1 files changed, 27 insertions, 33 deletions

diff --git a/src/mongo/db/snapshot_window_options.h b/src/mongo/db/snapshot_window_options.h
index 683c48c9253..93b3de975c9 100644
--- a/src/mongo/db/snapshot_window_options.h
+++ b/src/mongo/db/snapshot_window_options.h
@@ -36,9 +36,9 @@
 namespace mongo {
 
 /**
- * This is a collection of parameters that affect how much snapshot history the storage engine will
- * maintain to support point-in-time transactions (read or write). This is referred to as the
- * snapshot window.
+ * These are parameters that affect how much snapshot history the storage engine will keep to
+ * support point-in-time transactions (read or write). This is referred to as the snapshot window.
+ * The window is between the stable timestamp and the oldest timestamp.
  */
 struct SnapshotWindowParams {
 
@@ -59,53 +59,47 @@ struct SnapshotWindowParams {
     // Note that this is the history window we attempt to maintain, but our current system state may
     // not always reflect it: the window can only change as more writes come in, so it can take time
     // for the actual window size to catch up with a change. This value guides actions whenever the
-    // system goes to update the oldest_timestamp value.
+    // system goes to update the oldest_timestamp value (usually when the stable_timestamp is
+    // updated).
     AtomicWord<int> targetSnapshotHistoryWindowInSeconds{
         maxTargetSnapshotHistoryWindowInSeconds.load()};
 
-    // cachePressureThreshold (startup & runtime server paramter, range [0, 100]).
-    //
-    // Dictates what percentage of cache in use is considered too high. This setting helps preempt
-    // storage cache pressure immobilizing the system. Attempts to increase
-    // targetSnapshotHistoryWindowInSeconds will be ignored when the cache pressure reaches this
-    // threshold. Additionally, a periodic task will decrease targetSnapshotHistoryWindowInSeconds
-    // when cache pressure exceeds the threshold.
-    AtomicWord<int> cachePressureThreshold{50};
-
     // snapshotWindowMultiplicativeDecrease (startup & runtime server paramter, range (0,1)).
     //
-    // Controls by what multiplier the target snapshot history window setting is decreased when
-    // cache pressure becomes too high, per the cachePressureThreshold setting.
+    // Controls by what multiplier the target snapshot history window size setting is decreased
+    // when there is cache pressure.
     AtomicDouble snapshotWindowMultiplicativeDecrease{0.75};
 
     // snapshotWindowAdditiveIncreaseSeconds (startup & runtime server paramter, range 1+).
     //
-    // Controls by how much the target snapshot history window setting is increased when cache
-    // pressure is OK, per cachePressureThreshold, and we need to service older snapshots for global
-    // point-in-time reads.
-    AtomicWord<int> snapshotWindowAdditiveIncreaseSeconds{2};
+    // Controls by how much the target snapshot history window size setting is increased when we
+    // need to service older snapshots for global point-in-time reads.
+    AtomicWord<int> snapshotWindowAdditiveIncreaseSeconds{1};
 
     // minMillisBetweenSnapshotWindowInc (startup & runtime server paramter, range 0+).
-    // minMillisBetweenSnapshotWindowDec (startup & runtime server paramter, range 0+).
     //
-    // Controls how often attempting to increase/decrease the target snapshot window will have an
-    // effect. Multiple callers within minMillisBetweenSnapshotWindowInc will have the same effect
-    // as one. This protects the system because it takes time for the target snapshot window to
-    // affect the actual storage engine snapshot window. The stable timestamp must move forward for
-    // the window between it and oldest timestamp to grow or shrink.
+    // Controls how often attempting to increase the target snapshot window will have an effect.
+    // Multiple callers within minMillisBetweenSnapshotWindowInc will have the same effect as one.
+    // This protects the system because it takes time for the target snapshot window to affect the
+    // actual storage engine snapshot window. The stable timestamp must move forward for the window
+    // between it and oldest timestamp to grow or shrink.
     AtomicWord<int> minMillisBetweenSnapshotWindowInc{500};
-    AtomicWord<int> minMillisBetweenSnapshotWindowDec{500};
 
-    // checkCachePressurePeriodSeconds (startup & runtime server paramter, range 1+)
+    // decreaseHistoryIfNotNeededPeriodSeconds (startup & runtime server paramter, range 1+)
     //
     // Controls the period of the task that checks for cache pressure and decreases
-    // targetSnapshotHistoryWindowInSeconds if the pressure is above cachePressureThreshold. The
-    // target window size setting must not be decreased too fast because time must be allowed for
-    // the storage engine to attempt to act on the new setting.
-    AtomicWord<int> checkCachePressurePeriodSeconds{5};
+    // targetSnapshotHistoryWindowInSeconds if there has been pressure and no new SnapshotTooOld
+    // errors.
+    //
+    // This should not run very frequently. It is preferable to increase the window size, and cache
+    // pressure, rather than failing PIT reads.
+    AtomicWord<int> decreaseHistoryIfNotNeededPeriodSeconds{15};
+
+    static inline MutableObserverRegistry<decltype(
+        decreaseHistoryIfNotNeededPeriodSeconds)::WordType>
+        observeDecreaseHistoryIfNotNeededPeriodSeconds;
 
-    static inline MutableObeserverRegistry<decltype(checkCachePressurePeriodSeconds)::WordType>
-        observeCheckCachePressurePeriodSeconds;
+    AtomicWord<long long> snapshotTooOldErrorCount{0};
 };
 
 extern SnapshotWindowParams snapshotWindowParams;