8340426: ZGC: Move defragment out of the allocation path

Reviewed-by: aboldtch, jsikstro, eosterlund

src/hotspot/share/gc/z/zHeap.cpp

@@ -241,10 +241,10 @@ void ZHeap::undo_alloc_page(ZPage* page) {
   log_trace(gc)("Undo page allocation, thread: " PTR_FORMAT " (%s), page: " PTR_FORMAT ", size: " SIZE_FORMAT,
                 p2i(Thread::current()), ZUtils::thread_name(), p2i(page), page->size());
 
-  free_page(page);
+  free_page(page, false /* allow_defragment */);
 }
 
-void ZHeap::free_page(ZPage* page) {
+void ZHeap::free_page(ZPage* page, bool allow_defragment) {
   // Remove page table entry
   _page_table.remove(page);
 
@@ -253,7 +253,7 @@ void ZHeap::free_page(ZPage* page) {
   }
 
   // Free page
-  _page_allocator.free_page(page);
+  _page_allocator.free_page(page, allow_defragment);
 }
 
 size_t ZHeap::free_empty_pages(const ZArray<ZPage*>* pages) {

src/hotspot/share/gc/z/zHeap.hpp

@@ -104,7 +104,7 @@ public:
   // Page allocation
   ZPage* alloc_page(ZPageType type, size_t size, ZAllocationFlags flags, ZPageAge age);
   void undo_alloc_page(ZPage* page);
-  void free_page(ZPage* page);
+  void free_page(ZPage* page, bool allow_defragment);
   size_t free_empty_pages(const ZArray<ZPage*>* pages);
 
   // Object allocation

src/hotspot/share/gc/z/zPageAllocator.cpp

@@ -275,7 +275,7 @@ bool ZPageAllocator::prime_cache(ZWorkers* workers, size_t size) {
     workers->run_all(&task);
   }
 
-  free_page(page);
+  free_page(page, false /* allow_defragment */);
 
   return true;
 }
@@ -462,6 +462,38 @@ void ZPageAllocator::destroy_page(ZPage* page) {
   safe_destroy_page(page);
 }
 
+bool ZPageAllocator::should_defragment(const ZPage* page) const {
+  // A small page can end up at a high address (second half of the address space)
+  // if we've split a larger page or we have a constrained address space. To help
+  // fight address space fragmentation we remap such pages to a lower address, if
+  // a lower address is available.
+  return page->type() == ZPageType::small &&
+         page->start() >= to_zoffset(_virtual.reserved() / 2) &&
+         page->start() > _virtual.lowest_available_address();
+}
+
+ZPage* ZPageAllocator::defragment_page(ZPage* page) {
+  // Harvest the physical memory (which is committed)
+  ZPhysicalMemory pmem;
+  ZPhysicalMemory& old_pmem = page->physical_memory();
+  pmem.add_segments(old_pmem);
+  old_pmem.remove_segments();
+
+  _unmapper->unmap_and_destroy_page(page);
+
+  // Allocate new virtual memory at a low address
+  const ZVirtualMemory vmem = _virtual.alloc(pmem.size(), true /* force_low_address */);
+
+  // Create the new page and map it
+  ZPage* new_page = new ZPage(ZPageType::small, vmem, pmem);
+  map_page(new_page);
+
+  // Update statistics
+  ZStatInc(ZCounterDefragment);
+
+  return new_page;
+}
+
 bool ZPageAllocator::is_alloc_allowed(size_t size) const {
   const size_t available = _current_max_capacity - _used - _claimed;
   return available >= size;
@@ -623,16 +655,6 @@ ZPage* ZPageAllocator::alloc_page_create(ZPageAllocation* allocation) {
   return new ZPage(allocation->type(), vmem, pmem);
 }
 
-bool ZPageAllocator::should_defragment(const ZPage* page) const {
-  // A small page can end up at a high address (second half of the address space)
-  // if we've split a larger page or we have a constrained address space. To help
-  // fight address space fragmentation we remap such pages to a lower address, if
-  // a lower address is available.
-  return page->type() == ZPageType::small &&
-         page->start() >= to_zoffset(_virtual.reserved() / 2) &&
-         page->start() > _virtual.lowest_available_address();
-}
-
 bool ZPageAllocator::is_alloc_satisfied(ZPageAllocation* allocation) const {
   // The allocation is immediately satisfied if the list of pages contains
   // exactly one page, with the type and size that was requested. However,
@@ -652,12 +674,6 @@ bool ZPageAllocator::is_alloc_satisfied(ZPageAllocation* allocation) const {
     return false;
   }
 
-  if (should_defragment(page)) {
-    // Defragment address space
-    ZStatInc(ZCounterDefragment);
-    return false;
-  }
-
   // Allocation immediately satisfied
   return true;
 }
@@ -773,6 +789,18 @@ void ZPageAllocator::satisfy_stalled() {
   }
 }
 
