High and low memory explanation
Article ID: 168029
Updated On:
Products
XOS
Issue/Introduction
High memory and low memory explanation.When running show module status command from CLI or cat /proc/meminfo command on the module, the output lists values for High and Low memory (output cropped):
total: used: free: shared: buffers: cached:
Mem: 1049051136 765775872 283275264 0 158441472 376979456
Swap: 1036115968 0 1036115968
MemTotal: 1024464 kB
MemFree: 276636 kB
MemShared: 0 kB
Buffers: 154728 kB
Cached: 368144 kB
...
HighTotal: 131072 kB
HighFree: 3064 kB
LowTotal: 893392 kB
LowFree: 273572 kB
Mem: 1049051136 765775872 283275264 0 158441472 376979456
Swap: 1036115968 0 1036115968
MemTotal: 1024464 kB
MemFree: 276636 kB
MemShared: 0 kB
Buffers: 154728 kB
Cached: 368144 kB
...
HighTotal: 131072 kB
HighFree: 3064 kB
LowTotal: 893392 kB
LowFree: 273572 kB
This article attempts to clarify Low and High memory details.
Cause
On a 32-bit Linux based operating system, CPU can address maximum of 4GB of memory. The memory is further divided in Low memory (or Normal memory), which is directly mapped to kernel's part of the address space and High memory, which has no direct kernel mapping. In other words:
High memory normally starts above 896MB. However on a Blue Coat X-Series chassis, the thresholds are different.
- The kernel itself (including its active modules, e.g. Check Point kernel modules) can only make use of LOW memory.
- User processes on the system (anything that is not kernel itself) can potentially make use of LOW and HIGH memory.
Because of this Low memory limitation OoM (Out of Memory killer) can be invoked even if there is a plenty of free memory in total. This situation occurs when the Low memory is exhausted and kernel needs to allocate more memory. However, it is a highly unusual situation to have a lot of high memory free but run out of low memory. More common is to see high AND low memory heading towards zero.
Because of this Low memory limitation OoM (Out of Memory killer) can be invoked even if there is a plenty of free memory in total. This situation occurs when the Low memory is exhausted and kernel needs to allocate more memory. However, it is a highly unusual situation to have a lot of high memory free but run out of low memory. More common is to see high AND low memory heading towards zero.
High memory normally starts above 896MB. However on a Blue Coat X-Series chassis, the thresholds are different.
Resolution
Please note that on a 64-bit OS, because there is more than enough virtual memory address space, Low memory equals Total memory. When running show module status command from CLI, Low and High memory values are not present.
When running cat /proc/meminfo command on the module, HighTotal and HighFree will always be zero:
Slot 14:
SDRAM 1 Size 1048576(KB)
SDRAM 2 Size 1048576(KB)
SDRAM 3 Size 1048576(KB)
SDRAM 4 Size 1048576(KB)
SDRAM Total Size 4194304(KB)
Reserved Memory 602536(KB)
Total Memory 3591768(KB)
Used Memory 453144(KB)
Free Memory 3138624(KB)
Shared Memory 0(KB)
Buffers Memory 139340(KB)
Cached Memory 187200(KB)
Memory Utilization 3.52%
When running cat /proc/meminfo command on the module, HighTotal and HighFree will always be zero:
# cat /proc/meminfo
MemTotal: 3591768 kB
MemFree: 135968 kB
Buffers: 378312 kB
Cached: 1754432 kB
SwapCached: 68 kB
Active: 965532 kB
Inactive: 2080476 kB
HighTotal: 0 kB
HighFree: 0 kB
Workaround
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