Document memory coordinator

base/memory/memory_coordinator_client.h

Index: base/memory/memory_coordinator_client.h
diff --git a/base/memory/memory_coordinator_client.h b/base/memory/memory_coordinator_client.h
index da7104a8d94395decb299f15e86aa79069ce2e32..d1ea2106bc9316c8826c7bcbba19d332c0a40056 100644
--- a/base/memory/memory_coordinator_client.h
+++ b/base/memory/memory_coordinator_client.h
@@ -9,6 +9,15 @@
 
 namespace base {
 
+// OVERVIEW:
+//
+// MemoryCoordinatorClient is an interface which a component can implement to
+// respond to memory state changes. Unlike MemoryPressureListener, this is a
+// stateful mechanism and clients that receive notifications only when memory

[chrisha, 2016/09/26 14:10:45]

clients receive

(remove "that")

[bashi, 2016/09/26 23:08:10]

Done.

+// states are changed. State transitions are throttled to avoid thrashing; the
+// exact throttling period is platform dependent, but will be at least 5-10
+// seconds.

[chrisha, 2016/09/26 14:10:45]

Mention that clients are expected to make changes in memory usage that persist
for the duration of the memory state.

[Ryan Sleevi, 2016/09/26 15:05:17]

Interesting. So to confirm - if a memory state is THROTTLED, for example, a
component should disable all caching until it receives notification of NORMAL,
correct?

[bashi, 2016/09/26 23:08:10]

Done.

+
 // MemoryState is an indicator that processes can use to guide their memory
 // allocation policies. For example, a process that receives the suspended
 // state can use that as as signal to drop memory caches.
@@ -24,12 +33,18 @@ enum class MemoryState {
 };
 
 // This is an interface for components which can respond to memory status
-// changes.
+// changes. An initial state is NORMAL. See MemoryCoordinatorClientRegistry for
+// threading guarantees and ownership management.
 class BASE_EXPORT MemoryCoordinatorClient {
  public:
   virtual ~MemoryCoordinatorClient() {}
 
-  // Called when memory state has changed.
+  // Called when memory state has changed. Any transition can occur except for
+  // UNKNOWN. General guidelines are:
+  //  * NORMAL:    Restore the default settings for memory allocation/usage if
+  //               it has changed.
+  //  * THROTTLED: Use smaller limits for memory allocations and caches.
+  //  * SUSPENDED: Purge memory.
   virtual void OnMemoryStateChange(MemoryState state) = 0;
 };