docs/data-sources/memory-counters.md - third_party/perfetto - Git at Google

 # Memory counters and events

 Perfetto allows to gather a number of memory events and counters on
 Android and Linux. These events come from kernel interfaces, both ftrace and
 /proc interfaces, and are of two types: polled counters and events pushed by
 the kernel in the ftrace buffer.

 ## Per-process polled counters

 The process stats data source allows to poll `/proc/<pid>/status` and
 `/proc/<pid>/oom_score_adj` at user-defined intervals.

 See [`man 5 proc`][man-proc] for their semantic.

 ### UI

 ![](/docs/images/proc_stat.png "UI showing trace data collected by process stats pollers")

 ### SQL

 ```sql
 select c.ts, c.value, t.name as counter_name, p.name as proc_name, p.pid
 from counter as c left join process_counter_track as t on c.track_id = t.id
 left join process as p using (upid)
 where t.name like 'mem.%'
 ```
 ts | counter_name | value_kb | proc_name | pid
 ---|--------------|----------|-----------|----
 261187015027350 | mem.virt | 1326464 | com.android.vending | 28815
 261187015027350 | mem.rss | 85592 | com.android.vending | 28815
 261187015027350 | mem.rss.anon | 36948 | com.android.vending | 28815
 261187015027350 | mem.rss.file | 46560 | com.android.vending | 28815
 261187015027350 | mem.swap | 6908 | com.android.vending | 28815
 261187015027350 | mem.rss.watermark | 102856 | com.android.vending | 28815
 261187090251420 | mem.virt | 1326464 | com.android.vending | 28815

 ### TraceConfig

 To collect process stat counters every X ms set `proc_stats_poll_ms = X` in
 your process stats config. X must be greater than 100ms to avoid excessive CPU
 usage. Details about the specific counters being collected can be found in the
 [ProcessStats reference](/docs/reference/trace-packet-proto.autogen#ProcessStats).

 ```protobuf
 data_sources: {
     config {
         name: "linux.process_stats"
         process_stats_config {
             scan_all_processes_on_start: true
             proc_stats_poll_ms: 1000
         }
     }
 }
 ```

 ## Per-process memory events (ftrace)

 ### rss_stat

 Recent versions of the Linux kernel allow to report ftrace events when the
 Resident Set Size (RSS) mm counters change. This is the same counter available
 in `/proc/pid/status` as `VmRSS`. The main advantage of this event is that by
 being an event-driven push event it allows to detect very short memory usage
 bursts that would be otherwise undetectable by using /proc counters.

 Memory usage peaks of hundreds of MB can have dramatically negative impact on
 Android, even if they last only few ms, as they can cause mass low memory kills
 to reclaim memory.

 The kernel feature that supports this has been introduced in the Linux Kernel
 in [b3d1411b6] and later improved by [e4dcad20]. They are available in upstream
 since Linux v5.5-rc1. This patch has been backported in several Google Pixel
 kernels running Android 10 (Q).

 [b3d1411b6]: https://github.com/torvalds/linux/commit/b3d1411b6726ea6930222f8f12587d89762477c6
 [e4dcad20]: https://github.com/torvalds/linux/commit/e4dcad204d3a281be6f8573e0a82648a4ad84e69

 ### mm_event

 `mm_event` is an ftrace event that captures statistics about key memory events
 (a subset of the ones exposed by `/proc/vmstat`). Unlike RSS-stat counter
 updates, mm events are extremely high volume and tracing them individually would
 be unfeasible. `mm_event` instead reports only periodic histograms in the trace,
 reducing sensibly the overhead.

 `mm_event` is available only on some Google Pixel kernels running Android 10 (Q)
 and beyond.

