From 0ef1e13db4f3d217ddc6de7ffc46842a4fc9889a Mon Sep 17 00:00:00 2001
From: Louis Williams <louis.williams@mongodb.com>
Date: Wed, 14 Feb 2018 17:11:57 -0500
Subject: SERVER-33122 Add option to disable WiredTiger cursor caching

---
 .../wiredtiger/wiredtiger_session_cache.cpp        | 46 ++++++++++++++--------
 .../storage/wiredtiger/wiredtiger_session_cache.h  |  2 +-
 2 files changed, 30 insertions(+), 18 deletions(-)

diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_session_cache.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_session_cache.cpp
index 31889025b92..79b4ad90630 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_session_cache.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_session_cache.cpp
@@ -38,6 +38,7 @@
 #include "mongo/base/error_codes.h"
 #include "mongo/db/global_settings.h"
 #include "mongo/db/repl/repl_settings.h"
+#include "mongo/db/server_parameters.h"
 #include "mongo/db/storage/journal_listener.h"
 #include "mongo/db/storage/wiredtiger/wiredtiger_kv_engine.h"
 #include "mongo/db/storage/wiredtiger/wiredtiger_util.h"
@@ -48,13 +49,32 @@
 
 namespace mongo {
 
+AtomicInt32 kWiredTigerCursorCacheSize(10000);
+
+class WiredTigerCursorCacheSize
+    : public ExportedServerParameter<std::int32_t, ServerParameterType::kStartupAndRuntime> {
+public:
+    WiredTigerCursorCacheSize()
+        : ExportedServerParameter<std::int32_t, ServerParameterType::kStartupAndRuntime>(
+              ServerParameterSet::getGlobal(),
+              "wiredTigerCursorCacheSize",
+              &kWiredTigerCursorCacheSize) {}
+
+    virtual Status validate(const std::int32_t& potentialNewValue) {
+        if (potentialNewValue < 0) {
+            return Status(ErrorCodes::BadValue,
+                          str::stream()
+                              << "wiredTigerCursorCacheSize must be greater than or equal "
+                              << "to 0, but attempted to set to: "
+                              << potentialNewValue);
+        }
+
+        return Status::OK();
+    }
+} WiredTigerCursorCacheSizeSetting;
+
 WiredTigerSession::WiredTigerSession(WT_CONNECTION* conn, uint64_t epoch, uint64_t cursorEpoch)
-    : _epoch(epoch),
-      _cursorEpoch(cursorEpoch),
-      _session(NULL),
-      _cursorGen(0),
-      _cursorsCached(0),
-      _cursorsOut(0) {
+    : _epoch(epoch), _cursorEpoch(cursorEpoch), _session(NULL), _cursorGen(0), _cursorsOut(0) {
     invariantWTOK(conn->open_session(conn, NULL, "isolation=snapshot", &_session));
 }
 
@@ -67,7 +87,6 @@ WiredTigerSession::WiredTigerSession(WT_CONNECTION* conn,
       _cache(cache),
       _session(NULL),
       _cursorGen(0),
-      _cursorsCached(0),
       _cursorsOut(0) {
     invariantWTOK(conn->open_session(conn, NULL, "isolation=snapshot", &_session));
 }
@@ -85,7 +104,6 @@ WT_CURSOR* WiredTigerSession::getCursor(const std::string& uri, uint64_t id, boo
             WT_CURSOR* c = i->_cursor;
             _cursors.erase(i);
             _cursorsOut++;
-            _cursorsCached--;
             return c;
         }
     }
@@ -109,17 +127,11 @@ void WiredTigerSession::releaseCursor(uint64_t id, WT_CURSOR* cursor) {
 
     // Cursors are pushed to the front of the list and removed from the back
     _cursors.push_front(WiredTigerCachedCursor(id, _cursorGen++, cursor));
-    _cursorsCached++;
-
-    // "Old" is defined as not used in the last N**2 operations, if we have N cursors cached.
-    // The reasoning here is to imagine a workload with N tables performing operations randomly
-    // across all of them (i.e., each cursor has 1/N chance of used for each operation).  We
-    // would like to cache N cursors in that case, so any given cursor could go N**2 operations
-    // in between use.
-    while (_cursorGen - _cursors.back()._gen > 10000) {
+
+    std::uint64_t cursorCacheSize = static_cast<std::uint64_t>(kWiredTigerCursorCacheSize.load());
+    while (!_cursors.empty() && _cursorGen - _cursors.back()._gen > cursorCacheSize) {
         cursor = _cursors.back()._cursor;
         _cursors.pop_back();
-        _cursorsCached--;
         invariantWTOK(cursor->close(cursor));
     }
 }
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_session_cache.h b/src/mongo/db/storage/wiredtiger/wiredtiger_session_cache.h
index 12c61fb0db3..4df2d7e5db7 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_session_cache.h
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_session_cache.h
@@ -137,7 +137,7 @@ private:
     WT_SESSION* _session;            // owned
     CursorCache _cursors;            // owned
     uint64_t _cursorGen;
-    int _cursorsCached, _cursorsOut;
+    int _cursorsOut;
 };
 
 /**
-- 
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