MB-40531: [BP] Prefer paging from replicas if possible

Backport of http://review.couchbase.org/c/kv_engine/+/134806

Prior to this change, the pager would try to page out X% of items in
replica vbuckets, but if this did not bring memory usage below the low
water mark, it would flip phase to ACTIVE_AND_PENDING_ONLY - meaning
the next run of the pager would page immediately try to page from
active and pending vbuckets.

Now, active/pending vbuckets will only be considered if paging _every_
possible item from replicas would not free enough memory to drop below
the low water mark.

Note: On branch 'alice' HIFI MRU is the default eviction policy, but
2Bit LRU is still present. This change applis to HIFI MRU only.

Change-Id: I47347fe1680a6c2316154e4d9c511a1b54489d82
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/142512
Well-Formed: Build Bot <[email protected]>
Reviewed-by: Daniel Owen <[email protected]>
Tested-by: Paolo Cocchi <[email protected]>

Author: paolococchi

engines/ep/src/ep_bucket.h

@@ -147,6 +147,10 @@ class EPBucket : public KVBucket {
         return true;
     }
 
+    bool canEvictFromReplicas() override {
+        return true;
+    }
+
 protected:
     class ValueChangedListener;
 

engines/ep/src/ephemeral_bucket.h

@@ -133,6 +133,10 @@ class EphemeralBucket : public KVBucket {
      */
     static void reconfigureForEphemeral(Configuration& config);
 
+    bool canEvictFromReplicas() override {
+        return false;
+    }
+
 protected:
     std::unique_ptr<VBucketCountVisitor> makeVBCountVisitor(
             vbucket_state_t state) override;

engines/ep/src/hash_table.h

@@ -854,6 +854,15 @@ class HashTable {
      */
     void dump() const;
 
+    /**
+     * Generates a new value that is either the same or higher than the input
+     * value.  It is intended to be used to increment the frequency counter of a
+     * storedValue.
+     * @param value  The value counter to try to generate an increment for.
+     * @returns      The new value that is the same or higher than value.
+     */
+    uint8_t generateFreqValue(uint8_t value);
+
 private:
     // The container for actually holding the StoredValues.
     using table_type = std::vector<StoredValue::UniquePtr>;
@@ -951,15 +960,6 @@ class HashTable {
 
     void clear_UNLOCKED(bool deactivate);
 
-    /**
-     * Generates a new value that is either the same or higher than the input
-     * value.  It is intended to be used to increment the frequency counter of a
-     * storedValue.
-     * @param value  The value counter to try to generate an increment for.
-     * @returns      The new value that is the same or higher than value.
-     */
-    uint8_t generateFreqValue(uint8_t value);
-
     DISALLOW_COPY_AND_ASSIGN(HashTable);
 };