How to calculate used memory on Linux?

Question

Hi, wanted to get more information on how to get the real used memory value on a droplet with CentOS 7 x64.

I was reviewing how DigitalOcean calculates used memory in the monitoring graphs and they do it “used memory is calculated by subtracting free memory and memory used for caching from the total memory amount” from the information exposed in /proc/meminfo.

This is the correct way to calculate used memory on a Linux server?

I see other people calculating used memory as “dividing active memory by total memory amount” from the same information exposed in /proc/meminfo.

Any ideas on this? What do you think? As you can see I have little knowledge on this issue.

Thank you.

Answer 1

Shiroka February 7, 2020

Show answer This answer has been marked as resolved by Shiroka.

danielpcharrua February 7, 2020

Hi @Shiroka, for the red font you can use the Highlight button on the top panel when you are writing a message, beside Bold, Italics, lists, href buttons....

I’m still trying to fully understand memory usage on linux with the /proc/meminfo output.
Today I’ve found a linux kernel commit (MemAvailable) talking about memory available for applications.

MemAvailable:
An estimate of how much memory is available for starting new applications, without swapping. Calculated from MemFree, SReclaimable, the size of the file LRU lists, and the low watermarks in each zone.
The estimate takes into account that the system needs some page cache to function well, and that not all reclaimable slab will be reclaimable, due to items being in use. The impact of those factors will vary from system to system.

So by definition (if I don’t understand it wrong) I can use this value to know my server has available memory for running new applications.

Now I have to decide what value should I use for memory usage, if used memory or available memory. I’m trying to create a server monitor to see memory usage.

DigitalOcean uses used memory value on monitoring system.
If I see actual values on my server, it has 53% used memory (according to DigitalOcean monitoring), this is about 2Gb for my 4Gb memory. This idea gives me I have around 2Gb available to run new applications.

But on the other path, using the MemAvailable value I have 2.6Gb available for running new applications.

So here we go again, what value do you think I should use to monitor memory on my server?

Thank you.
Reply Report
- Shiroka February 7, 2020
  
  MemAvailable,shows the system’s available memory. Some applications will automatically adjust the amount of memory requested based on the system’s available memory. Therefore,a statistical value that records the amount of available memory is required.MemFree is not applicable because MemFree cannot represent all available memory.Some of the memory can be recycled although it has been used,such as cache / bufferand slab can be recycled, so this part of recyclable memory plus MemFree is the system’s available memory,that is, MemAvailable.
  
  DigitalOcean’s Droplets console maybe just shows the MemTotal so the real memory been using is 100 - MemTotal,it’s little larger than DigitalOcean because the monitor doesn’t calculate the recyclable memory.
  
  Kind regards.
  
  Reply Report

Answer 2

danielpcharrua February 7, 2020

Hi @Shiroka, for the red font you can use the Highlight button on the top panel when you are writing a message, beside Bold, Italics, lists, href buttons....

I’m still trying to fully understand memory usage on linux with the /proc/meminfo output.
Today I’ve found a linux kernel commit (MemAvailable) talking about memory available for applications.

MemAvailable:
An estimate of how much memory is available for starting new applications, without swapping. Calculated from MemFree, SReclaimable, the size of the file LRU lists, and the low watermarks in each zone.
The estimate takes into account that the system needs some page cache to function well, and that not all reclaimable slab will be reclaimable, due to items being in use. The impact of those factors will vary from system to system.

So by definition (if I don’t understand it wrong) I can use this value to know my server has available memory for running new applications.

Now I have to decide what value should I use for memory usage, if used memory or available memory. I’m trying to create a server monitor to see memory usage.

DigitalOcean uses used memory value on monitoring system.
If I see actual values on my server, it has 53% used memory (according to DigitalOcean monitoring), this is about 2Gb for my 4Gb memory. This idea gives me I have around 2Gb available to run new applications.

But on the other path, using the MemAvailable value I have 2.6Gb available for running new applications.

So here we go again, what value do you think I should use to monitor memory on my server?

Thank you.
Reply Report
- Shiroka February 7, 2020
  
  MemAvailable,shows the system’s available memory. Some applications will automatically adjust the amount of memory requested based on the system’s available memory. Therefore,a statistical value that records the amount of available memory is required.MemFree is not applicable because MemFree cannot represent all available memory.Some of the memory can be recycled although it has been used,such as cache / bufferand slab can be recycled, so this part of recyclable memory plus MemFree is the system’s available memory,that is, MemAvailable.
  
  DigitalOcean’s Droplets console maybe just shows the MemTotal so the real memory been using is 100 - MemTotal,it’s little larger than DigitalOcean because the monitor doesn’t calculate the recyclable memory.
  
  Kind regards.
  
  Reply Report

Answer 3

danielpcharrua February 7, 2020

This is the /proc/meminfo output:

MemTotal:        3880364 kB
MemFree:          222412 kB
MemAvailable:    2666848 kB
Buffers:              28 kB
Cached:          1536256 kB
SwapCached:            0 kB
Active:          1341356 kB
Inactive:         758104 kB
Active(anon):     654492 kB
Inactive(anon):    38632 kB
Active(file):     686864 kB
Inactive(file):   719472 kB

Reply Report

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