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Relic Solution: How Infrastructure Calculates the Percentage of Available Memory


If you have ever wondered how New Relic calculates the percentage of available memory, we’ve got you covered. Here’s how it works:

The infrastructure agent uses cat /proc/meminfo to retrieve memory metrics from your server.
Here’s an example of what it returns:

$ cat /proc/meminfo
MemTotal:         997820 kB
MemFree:           74028 kB
MemAvailable:     337608 kB
Buffers:           17720 kB
Cached:           366776 kB
SwapCached:         9500 kB
Active:           358876 kB
Inactive:         305120 kB
Active(anon):     119956 kB
Inactive(anon):   172352 kB
Active(file):     238920 kB
Inactive(file):   132768 kB
Unevictable:          32 kB
Mlocked:              32 kB
SwapTotal:       1046524 kB
SwapFree:         428956 kB
Dirty:              1452 kB
Writeback:             0 kB

With this information, the agent calculates memory metrics in 2 possible ways, depending on the version of the kernel.
On kernels version 3.14+, a new metric was introduced (MemAvailable) which provides an estimate of how much memory is available for starting new applications, without swapping.
If this metric is available the agent will calculate memory % as follows:

UsedMemory     = (MemTotal - MemAvailable)
UsedPercentage = (MemTotal - MemAvailable) / MemTotal * 100.0

On older kernels prior to 3.14, we take into account the amount of available memory plus buffers and cached:

AvailableMemory = MemFree + MemBuffers + MemCached

and the same calculation is then applied:

UsedMemory     = (MemTotal - AvailableMemory)
UsedPercentage = (MemTotal - AvailableMemory) / MemTotal * 100.0

The reason for this is that Linux considers Buffers and Cache as written memory, however it still considers them as available, so New Relic does too. They are considered available because the system should store frequently accessed data as buffers/cache so that it doesn’t have to retrieve them from the hard drive every time they are needed. However, if a process needs more memory, all of these memory types are promptly surrendered to the process just as if they were not written.

For more information on this, please see this external documentation:

What about my Windows Server???

In windows the Infrastructure agent does a direct system call to the exported method GlobalMemoryStatusEx from kernel32.dll

It then reads ullTotalPhys and ullAvailPhys for Total and Available memory values respectively as well as dwMemoryLoad for the percentage of used memory

For more information on check this MSDN article:

I hope that helps to explain how memory usage is calculated in New Relic Infrastructure!