 When `mm_event` is enabled, the following mm event types are recorded:

 * mem.mm.min_flt: Minor page faults
 * mem.mm.maj_flt: Major page faults
 * mem.mm.swp_flt: Page faults served by swapcache
 * mem.mm.read_io: Read page faults backed by I/O
 * mem.mm..compaction: Memory compaction events
 * mem.mm.reclaim: Memory reclaim events

 For each event type, the event records:

 * count: how many times the event happened since the previous event.
 * min_lat: the smallest latency (the duration of the mm event) recorded since
   the previous event.
 * max_lat: the highest latency recorded since the previous event.

 ### UI

 ![rss_stat and mm_event](/docs/images/rss_stat_and_mm_event.png)

 ### SQL

 At the SQL level, these events are imported and exposed in the same way as
 the corresponding polled events. This allows to collect both types of events
 (pushed and polled) and treat them uniformly in queries and scripts.

 ```sql
 select c.ts, c.value, t.name as counter_name, p.name as proc_name, p.pid
 from counter as c left join process_counter_track as t on c.track_id = t.id
 left join process as p using (upid)
 where t.name like 'mem.%'
 ```

 ts | value | counter_name | proc_name | pid
 ---|-------|--------------|-----------|----
 777227867975055 | 18358272 | mem.rss.anon | com.google.android.apps.safetyhub | 31386
 777227865995315 | 5 | mem.mm.min_flt.count | com.google.android.apps.safetyhub | 31386
 777227865995315 | 8 | mem.mm.min_flt.max_lat | com.google.android.apps.safetyhub | 31386
 777227865995315 | 4 | mem.mm.min_flt.avg_lat | com.google.android.apps.safetyhub | 31386
 777227865998023 | 3 | mem.mm.swp_flt.count | com.google.android.apps.safetyhub | 31386

 ### TraceConfig

 ```protobuf
 data_sources: {
     config {
         name: "linux.ftrace"
         ftrace_config {
             ftrace_events: "kmem/rss_stat"
             ftrace_events: "mm_event/mm_event_record"
         }
     }
 }

 # This is for getting Thread<>Process associations and full process names.
 data_sources: {
     config {
         name: "linux.process_stats"
     }
 }
 ```

 ## System-wide polled counters

 This data source allows periodic polling of system data from:

 - `/proc/stat`
 - `/proc/vmstat`
 - `/proc/meminfo`

 See [`man 5 proc`][man-proc] for their semantic.

 ### UI

 ![System Memory Counters](/docs/images/sys_stat_counters.png
 "Example of system memory counters in the UI")

 The polling period and specific counters to include in the trace can be set in the trace config.

 ### SQL

 ```sql
 select c.ts, t.name, c.value / 1024 as value_kb from counters as c left join counter_track as t on c.track_id = t.id
 ```

 ts | name | value_kb
 ---|------|---------
 775177736769834 | MemAvailable | 1708956
 775177736769834 | Buffers | 6208
 775177736769834 | Cached | 1352960
 775177736769834 | SwapCached | 8232
 775177736769834 | Active | 1021108
 775177736769834 | Inactive(file) | 351496

 ### TraceConfig

 The set of supported counters is available in the
 [TraceConfig reference](/docs/reference/trace-config-proto.autogen#SysStatsConfig)

 ```protobuf
 data_sources: {
     config {
         name: "linux.sys_stats"
         sys_stats_config {
             meminfo_period_ms: 1000
             meminfo_counters: MEMINFO_MEM_TOTAL
             meminfo_counters: MEMINFO_MEM_FREE
             meminfo_counters: MEMINFO_MEM_AVAILABLE

             vmstat_period_ms: 1000
             vmstat_counters: VMSTAT_NR_FREE_PAGES
             vmstat_counters: VMSTAT_NR_ALLOC_BATCH
             vmstat_counters: VMSTAT_NR_INACTIVE_ANON
             vmstat_counters: VMSTAT_NR_ACTIVE_ANON

             stat_period_ms: 1000
             stat_counters: STAT_CPU_TIMES
             stat_counters: STAT_FORK_COUNT
         }
     }
 }
 ```


 ## Low-memory Kills (LMK)

 #### Background

 The Android framework kills apps and services, especially background ones, to
 make room for newly opened apps when memory is needed. These are known as low
 memory kills (LMK).

