diff --git a/util/concurrency/value.h b/util/concurrency/value.h
index d75a0e58ce6e6..8c6820a7628ec 100644
--- a/util/concurrency/value.h
+++ b/util/concurrency/value.h
@@ -20,7 +20,7 @@
 
 #pragma once
 
-#include "mutex.h"
+#include "spin_lock.h"
 
 namespace mongo {
 
@@ -45,35 +45,40 @@ namespace mongo {
         }
     };
 
+    /** there is now one mutex per DiagStr.  If you have hundreds or millions of
+        DiagStrs you'll need to do something different.
+    */
     class DiagStr {
+        mutable SpinLock m;
         string _s;
-        static SimpleMutex m;
     public:
         DiagStr(const DiagStr& r) : _s(r.get()) { }
+        DiagStr(const string& r) : _s(r) { }
         DiagStr() { }
         bool empty() const { 
-            SimpleMutex::scoped_lock lk(m);
+            scoped_spinlock lk(m);
             return _s.empty();
         }
         string get() const { 
-            SimpleMutex::scoped_lock lk(m);
+            scoped_spinlock lk(m);
             return _s;
         }
-
         void set(const char *s) {
-            SimpleMutex::scoped_lock lk(m);
+            scoped_spinlock lk(m);
             _s = s;
         }
         void set(const string& s) { 
-            SimpleMutex::scoped_lock lk(m);
+            scoped_spinlock lk(m);
             _s = s;
         }
         operator string() const { return get(); }
         void operator=(const string& s) { set(s); }
         void operator=(const DiagStr& rhs) { 
-            SimpleMutex::scoped_lock lk(m);
-            _s = rhs.get();
+            set( rhs.get() );
         }
+
+        // == is not defined.  use get() == ... instead.  done this way so one thinks about if composing multiple operations
+        bool operator==(const string& s) const; 
     };
 
     /** Thread safe map.  
diff --git a/util/concurrency/vars.cpp b/util/concurrency/vars.cpp
index 669725a149200..b561ccce00343 100644
--- a/util/concurrency/vars.cpp
+++ b/util/concurrency/vars.cpp
@@ -22,8 +22,6 @@
 
 namespace mongo {
 
-    SimpleMutex DiagStr::m("diags");
-
     // intentional leak. otherwise destructor orders can be problematic at termination.
     MutexDebugger &mutexDebugger = *(new MutexDebugger());
 
diff --git a/util/net/sock.cpp b/util/net/sock.cpp
index 1b5828c24ea38..b4bbf524c6e14 100644
--- a/util/net/sock.cpp
+++ b/util/net/sock.cpp
@@ -286,10 +286,9 @@ namespace mongo {
     // ------ hostname -------------------
 
     string hostbyname(const char *hostname) {
-        {
+        if( *hostname == '#' ) {
             string s = dynHostNames.get(hostname);
-            if( !s.empty() )
-                return s;
+            return s;
         }
 
         string addr =  SockAddr(hostname, 0).getAddr();
@@ -311,7 +310,7 @@ namespace mongo {
         return buf;
     }
 
-    string _hostNameCached;
+    DiagStr _hostNameCached;
     static void _hostNameCachedInit() {
         _hostNameCached = getHostName();
     }