Startup speed (#665)

* Refactor buddy allocator

Make it clearer the structure of add_block by pulling out remove_buddy.

* Give buddy a few elements so don't have to touch pagemap earlie on.

* Only use do and dont dump on pagemap

The do and dont dump calls were costings a lot during start up of snmalloc.  This reduces the times they are called to only be for the pagemap.

src/snmalloc/backend_helpers/buddy.h

@@ -15,7 +15,13 @@ namespace snmalloc
   template<typename Rep, size_t MIN_SIZE_BITS, size_t MAX_SIZE_BITS>
   class Buddy
   {
-    std::array<RBTree<Rep>, MAX_SIZE_BITS - MIN_SIZE_BITS> trees{};
+    struct Entry
+    {
+      typename Rep::Contents cache[3];
+      RBTree<Rep> tree{};
+    };
+
+    std::array<Entry, MAX_SIZE_BITS - MIN_SIZE_BITS> entries{};
     // All RBtrees at or above this index should be empty.
     size_t empty_at_or_above{0};
 
@@ -42,13 +48,54 @@ namespace snmalloc
     void invariant()
     {
 #ifndef NDEBUG
-      for (size_t i = empty_at_or_above; i < trees.size(); i++)
+      for (size_t i = empty_at_or_above; i < entries.size(); i++)
       {
-        SNMALLOC_ASSERT(trees[i].is_empty());
+        SNMALLOC_ASSERT(entries[i].tree.is_empty());
+        // TODO check cache is empty
       }
 #endif
     }
 
+    bool remove_buddy(typename Rep::Contents addr, size_t size)
+    {
+      auto idx = to_index(size);
+
+      // Empty at this range.
+      if (idx >= empty_at_or_above)
+        return false;
+
+      auto buddy = Rep::buddy(addr, size);
+
+      // Check local cache first
+      for (auto& e : entries[idx].cache)
+      {
+        if (Rep::equal(buddy, e))
+        {
+          if (!Rep::can_consolidate(addr, size))
+            return false;
+
+          e = entries[idx].tree.remove_min();
+          return true;
+        }
+      }
+
+      auto path = entries[idx].tree.get_root_path();
+      bool contains_buddy = entries[idx].tree.find(path, buddy);
+
+      if (!contains_buddy)
+        return false;
+
+      // Only check if we can consolidate after we know the buddy is in
+      // the buddy allocator.  This is required to prevent possible segfaults
+      // from looking at the buddies meta-data, which we only know exists
+      // once we have found it in the red-black tree.
+      if (!Rep::can_consolidate(addr, size))
+        return false;
+
+      entries[idx].tree.remove_path(path);
+      return true;
+    }
+
   public:
     constexpr Buddy() = default;
     /**
@@ -63,48 +110,39 @@ namespace snmalloc
      */
     typename Rep::Contents add_block(typename Rep::Contents addr, size_t size)
     {
-      auto idx = to_index(size);
-      empty_at_or_above = bits::max(empty_at_or_above, idx + 1);
-
       validate_block(addr, size);
 
-      auto buddy = Rep::buddy(addr, size);
+      if (remove_buddy(addr, size))
+      {
+        // Add to next level cache
+        size *= 2;
+        addr = Rep::align_down(addr, size);
+        if (size == bits::one_at_bit(MAX_SIZE_BITS))
+        {
+          // Invariant should be checked on all non-tail return paths.
+          // Holds trivially here with current design.
+          invariant();
+          // Too big for this buddy allocator.
+          return addr;
+        }
+        return add_block(addr, size);
+      }
 
-      auto path = trees[idx].get_root_path();
-      bool contains_buddy = trees[idx].find(path, buddy);
+      auto idx = to_index(size);
+      empty_at_or_above = bits::max(empty_at_or_above, idx + 1);
 
