MM Events - Historical Memory Statistics

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Devices that launch on Android 12 and higher utilize mm_events, a set of memory-related statistics that get captured periodically while a system experiences memory pressure. mm_events is integrated with perfetto tracing mechanisms, and because it’s activated only when memory pressure is detected, its added performance overhead is minimal. The statistics collection starts when the kernel's kswapd, direct reclaim, or compaction mechanisms get activated, and it stays active for a configurable period of time to capture statistics at regular intervals.

Instead of providing a one-time snapshot of the system memory state when a bug report is filed, mm_events shows a brief historical view of the memory statistics during periods of memory pressure. The captured statistics are listed in the following tables.

vmstat fields

nr_free_pagesnr_slab_reclaimable
nr_slab_unreclaimablenr_active_file
nr_inactive_filenr_active_anon
nr_inactive_anonworkingset_refault
workingset_activatenr_file_pages
pgpginpgpgout
pswpinpswpout
pgsteal_kswapd_dmapgsteal_kswapd_normal
pgsteal_kswapd_movablepgsteal_direct_dma
pgsteal_direct_normalpgsteal_direct_movable
pgscan_kswapd_dmapgscan_kswapd_normal
pgscan_kswapd_movablepgscan_direct_dma
pgscan_direct_normalpgscan_direct_movable
compact_migrate_scannedcompact_free_scanned

mm-related trace events

vmscan/mm_vmscan_kswapd_wakevmscan/mm_vmscan_kswapd_sleep
vmscan/mm_vmscan_direct_reclaim_beginvmscan/mm_vmscan_direct_reclaim_end
compaction/mm_compaction_begincompaction/mm_compaction_end

Analyzing mm_events data

If mm_events is enabled, the bug reports for the events that get captured soon after a device begins to experience high memory pressure provide the historical mm_events stats, in the form of a zipped report in FS/data/misc/perfetto-traces/bugreport/systrace.pftrace.

Both the vmstat data and ftrace events can be viewed for analysis using the Perfetto UI.

vmstat data

Upload the systrace.pftrace file to Perfetto UI to see the vmstat data graphed on a timeline as shown below.

vmstat data graphed on a timeline

Figure 1. Timeline of vmstat graphical data

ftrace events

The captured ftrace mm_events aren’t shown graphically on the timeline. To view them, click the Query SQL tab, located as shown in Figure 2.

Locate the Query SQL tab and click to view ftrace and mm_events captured and graphed

Figure 2. Click Query (SQL) to access

Enabling mm_events

To enable mm_events, set sysprop persist.mm_events.enabled=true from vendor init.rc.

The following are in place to mitigate the memory and CPU footprint of mm_events:

  • An mm-events ftrace instance uses a 4 KB buffer per CPU.
  • The kmem_activity trigger is rate-limited to once per minute.
  • Only 1 mm-events trace session can be active at any time.

Customization

mm_events uses a perfetto trace config file to specify which stats to capture during the tracing session.

You can provide a custom Perfetto trace config in /vendor/etc/mm_events.cfg. For a description of the available trace config fields, see the Perfetto Docs. For a sample trace configuration, see this mm_events.cfg example.

The important fields to include in your trace config to ensure it gets triggered by memory pressure are shown in the snippet below.

# Ensures only 1 tracing session with this key can be active
unique_session_name: "perfetto_mm_events_session"

# If a trace with bugreport_score > 0 is running,
# the captured data is made available in the zipped bugreport
# at FS/data/misc/perfetto-traces/bugreport/systrace.pftrace
bugreport_score: 100

trigger_config {
  trigger_mode: START_TRACING
  trigger_timeout_ms: 3600000   # 1 hour
  triggers {
    # kmem_activity trigger gets activated when memory pressure
    # is detected
    name: "kmem_activity"
    stop_delay_ms: 360000       # 6 mins
  }
}

In this configuration mm_events initiates the perfetto kmem_activity trigger and the Perfetto trace session begins capturing vm_stats and ftrace memory events until the end of the configured stop_delay_ms period, 36000 ms (6 minutes). The trigger timeout is set to a large value (in this case, 1 hour) and mm_events config is periodically rearmed to ensure that mm_events is always enabled. A bug report gets generated as a result, containing the type of data shown in Figure 1 and Figure 2.