Warn on _compiled_cache growth

Added warnings to the LRU "compiled cache" used by the :class:`.Mapper`
(and ultimately will be for other ORM-based LRU caches) such that
when the cache starts hitting its size limits, the application will
emit a warning that this is a performance-degrading situation that
may require attention.   The LRU caches can reach their size limits
primarily if an application is making use of an unbounded number
of :class:`.Engine` objects, which is an antipattern.  Otherwise,
this may suggest an issue that should be brought to the SQLAlchemy
developer's attention.

Additionally, adjusted the test_memusage algorithm again as the
previous one could still allow a growing memory size to be missed.

Change-Id: I020d1ceafb7a08f6addfa990a1e7acd09f933240

2 files changed, 26 insertions, 2 deletions

diff --git a/lib/sqlalchemy/orm/mapper.py b/lib/sqlalchemy/orm/mapper.py
index 27456c35b..28a5c5b9b 100644
--- a/lib/sqlalchemy/orm/mapper.py
+++ b/lib/sqlalchemy/orm/mapper.py
@@ -2718,7 +2718,20 @@ class Mapper(InspectionAttr):
 
     @_memoized_configured_property
     def _compiled_cache(self):
-        return util.LRUCache(self._compiled_cache_size)
+        return util.LRUCache(self._compiled_cache_size,
+                             size_alert=self._alert_lru_cache_limit)
+
+    def _alert_lru_cache_limit(self):
+        util.warn(
+            "Compiled statement cache for mapper %s is "
+            "reaching its size threshold of %d, based on _compiled_cache_size "
+            "of %d.  Please refer to "
+            "http://docs.sqlalchemy.org/en/latest/faq/performance.html"
+            "#faq_compiled_cache_threshold"
+            " for best practices." %
+            (self,
+             self._compiled_cache.size_threshold,
+             self._compiled_cache_size))
 
     @_memoized_configured_property
     def _sorted_tables(self):
diff --git a/lib/sqlalchemy/util/_collections.py b/lib/sqlalchemy/util/_collections.py
index bdd40fc79..d94af5f62 100644
--- a/lib/sqlalchemy/util/_collections.py
+++ b/lib/sqlalchemy/util/_collections.py
@@ -868,9 +868,12 @@ class LRUCache(dict):
 
     """
 
-    def __init__(self, capacity=100, threshold=.5):
+    __slots__ = 'capacity', 'threshold', 'size_alert', '_counter', '_mutex'
+
+    def __init__(self, capacity=100, threshold=.5, size_alert=None):
         self.capacity = capacity
         self.threshold = threshold
+        self.size_alert = size_alert
         self._counter = 0
         self._mutex = threading.Lock()
 
@@ -910,11 +913,19 @@ class LRUCache(dict):
             item[1] = value
         self._manage_size()
 
+    @property
+    def size_threshold(self):
+        return self.capacity + self.capacity * self.threshold
+
     def _manage_size(self):
         if not self._mutex.acquire(False):
             return
         try:
+            size_alert = bool(self.size_alert)
             while len(self) > self.capacity + self.capacity * self.threshold:
+                if size_alert:
+                    size_alert = False
+                    self.size_alert()
                 by_counter = sorted(dict.values(self),
                                     key=operator.itemgetter(2),
                                     reverse=True)