 Note LMKs are not always the symptom of a performance problem. The rule of thumb
 is that the severity (as in: user perceived impact) is proportional to the state
 of the app being killed. The app state can be derived in a trace from the OOM
 adjustment score.

 A LMK of a foreground app or service is typically a big concern. This happens
 when the app that the user was using disappeared under their fingers, or their
 favorite music player service suddenly stopped playing music.

 A LMK of a cached app or service, instead, is frequently business-as-usual and
 in most cases won't be noticed by the end user until they try to go back to
 the app, which will then cold-start.

 The situation in between these extremes is more nuanced. LMKs of cached
 apps/service can be still problematic if it happens in storms (i.e. observing
 that most processes get LMK-ed in a short time frame) and are often the symptom
 of some component of the system causing memory spikes.

 ### lowmemorykiller vs lmkd

 #### In-kernel lowmemorykiller driver
 In Android, LMK used to be handled by an ad-hoc kernel-driver,
 Linux's [drivers/staging/android/lowmemorykiller.c](https://github.com/torvalds/linux/blob/v3.8/drivers/staging/android/lowmemorykiller.c).
 This driver uses to emit the ftrace event `lowmemorykiller/lowmemory_kill`
 in the trace.

 #### Userspace lmkd

 Android 9 introduced a userspace native daemon that took over the LMK
 responsibility: `lmkd`. Not all devices running Android 9 will
 necessarily use `lmkd` as the ultimate choice of in-kernel vs userspace is
 up to the phone manufacturer, their kernel version and kernel config.

 On Google Pixel phones, `lmkd`-side killing is used since Pixel 2 running
 Android 9.

 See https://source.android.com/devices/tech/perf/lmkd for details.

 `lmkd` emits a userspace atrace counter event called `kill_one_process`.

 #### Android LMK vs Linux oomkiller

 LMKs on Android, whether the old in-kernel `lowmemkiller` or the newer `lmkd`,
 use a completely different mechanism than the standard
 [Linux kernel's OOM Killer](https://linux-mm.org/OOM_Killer).
 Perfetto at the moment supports only Android LMK events (Both in-kernel and
 user-space) and does not support tracing of Linux kernel OOM Killer events.
 Linux OOMKiller events are still theoretically possible on Android but extremely
 unlikely to happen. If they happen, they are more likely the symptom of a
 mis-configured BSP.

 ### UI

 Newer userspace LMKs are available in the UI under the `lmkd` track
 in the form of a counter. The counter value is the PID of the killed process
 (in the example below, PID=27985).

 ![Userspace lmkd](/docs/images/lmk_lmkd.png "Example of a LMK caused by lmkd")

 TODO: we are working on a better UI support for LMKs.

 ### SQL

 Both newer lmkd and legacy kernel-driven lowmemorykiller events are normalized
 at import time and available under the `mem.lmk` key in the `instants` table.

 ```sql
 SELECT ts, process.name, process.pid
 FROM instant
 JOIN process_track ON instant.track_id = process_track.id
 JOIN process USING (upid)
 WHERE instant.name = 'mem.lmk'
 ```

 | ts | name | pid |
 |----|------|-----|
 | 442206415875043 | roid.apps.turbo | 27324 |
 | 442206446142234 | android.process.acore | 27683 |
 | 442206462090204 | com.google.process.gapps | 28198 |

 ### TraceConfig

 To enable tracing of low memory kills add the following options to trace config:

 ```protobuf
 data_sources: {
     config {
         name: "linux.ftrace"
         ftrace_config {
             # For old in-kernel events.
             ftrace_events: "lowmemorykiller/lowmemory_kill"

             # For new userspace lmkds.
             atrace_apps: "lmkd"

             # This is not strictly required but is useful to know the state
             # of the process (FG, cached, ...) before it got killed.
             ftrace_events: "oom/oom_score_adj_update"
         }
     }
 }
 ```

 ## {#oom-adj} App states and OOM adjustment score

 The Android app state can be inferred in a trace from the process
 `oom_score_adj`. The mapping is not 1:1, there are more states than
 oom_score_adj value groups and the `oom_score_adj` range for cached processes
 spans from 900 to 1000.

