A dirns class that is an array of xns instances.

An xns must fit in a page, but that doesn't allow enough buckets to avoid physical sharing in dirbench. This is basically a work-around for the one page limit.

A dirns class that is an array of xns instances.
6bf61f8d · Silas Boyd-Wickizer · cb7ca72d · 6bf61f8d
--- a/include/dirns.hh
+++ b/include/dirns.hh
@@ -2,26 +2,76 @@

 u64 namehash(const strbuf<DIRSIZ>&);

+#define NINS      251
+
+// Prevent potentially non-scalable bucket groupings
+static_assert(NINS != NHASH, "Bad NINS choice");
+
 class dirns : public rcu_freed
 {
+  typedef xns<strbuf<DIRSIZ>, u32, namehash> ins;
+
 public:
-  dirns() : rcu_freed("dirns"), ns_(false) {}
+  dirns() : rcu_freed("dirns") {
+    for (int i = 0; i < NINS; i++)
+      a_[i].store(nullptr);
+  }
+
+  ~dirns() {
+    for (int i = 0; i < NINS; i++)
+      if (a_[i].load() != nullptr) {
+        delete a_[i].load();
+        a_[i].store(nullptr);
+      }
+  }

  bool remove(const strbuf<DIRSIZ>& name, const u32* vp = nullptr) {
-    return ns_.remove(name, vp);
+    u32 i = ah(name);
+    if (a_[i].load() != nullptr)
+      return a_[i].load()->remove(name, vp);
+    return false;
  }

-  int  insert(const strbuf<DIRSIZ>& name, const u32& inum) {
-    return ns_.insert(name, inum);
+  int insert(const strbuf<DIRSIZ>& name, const u32& inum) {
+    ins* ns;
+    u32 i;
+
+    i = ah(name);
+
+  retry:
+    if (a_[i].load() == nullptr) {
+      ns = new ins(false);
+      if (!cmpxch(&a_[i], (decltype(ns)) nullptr, ns)) {
+        delete ns;
+        goto retry;
+      }
+    } else {
+      ns = a_[i];
+    }
+    
+    return ns->insert(name, inum);
  }

-  u32  lookup(const strbuf<DIRSIZ>& name) {
-    return ns_.lookup(name);
+  u32 lookup(const strbuf<DIRSIZ>& name) const {
+    ins* ns;
+    u32 i;
+
+    i = ah(name);
+    ns = a_[i].load();
+    if (ns != nullptr)
+      return ns->lookup(name);
+    return 0;
  }

  template<class CB>
-  void enumerate(CB cb) {
-    return ns_.enumerate(cb);
+  void enumerate(CB cb) const {
+    // XXX(sbw) not the same semantics as xns::enumerate.
+    // This continues if cb returns true.
+    for (int i = 0; i < NINS; i++) {
+      ins* ns = a_[i].load();
+      if (ns != nullptr)
+        ns->enumerate(cb);
+    }
  }

  virtual void do_gc(void) {
@@ -31,5 +81,10 @@ public:
  NEW_DELETE_OPS(dirns);

 private:
-  xns<strbuf<DIRSIZ>, u32, namehash> ns_;
+  std::atomic<ins*> a_[NINS];
+
+  u32 ah(const strbuf<DIRSIZ>& name) const {
+    u64 h = namehash(name);
+    return h % NINS;
+  }
 };