff000-0x000000003fefffff] ACPI NVS[ 0.000000] ACPI: SRAT 0x000000003FEE9757 0008A8 (v02 VMWARE MEMPLUG 06040000 VMW 00000001)[ 0.000000] ACPI: Reserving FACP table memory at [mem 0x3fefee73-0x3fefef66][ 0.000000] ACPI: Reserving DSDT table memory at [mem 0x3feeaa08-0x3fefee72][ 0.000000] ACPI: Reserving FACS table memory at [mem 0x3fefffc0-0x3fefffff][ 0.000000] ACPI: Reserving FACS table memory at [mem 0x3fefffc0-0x3fefffff][ 0.000000] ACPI: Reserving BOOT table memory at [mem 0x3feea9e0-0x3feeaa07][ 0.000000] ACPI: Reserving APIC table memory at [mem 0x3feea29e-0x3feea9df][ 0.000000] ACPI: Reserving MCFG table memory at [mem 0x3feea262-0x3feea29d][ 0.000000] ACPI: Reserving SRAT table memory at [mem 0x3fee9757-0x3fee9ffe][ 0.000000] ACPI: Reserving HPET table memory at [mem 0x3fee971f-0x3fee9756][ 0.000000] ACPI: Reserving WAET table memory at [mem 0x3fee96f7-0x3fee971e][ 0.000000] ACPI: SRAT: Node 0 PXM 0 [mem 0x00000000-0x0009ffff][ 0.000000] ACPI: SRAT: Node 0 PXM 0 [mem 0x00100000-0x3fffffff][ 0.000000] ACPI: SRAT: Node 0 PXM 0 [mem 0x40000000-0xbfffffff] hotplug[ 0.018861] ACPI: PM: Registering ACPI NVS region [mem 0x3feff000-0x3fefffff] (4096 bytes)3.reserved分配环境
Reserved：是Kernel可决定将该区间映射到其他地方，好比PCI装备。通过读取/proc/iomem可以查察物理地点空间的映射环境,可以知道这些reserved的空间，是怎样进一步分配给差别的装备来使用的。
# more /proc/iomem |grep -i Reserved00000000-00000fff : Reserved #0~4095=40960009ec00-0009ffff : Reserved #650240~655359=5120000dc000-000fffff : Reserved #901120~1048575=147456 000f0000-000fffff : System ROM f0000000-f7ffffff : PCI MMCONFIG 0000 [bus 00-7f] f0000000-f7ffffff : Reserved #4026531840~4160749567=134217728 f0000000-f7ffffff : pnp 00:054026531840fec00000-fec0ffff : Reserved #4273995776~4274061311=65536 fec00000-fec003ff : IOAPIC 0fee00000-fee00fff : Local APIC fee00000-fee00fff : Reserved #4276092928~4276097023=4096fffe0000-ffffffff : Reserved #4294836224~4294967295=131071总计：134,575,104
涉及Kernel分配Reserved，后续再分析，尤其从对比RHEL/Rocky/AlmaLinux8动手。
参考URL
https://www.jianshu.com/p/fd7f374976a6 |