 The mapping can be inferred from the
 [ActivityManager's ProcessList sources](https://cs.android.com/android/platform/superproject/+/android10-release:frameworks/base/services/core/java/com/android/server/am/ProcessList.java;l=126)

 ```java
 // This is a process only hosting activities that are not visible,
 // so it can be killed without any disruption.
 static final int CACHED_APP_MAX_ADJ = 999;
 static final int CACHED_APP_MIN_ADJ = 900;

 // This is the oom_adj level that we allow to die first. This cannot be equal to
 // CACHED_APP_MAX_ADJ unless processes are actively being assigned an oom_score_adj of
 // CACHED_APP_MAX_ADJ.
 static final int CACHED_APP_LMK_FIRST_ADJ = 950;

 // The B list of SERVICE_ADJ -- these are the old and decrepit
 // services that aren't as shiny and interesting as the ones in the A list.
 static final int SERVICE_B_ADJ = 800;

 // This is the process of the previous application that the user was in.
 // This process is kept above other things, because it is very common to
 // switch back to the previous app.  This is important both for recent
 // task switch (toggling between the two top recent apps) as well as normal
 // UI flow such as clicking on a URI in the e-mail app to view in the browser,
 // and then pressing back to return to e-mail.
 static final int PREVIOUS_APP_ADJ = 700;

 // This is a process holding the home application -- we want to try
 // avoiding killing it, even if it would normally be in the background,
 // because the user interacts with it so much.
 static final int HOME_APP_ADJ = 600;

 // This is a process holding an application service -- killing it will not
 // have much of an impact as far as the user is concerned.
 static final int SERVICE_ADJ = 500;

 // This is a process with a heavy-weight application.  It is in the
 // background, but we want to try to avoid killing it.  Value set in
 // system/rootdir/init.rc on startup.
 static final int HEAVY_WEIGHT_APP_ADJ = 400;

 // This is a process currently hosting a backup operation.  Killing it
 // is not entirely fatal but is generally a bad idea.
 static final int BACKUP_APP_ADJ = 300;

 // This is a process bound by the system (or other app) that's more important than services but
 // not so perceptible that it affects the user immediately if killed.
 static final int PERCEPTIBLE_LOW_APP_ADJ = 250;

 // This is a process only hosting components that are perceptible to the
 // user, and we really want to avoid killing them, but they are not
 // immediately visible. An example is background music playback.
 static final int PERCEPTIBLE_APP_ADJ = 200;

 // This is a process only hosting activities that are visible to the
 // user, so we'd prefer they don't disappear.
 static final int VISIBLE_APP_ADJ = 100;

 // This is a process that was recently TOP and moved to FGS. Continue to treat it almost
 // like a foreground app for a while.
 // @see TOP_TO_FGS_GRACE_PERIOD
 static final int PERCEPTIBLE_RECENT_FOREGROUND_APP_ADJ = 50;

 // This is the process running the current foreground app.  We'd really
 // rather not kill it!
 static final int FOREGROUND_APP_ADJ = 0;

 // This is a process that the system or a persistent process has bound to,
 // and indicated it is important.
 static final int PERSISTENT_SERVICE_ADJ = -700;

 // This is a system persistent process, such as telephony.  Definitely
 // don't want to kill it, but doing so is not completely fatal.
 static final int PERSISTENT_PROC_ADJ = -800;

 // The system process runs at the default adjustment.
 static final int SYSTEM_ADJ = -900;

 // Special code for native processes that are not being managed by the system (so
 // don't have an oom adj assigned by the system).
 static final int NATIVE_ADJ = -1000;
 ```

 [man-proc]: https://manpages.debian.org/stretch/manpages/proc.5.en.html
	# Memory counters and events

	Perfetto allows to gather a number of memory events and counters on
	Android and Linux. These events come from kernel interfaces, both ftrace and
	/proc interfaces, and are of two types: polled counters and events pushed by
	the kernel in the ftrace buffer.