-      if (contains_buddy)
+      for (auto& e : entries[idx].cache)
       {
-        // Only check if we can consolidate after we know the buddy is in
-        // the buddy allocator.  This is required to prevent possible segfaults
-        // from looking at the buddies meta-data, which we only know exists
-        // once we have found it in the red-black tree.
-        if (Rep::can_consolidate(addr, size))
+        if (Rep::equal(Rep::null, e))
         {
-          trees[idx].remove_path(path);
-
-          // Add to next level cache
-          size *= 2;
-          addr = Rep::align_down(addr, size);
-          if (size == bits::one_at_bit(MAX_SIZE_BITS))
-          {
-            // Invariant should be checked on all non-tail return paths.
-            // Holds trivially here with current design.
-            invariant();
-            // Too big for this buddy allocator.
-            return addr;
-          }
-          return add_block(addr, size);
+          e = addr;
+          return Rep::null;
         }
-
-        // Re-traverse as the path was to the buddy,
-        // but the representation says we cannot combine.
-        // We must find the correct place for this element.
-        // Something clever could be done here, but it's not worth it.
-        //        path = trees[idx].get_root_path();
-        trees[idx].find(path, addr);
       }
-      trees[idx].insert_path(path, addr);
+
+      auto path = entries[idx].tree.get_root_path();
+      entries[idx].tree.find(path, addr);
+      entries[idx].tree.insert_path(path, addr);
       invariant();
       return Rep::null;
     }
@@ -121,7 +159,15 @@ namespace snmalloc
       if (idx >= empty_at_or_above)
         return Rep::null;
 
-      auto addr = trees[idx].remove_min();
+      auto addr = entries[idx].tree.remove_min();
+      for (auto& e : entries[idx].cache)
+      {
+        if (Rep::equal(Rep::null, addr) || Rep::compare(e, addr))
+        {
+          addr = std::exchange(e, addr);
+        }
+      }
+
       if (addr != Rep::null)
       {
         validate_block(addr, size);

src/snmalloc/ds/pagemap.h

@@ -68,6 +68,10 @@ namespace snmalloc
       auto page_end = pointer_align_up<OS_PAGE_SIZE, char>(last);
       size_t using_size = pointer_diff(page_start, page_end);
       PAL::template notify_using<NoZero>(page_start, using_size);
+      if constexpr (pal_supports<CoreDump, PAL>)
+      {
+        PAL::notify_do_dump(page_start, using_size);
+      }
     }
 
     constexpr FlatPagemap() = default;
@@ -192,6 +196,12 @@ namespace snmalloc
 
       auto new_body_untyped = PAL::reserve(request_size);
 
+      if constexpr (pal_supports<CoreDump, PAL>)
+      {
+        // Pagemap should not be in core dump except where it is non-zero.
+        PAL::notify_do_not_dump(new_body_untyped, request_size);
+      }
+
       if (new_body_untyped == nullptr)
       {
         PAL::error("Failed to initialise snmalloc.");

src/snmalloc/pal/pal_consts.h

@@ -55,6 +55,12 @@ namespace snmalloc
      * This Pal provides a millisecond time source
      */
     Time = (1 << 5),
+
+    /**
+     * This Pal provides selective core dumps, so
+     * modify which parts get dumped.
+     */
+    CoreDump = (1 << 6),
   };
 
   /**

src/snmalloc/pal/pal_freebsd.h

@@ -43,7 +43,8 @@ namespace snmalloc
      * field is declared explicitly to remind anyone modifying this class to
      * add new features that they should add any required feature flags.
      */
-    static constexpr uint64_t pal_features = PALBSD_Aligned::pal_features;
+    static constexpr uint64_t pal_features =
+      PALBSD_Aligned::pal_features | CoreDump;
 
     /**
      * FreeBSD uses atypically small address spaces on its 64 bit RISC machines.
@@ -68,10 +69,8 @@ namespace snmalloc
     /**
      * Notify platform that we will not be using these pages.
      *
-     * We use the `MADV_FREE` and `NADV_NOCORE` flags to `madvise`.  The first
-     * allows the system to discard the page and replace it with a CoW mapping
-     * of the zero page.  The second prevents this mapping from appearing in
-     * core files.
+     * We use the `MADV_FREE` flag to `madvise`. This allows the system to
+     * discard the page and replace it with a CoW mapping of the zero page.
      */
     static void notify_not_using(void* p, size_t size) noexcept
     {
@@ -80,7 +79,6 @@ namespace snmalloc
       if constexpr (DEBUG)
         memset(p, 0x5a, size);
 
