Used memory should be based on operating system's internals (#2534)

Author: rogercoll

.chloggen/clarify_used_memory.yaml

@@ -0,0 +1,22 @@
+# Use this changelog template to create an entry for release notes.
+#
+# If your change doesn't affect end users you should instead start
+# your pull request title with [chore] or use the "Skip Changelog" label.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the area of concern in the attributes-registry, (e.g. http, cloud, db)
+component: system
+
+# A brief description of the change. Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Used memory should be based on operating system's internals
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+# The values here must be integers.
+issues: [2534]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:

docs/registry/attributes/system.md

@@ -69,7 +69,9 @@ Describes System Memory attributes
 | `buffers` | buffers | ![Development](https://img.shields.io/badge/-development-blue) |
 | `cached` | cached | ![Development](https://img.shields.io/badge/-development-blue) |
 | `free` | free | ![Development](https://img.shields.io/badge/-development-blue) |
-| `used` | used | ![Development](https://img.shields.io/badge/-development-blue) |
+| `used` | Actual used virtual memory in bytes. [1] | ![Development](https://img.shields.io/badge/-development-blue) |
+
+**[1]:** Calculation based on the operating system metrics. On Linux, this corresponds to "MemTotal - MemAvailable" from /proc/meminfo, which more accurately reflects memory in active use by applications compared to older formulas based on free, cached, and buffers. If MemAvailable is not available, a fallback to those older formulas may be used.
 
 ## System Paging Attributes
 

docs/system/system-metrics.md

@@ -272,10 +272,7 @@ This metric is [recommended][MetricRecommended].
 
 | Name     | Instrument Type | Unit (UCUM) | Description    | Stability | Entity Associations |
 | -------- | --------------- | ----------- | -------------- | --------- | ------ |
-| `system.memory.usage` | UpDownCounter | `By` | Reports memory in use by state. [1] | ![Development](https://img.shields.io/badge/-development-blue) | [`host`](/docs/registry/entities/host.md#host) |
-
-**[1]:** The sum over all `system.memory.state` values SHOULD equal the total memory
-available on the system, that is `system.memory.limit`.
+| `system.memory.usage` | UpDownCounter | `By` | Reports memory in use by state. | ![Development](https://img.shields.io/badge/-development-blue) | [`host`](/docs/registry/entities/host.md#host) |
 
 | Attribute  | Type | Description  | Examples  | [Requirement Level](https://opentelemetry.io/docs/specs/semconv/general/attribute-requirement-level/) | Stability |
 |---|---|---|---|---|---|
@@ -290,7 +287,9 @@ available on the system, that is `system.memory.limit`.
 | `buffers` | buffers | ![Development](https://img.shields.io/badge/-development-blue) |
 | `cached` | cached | ![Development](https://img.shields.io/badge/-development-blue) |
 | `free` | free | ![Development](https://img.shields.io/badge/-development-blue) |
-| `used` | used | ![Development](https://img.shields.io/badge/-development-blue) |
+| `used` | Actual used virtual memory in bytes. [1] | ![Development](https://img.shields.io/badge/-development-blue) |
+
+**[1]:** Calculation based on the operating system metrics. On Linux, this corresponds to "MemTotal - MemAvailable" from /proc/meminfo, which more accurately reflects memory in active use by applications compared to older formulas based on free, cached, and buffers. If MemAvailable is not available, a fallback to those older formulas may be used.
 
 <!-- markdownlint-restore -->
 <!-- prettier-ignore-end -->
@@ -310,9 +309,7 @@ This metric is [opt-in][MetricOptIn].
 
 | Name     | Instrument Type | Unit (UCUM) | Description    | Stability | Entity Associations |
 | -------- | --------------- | ----------- | -------------- | --------- | ------ |
-| `system.memory.limit` | UpDownCounter | `By` | Total memory available in the system. [1] | ![Development](https://img.shields.io/badge/-development-blue) | [`host`](/docs/registry/entities/host.md#host) |
-
-**[1]:** Its value SHOULD equal the sum of `system.memory.state` over all states.
+| `system.memory.limit` | UpDownCounter | `By` | Total virtual memory available in the system. | ![Development](https://img.shields.io/badge/-development-blue) | [`host`](/docs/registry/entities/host.md#host) |
 
 <!-- markdownlint-restore -->
 <!-- prettier-ignore-end -->
@@ -370,7 +367,9 @@ This metric is [recommended][MetricRecommended].
 | `buffers` | buffers | ![Development](https://img.shields.io/badge/-development-blue) |
 | `cached` | cached | ![Development](https://img.shields.io/badge/-development-blue) |
 | `free` | free | ![Development](https://img.shields.io/badge/-development-blue) |
-| `used` | used | ![Development](https://img.shields.io/badge/-development-blue) |
+| `used` | Actual used virtual memory in bytes. [1] | ![Development](https://img.shields.io/badge/-development-blue) |
+
+**[1]:** Calculation based on the operating system metrics. On Linux, this corresponds to "MemTotal - MemAvailable" from /proc/meminfo, which more accurately reflects memory in active use by applications compared to older formulas based on free, cached, and buffers. If MemAvailable is not available, a fallback to those older formulas may be used.
 
 <!-- markdownlint-restore -->
 <!-- prettier-ignore-end -->

model/system/metrics.yaml

@@ -105,9 +105,6 @@ groups:
         metric_value_type: int
     stability: development
     brief: "Reports memory in use by state."
-    note: |
-      The sum over all `system.memory.state` values SHOULD equal the total memory
-      available on the system, that is `system.memory.limit`.
     instrument: updowncounter
     unit: "By"
     attributes:
@@ -122,9 +119,7 @@ groups:
       code_generation:
         metric_value_type: int
     stability: development
-    brief: "Total memory available in the system."
-    note: |
-      Its value SHOULD equal the sum of `system.memory.state` over all states.
+    brief: "Total virtual memory available in the system."
     instrument: updowncounter
     unit: "By"
     attributes: []

model/system/registry.yaml

@@ -22,6 +22,13 @@ groups:
             - id: used
               value: 'used'
               stability: development
+              brief: Actual used virtual memory in bytes.
+              note: >
+                Calculation based on the operating system metrics.
+                On Linux, this corresponds to "MemTotal - MemAvailable" from /proc/meminfo, which
+                more accurately reflects memory in active use by applications compared to older
+                formulas based on free, cached, and buffers. If MemAvailable is not available,
+                a fallback to those older formulas may be used.
             - id: free
               value: 'free'
               stability: development