	## Per-process polled counters

	The process stats data source allows to poll `/proc/<pid>/status` and
	`/proc/<pid>/oom_score_adj` at user-defined intervals.

	See [`man 5 proc`][man-proc] for their semantic.

	### UI

	![](/docs/images/proc_stat.png "UI showing trace data collected by process stats pollers")

	### SQL

	```sql
	select c.ts, c.value, t.name as counter_name, p.name as proc_name, p.pid
	from counter as c left join process_counter_track as t on c.track_id = t.id
	left join process as p using (upid)
	where t.name like 'mem.%'
	```
	ts \| counter_name \| value_kb \| proc_name \| pid
	---\|--------------\|----------\|-----------\|----
	261187015027350 \| mem.virt \| 1326464 \| com.android.vending \| 28815
	261187015027350 \| mem.rss \| 85592 \| com.android.vending \| 28815
	261187015027350 \| mem.rss.anon \| 36948 \| com.android.vending \| 28815
	261187015027350 \| mem.rss.file \| 46560 \| com.android.vending \| 28815
	261187015027350 \| mem.swap \| 6908 \| com.android.vending \| 28815
	261187015027350 \| mem.rss.watermark \| 102856 \| com.android.vending \| 28815
	261187090251420 \| mem.virt \| 1326464 \| com.android.vending \| 28815

	### TraceConfig

	To collect process stat counters every X ms set `proc_stats_poll_ms = X` in
	your process stats config. X must be greater than 100ms to avoid excessive CPU
	usage. Details about the specific counters being collected can be found in the
	[ProcessStats reference](/docs/reference/trace-packet-proto.autogen#ProcessStats).

	```protobuf
	data_sources: {
	config {
	name: "linux.process_stats"
	process_stats_config {
	scan_all_processes_on_start: true
	proc_stats_poll_ms: 1000
	}
	}
	}
	```

	## Per-process memory events (ftrace)

	### rss_stat

	Recent versions of the Linux kernel allow to report ftrace events when the
	Resident Set Size (RSS) mm counters change. This is the same counter available
	in `/proc/pid/status` as `VmRSS`. The main advantage of this event is that by
	being an event-driven push event it allows to detect very short memory usage
	bursts that would be otherwise undetectable by using /proc counters.

	Memory usage peaks of hundreds of MB can have dramatically negative impact on
	Android, even if they last only few ms, as they can cause mass low memory kills
	to reclaim memory.

	The kernel feature that supports this has been introduced in the Linux Kernel
	in [b3d1411b6] and later improved by [e4dcad20]. They are available in upstream
	since Linux v5.5-rc1. This patch has been backported in several Google Pixel
	kernels running Android 10 (Q).

	[b3d1411b6]: https://github.com/torvalds/linux/commit/b3d1411b6726ea6930222f8f12587d89762477c6
	[e4dcad20]: https://github.com/torvalds/linux/commit/e4dcad204d3a281be6f8573e0a82648a4ad84e69

	### mm_event

	`mm_event` is an ftrace event that captures statistics about key memory events
	(a subset of the ones exposed by `/proc/vmstat`). Unlike RSS-stat counter
	updates, mm events are extremely high volume and tracing them individually would
	be unfeasible. `mm_event` instead reports only periodic histograms in the trace,
	reducing sensibly the overhead.

	`mm_event` is available only on some Google Pixel kernels running Android 10 (Q)
	and beyond.

	When `mm_event` is enabled, the following mm event types are recorded:

	* mem.mm.min_flt: Minor page faults
	* mem.mm.maj_flt: Major page faults
	* mem.mm.swp_flt: Page faults served by swapcache
	* mem.mm.read_io: Read page faults backed by I/O
	* mem.mm..compaction: Memory compaction events
	* mem.mm.reclaim: Memory reclaim events

	For each event type, the event records:

	* count: how many times the event happened since the previous event.
	* min_lat: the smallest latency (the duration of the mm event) recorded since
	the previous event.
	* max_lat: the highest latency recorded since the previous event.