-      madvise(p, size, MADV_NOCORE);
       madvise(p, size, MADV_FREE);
 
       if constexpr (mitigations(pal_enforce_access))
@@ -90,28 +88,19 @@ namespace snmalloc
     }
 
     /**
-     * Notify platform that we will be using these pages for reading.
-     *
-     * This is used only for pages full of zeroes and so we exclude them from
-     * core dumps.
+     * Notify platform that these pages should be included in a core dump.
      */
-    static void notify_using_readonly(void* p, size_t size) noexcept
+    static void notify_do_dump(void* p, size_t size) noexcept
     {
-      PALBSD_Aligned<PALFreeBSD>::notify_using_readonly(p, size);
-      madvise(p, size, MADV_NOCORE);
+      madvise(p, size, MADV_CORE);
     }
 
     /**
-     * Notify platform that we will be using these pages.
-     *
-     * We may have previously marked this memory as not being included in core
-     * files, so mark it for inclusion again.
+     * Notify platform that these pages should not be included in a core dump.
      */
-    template<ZeroMem zero_mem>
-    static void notify_using(void* p, size_t size) noexcept
+    static void notify_do_not_dump(void* p, size_t size) noexcept
     {
-      PALBSD_Aligned<PALFreeBSD>::notify_using<zero_mem>(p, size);
-      madvise(p, size, MADV_CORE);
+      madvise(p, size, MADV_NOCORE);
     }
 
 #  if defined(__CHERI_PURE_CAPABILITY__)

src/snmalloc/pal/pal_linux.h

@@ -27,7 +27,8 @@ namespace snmalloc
      *
      * We always make sure that linux has entropy support.
      */
-    static constexpr uint64_t pal_features = PALPOSIX::pal_features | Entropy;
+    static constexpr uint64_t pal_features =
+      PALPOSIX::pal_features | Entropy | CoreDump;
 
     static constexpr size_t page_size =
       Aal::aal_name == PowerPC ? 0x10000 : PALPOSIX::page_size;
@@ -58,7 +59,6 @@ namespace snmalloc
       void* p = PALPOSIX<PALLinux>::reserve(size);
       if (p)
       {
-        madvise(p, size, MADV_DONTDUMP);
 #  ifdef SNMALLOC_PAGEID
 #    ifndef PR_SET_VMA
 #      define PR_SET_VMA 0x53564d41
@@ -125,7 +125,6 @@ namespace snmalloc
       if constexpr (DEBUG)
         memset(p, 0x5a, size);
 
-      madvise(p, size, MADV_DONTDUMP);
       madvise(p, size, madvise_free_flags);
 
       if constexpr (mitigations(pal_enforce_access))
@@ -135,25 +134,19 @@ namespace snmalloc
     }
 
     /**
-     * Notify platform that we will be using these pages for reading.
-     *
-     * This is used only for pages full of zeroes and so we exclude them from
-     * core dumps.
+     * Notify platform that these pages should be included in a core dump.
      */
-    static void notify_using_readonly(void* p, size_t size) noexcept
+    static void notify_do_dump(void* p, size_t size) noexcept
     {
-      PALPOSIX<PALLinux>::notify_using_readonly(p, size);
-      madvise(p, size, MADV_DONTDUMP);
+      madvise(p, size, MADV_DODUMP);
     }
 
     /**
-     * Notify platform that we will be using these pages.
+     * Notify platform that these pages should not be included in a core dump.
      */
-    template<ZeroMem zero_mem>
-    static void notify_using(void* p, size_t size) noexcept
+    static void notify_do_not_dump(void* p, size_t size) noexcept
     {
-      PALPOSIX<PALLinux>::notify_using<zero_mem>(p, size);
-      madvise(p, size, MADV_DODUMP);
+      madvise(p, size, MADV_DONTDUMP);
     }
 
     static uint64_t get_entropy64()