	### UI

	![rss_stat and mm_event](/docs/images/rss_stat_and_mm_event.png)

	### SQL

	At the SQL level, these events are imported and exposed in the same way as
	the corresponding polled events. This allows to collect both types of events
	(pushed and polled) and treat them uniformly in queries and scripts.

	```sql
	select c.ts, c.value, t.name as counter_name, p.name as proc_name, p.pid
	from counter as c left join process_counter_track as t on c.track_id = t.id
	left join process as p using (upid)
	where t.name like 'mem.%'
	```

	ts \| value \| counter_name \| proc_name \| pid
	---\|-------\|--------------\|-----------\|----
	777227867975055 \| 18358272 \| mem.rss.anon \| com.google.android.apps.safetyhub \| 31386
	777227865995315 \| 5 \| mem.mm.min_flt.count \| com.google.android.apps.safetyhub \| 31386
	777227865995315 \| 8 \| mem.mm.min_flt.max_lat \| com.google.android.apps.safetyhub \| 31386
	777227865995315 \| 4 \| mem.mm.min_flt.avg_lat \| com.google.android.apps.safetyhub \| 31386
	777227865998023 \| 3 \| mem.mm.swp_flt.count \| com.google.android.apps.safetyhub \| 31386

	### TraceConfig

	```protobuf
	data_sources: {
	config {
	name: "linux.ftrace"
	ftrace_config {
	ftrace_events: "kmem/rss_stat"
	ftrace_events: "mm_event/mm_event_record"
	}
	}
	}

	# This is for getting Thread<>Process associations and full process names.
	data_sources: {
	config {
	name: "linux.process_stats"
	}
	}
	```

	## System-wide polled counters

	This data source allows periodic polling of system data from:

	- `/proc/stat`
	- `/proc/vmstat`
	- `/proc/meminfo`

	See [`man 5 proc`][man-proc] for their semantic.

	### UI

	![System Memory Counters](/docs/images/sys_stat_counters.png
	"Example of system memory counters in the UI")

	The polling period and specific counters to include in the trace can be set in the trace config.

	### SQL

	```sql
	select c.ts, t.name, c.value / 1024 as value_kb from counters as c left join counter_track as t on c.track_id = t.id
	```

	ts \| name \| value_kb
	---\|------\|---------
	775177736769834 \| MemAvailable \| 1708956
	775177736769834 \| Buffers \| 6208
	775177736769834 \| Cached \| 1352960
	775177736769834 \| SwapCached \| 8232
	775177736769834 \| Active \| 1021108
	775177736769834 \| Inactive(file) \| 351496

	### TraceConfig

	The set of supported counters is available in the
	[TraceConfig reference](/docs/reference/trace-config-proto.autogen#SysStatsConfig)

	```protobuf
	data_sources: {
	config {
	name: "linux.sys_stats"
	sys_stats_config {
	meminfo_period_ms: 1000
	meminfo_counters: MEMINFO_MEM_TOTAL
	meminfo_counters: MEMINFO_MEM_FREE
	meminfo_counters: MEMINFO_MEM_AVAILABLE

	vmstat_period_ms: 1000
	vmstat_counters: VMSTAT_NR_FREE_PAGES
	vmstat_counters: VMSTAT_NR_ALLOC_BATCH
	vmstat_counters: VMSTAT_NR_INACTIVE_ANON
	vmstat_counters: VMSTAT_NR_ACTIVE_ANON

	stat_period_ms: 1000
	stat_counters: STAT_CPU_TIMES
	stat_counters: STAT_FORK_COUNT
	}
	}
	}
	```



	## Low-memory Kills (LMK)

	#### Background

	The Android framework kills apps and services, especially background ones, to
	make room for newly opened apps when memory is needed. These are known as low
	memory kills (LMK).

	Note LMKs are not always the symptom of a performance problem. The rule of thumb
	is that the severity (as in: user perceived impact) is proportional to the state
	of the app being killed. The app state can be derived in a trace from the OOM
	adjustment score.

	A LMK of a foreground app or service is typically a big concern. This happens
	when the app that the user was using disappeared under their fingers, or their
	favorite music player service suddenly stopped playing music.

	A LMK of a cached app or service, instead, is frequently business-as-usual and
	in most cases won't be noticed by the end user until they try to go back to
	the app, which will then cold-start.

	The situation in between these extremes is more nuanced. LMKs of cached
	apps/service can be still problematic if it happens in storms (i.e. observing
	that most processes get LMK-ed in a short time frame) and are often the symptom
	of some component of the system causing memory spikes.

	### lowmemorykiller vs lmkd

	#### In-kernel lowmemorykiller driver
	In Android, LMK used to be handled by an ad-hoc kernel-driver,
	Linux's [drivers/staging/android/lowmemorykiller.c](https://github.com/torvalds/linux/blob/v3.8/drivers/staging/android/lowmemorykiller.c).
	This driver uses to emit the ftrace event `lowmemorykiller/lowmemory_kill`
	in the trace.

	#### Userspace lmkd

	Android 9 introduced a userspace native daemon that took over the LMK
	responsibility: `lmkd`. Not all devices running Android 9 will
	necessarily use `lmkd` as the ultimate choice of in-kernel vs userspace is
	up to the phone manufacturer, their kernel version and kernel config.

	On Google Pixel phones, `lmkd`-side killing is used since Pixel 2 running
	Android 9.

	See https://source.android.com/devices/tech/perf/lmkd for details.

	`lmkd` emits a userspace atrace counter event called `kill_one_process`.

	#### Android LMK vs Linux oomkiller

	LMKs on Android, whether the old in-kernel `lowmemkiller` or the newer `lmkd`,
	use a completely different mechanism than the standard
	[Linux kernel's OOM Killer](https://linux-mm.org/OOM_Killer).
	Perfetto at the moment supports only Android LMK events (Both in-kernel and
	user-space) and does not support tracing of Linux kernel OOM Killer events.
	Linux OOMKiller events are still theoretically possible on Android but extremely
	unlikely to happen. If they happen, they are more likely the symptom of a
	mis-configured BSP.

	### UI

	Newer userspace LMKs are available in the UI under the `lmkd` track
	in the form of a counter. The counter value is the PID of the killed process
	(in the example below, PID=27985).

	![Userspace lmkd](/docs/images/lmk_lmkd.png "Example of a LMK caused by lmkd")

	TODO: we are working on a better UI support for LMKs.

	### SQL

	Both newer lmkd and legacy kernel-driven lowmemorykiller events are normalized
	at import time and available under the `mem.lmk` key in the `instants` table.

	```sql
	SELECT ts, process.name, process.pid
	FROM instant
	JOIN process_track ON instant.track_id = process_track.id
	JOIN process USING (upid)
	WHERE instant.name = 'mem.lmk'
	```

	\| ts \| name \| pid \|
	\|----\|------\|-----\|
	\| 442206415875043 \| roid.apps.turbo \| 27324 \|
	\| 442206446142234 \| android.process.acore \| 27683 \|
	\| 442206462090204 \| com.google.process.gapps \| 28198 \|

	### TraceConfig

	To enable tracing of low memory kills add the following options to trace config:

	```protobuf
	data_sources: {
	config {
	name: "linux.ftrace"
	ftrace_config {
	# For old in-kernel events.
	ftrace_events: "lowmemorykiller/lowmemory_kill"

	# For new userspace lmkds.
	atrace_apps: "lmkd"

	# This is not strictly required but is useful to know the state
	# of the process (FG, cached, ...) before it got killed.
	ftrace_events: "oom/oom_score_adj_update"
	}
	}
	}
	```

	## {#oom-adj} App states and OOM adjustment score

	The Android app state can be inferred in a trace from the process
	`oom_score_adj`. The mapping is not 1:1, there are more states than
	oom_score_adj value groups and the `oom_score_adj` range for cached processes
	spans from 900 to 1000.

	The mapping can be inferred from the
	[ActivityManager's ProcessList sources](https://cs.android.com/android/platform/superproject/+/android10-release:frameworks/base/services/core/java/com/android/server/am/ProcessList.java;l=126)

	```java
	// This is a process only hosting activities that are not visible,
	// so it can be killed without any disruption.
	static final int CACHED_APP_MAX_ADJ = 999;
	static final int CACHED_APP_MIN_ADJ = 900;

	// This is the oom_adj level that we allow to die first. This cannot be equal to
	// CACHED_APP_MAX_ADJ unless processes are actively being assigned an oom_score_adj of
	// CACHED_APP_MAX_ADJ.
	static final int CACHED_APP_LMK_FIRST_ADJ = 950;

	// The B list of SERVICE_ADJ -- these are the old and decrepit
	// services that aren't as shiny and interesting as the ones in the A list.
	static final int SERVICE_B_ADJ = 800;

	// This is the process of the previous application that the user was in.
	// This process is kept above other things, because it is very common to
	// switch back to the previous app. This is important both for recent
	// task switch (toggling between the two top recent apps) as well as normal
	// UI flow such as clicking on a URI in the e-mail app to view in the browser,
	// and then pressing back to return to e-mail.
	static final int PREVIOUS_APP_ADJ = 700;

	// This is a process holding the home application -- we want to try
	// avoiding killing it, even if it would normally be in the background,
	// because the user interacts with it so much.
	static final int HOME_APP_ADJ = 600;

	// This is a process holding an application service -- killing it will not
	// have much of an impact as far as the user is concerned.
	static final int SERVICE_ADJ = 500;

	// This is a process with a heavy-weight application. It is in the
	// background, but we want to try to avoid killing it. Value set in
	// system/rootdir/init.rc on startup.
	static final int HEAVY_WEIGHT_APP_ADJ = 400;

	// This is a process currently hosting a backup operation. Killing it
	// is not entirely fatal but is generally a bad idea.
	static final int BACKUP_APP_ADJ = 300;

	// This is a process bound by the system (or other app) that's more important than services but
	// not so perceptible that it affects the user immediately if killed.
	static final int PERCEPTIBLE_LOW_APP_ADJ = 250;

	// This is a process only hosting components that are perceptible to the
	// user, and we really want to avoid killing them, but they are not
	// immediately visible. An example is background music playback.
	static final int PERCEPTIBLE_APP_ADJ = 200;

	// This is a process only hosting activities that are visible to the
	// user, so we'd prefer they don't disappear.
	static final int VISIBLE_APP_ADJ = 100;

	// This is a process that was recently TOP and moved to FGS. Continue to treat it almost
	// like a foreground app for a while.
	// @see TOP_TO_FGS_GRACE_PERIOD
	static final int PERCEPTIBLE_RECENT_FOREGROUND_APP_ADJ = 50;

	// This is the process running the current foreground app. We'd really
	// rather not kill it!
	static final int FOREGROUND_APP_ADJ = 0;

	// This is a process that the system or a persistent process has bound to,
	// and indicated it is important.
	static final int PERSISTENT_SERVICE_ADJ = -700;

	// This is a system persistent process, such as telephony. Definitely
	// don't want to kill it, but doing so is not completely fatal.
	static final int PERSISTENT_PROC_ADJ = -800;

	// The system process runs at the default adjustment.
	static final int SYSTEM_ADJ = -900;

	// Special code for native processes that are not being managed by the system (so
	// don't have an oom adj assigned by the system).
	static final int NATIVE_ADJ = -1000;
	```

	[man-proc]: https://manpages.debian.org/stretch/manpages/proc.5.en.html