+ZPage* ZPageAllocator::prepare_to_recycle(ZPage* page, bool allow_defragment) {
+  // Make sure we have a page that is safe to recycle
+  ZPage* const to_recycle = _safe_recycle.register_and_clone_if_activated(page);
+
+  // Defragment the page before recycle if allowed and needed
+  if (allow_defragment && should_defragment(to_recycle)) {
+    return defragment_page(to_recycle);
+  }
+
+  return to_recycle;
+}
+
 void ZPageAllocator::recycle_page(ZPage* page) {
   // Set time when last used
   page->set_last_used();
@@ -781,9 +809,11 @@ void ZPageAllocator::recycle_page(ZPage* page) {
   _cache.free_page(page);
 }
 
-void ZPageAllocator::free_page(ZPage* page) {
+void ZPageAllocator::free_page(ZPage* page, bool allow_defragment) {
   const ZGenerationId generation_id = page->generation_id();
-  ZPage* const to_recycle = _safe_recycle.register_and_clone_if_activated(page);
+
+  // Prepare page for recycling before taking the lock
+  ZPage* const to_recycle = prepare_to_recycle(page, allow_defragment);
 
   ZLocker<ZLock> locker(&_lock);
 
@@ -800,11 +830,12 @@ void ZPageAllocator::free_page(ZPage* page) {
 }
 
 void ZPageAllocator::free_pages(const ZArray<ZPage*>* pages) {
-  ZArray<ZPage*> to_recycle;
+  ZArray<ZPage*> to_recycle_pages;
 
   size_t young_size = 0;
   size_t old_size = 0;
 
+  // Prepare pages for recycling before taking the lock
   ZArrayIterator<ZPage*> pages_iter(pages);
   for (ZPage* page; pages_iter.next(&page);) {
     if (page->is_young()) {
@@ -812,7 +843,12 @@ void ZPageAllocator::free_pages(const ZArray<ZPage*>* pages) {
     } else {
       old_size += page->size();
     }
-    to_recycle.push(_safe_recycle.register_and_clone_if_activated(page));
+
+    // Prepare to recycle
+    ZPage* const to_recycle = prepare_to_recycle(page, true /* allow_defragment */);
+
+    // Register for recycling
+    to_recycle_pages.push(to_recycle);
   }
 
   ZLocker<ZLock> locker(&_lock);
@@ -823,7 +859,7 @@ void ZPageAllocator::free_pages(const ZArray<ZPage*>* pages) {
   decrease_used_generation(ZGenerationId::old, old_size);
 
   // Free pages
-  ZArrayIterator<ZPage*> iter(&to_recycle);
+  ZArrayIterator<ZPage*> iter(&to_recycle_pages);
   for (ZPage* page; iter.next(&page);) {
     recycle_page(page);
   }
@@ -833,11 +869,16 @@ void ZPageAllocator::free_pages(const ZArray<ZPage*>* pages) {
 }
 
 void ZPageAllocator::free_pages_alloc_failed(ZPageAllocation* allocation) {
-  ZArray<ZPage*> to_recycle;
+  ZArray<ZPage*> to_recycle_pages;
 
+  // Prepare pages for recycling before taking the lock
   ZListRemoveIterator<ZPage> allocation_pages_iter(allocation->pages());
   for (ZPage* page; allocation_pages_iter.next(&page);) {
-    to_recycle.push(_safe_recycle.register_and_clone_if_activated(page));
+    // Prepare to recycle
+    ZPage* const to_recycle = prepare_to_recycle(page, false /* allow_defragment */);
+
+    // Register for recycling
+    to_recycle_pages.push(to_recycle);
   }
 
   ZLocker<ZLock> locker(&_lock);
@@ -849,7 +890,7 @@ void ZPageAllocator::free_pages_alloc_failed(ZPageAllocation* allocation) {
   size_t freed = 0;
 
   // Free any allocated/flushed pages
-  ZArrayIterator<ZPage*> iter(&to_recycle);
+  ZArrayIterator<ZPage*> iter(&to_recycle_pages);
   for (ZPage* page; iter.next(&page);) {
     freed += page->size();
     recycle_page(page);

src/hotspot/share/gc/z/zPageAllocator.hpp

@@ -104,13 +104,15 @@ private:
 
   void destroy_page(ZPage* page);
 
+  bool should_defragment(const ZPage* page) const;
+  ZPage* defragment_page(ZPage* page);
+
   bool is_alloc_allowed(size_t size) const;
 
   bool alloc_page_common_inner(ZPageType type, size_t size, ZList<ZPage>* pages);
   bool alloc_page_common(ZPageAllocation* allocation);
   bool alloc_page_stall(ZPageAllocation* allocation);
   bool alloc_page_or_stall(ZPageAllocation* allocation);
-  bool should_defragment(const ZPage* page) const;
   bool is_alloc_satisfied(ZPageAllocation* allocation) const;
   ZPage* alloc_page_create(ZPageAllocation* allocation);
   ZPage* alloc_page_finalize(ZPageAllocation* allocation);
@@ -149,9 +151,10 @@ public:
   void reset_statistics(ZGenerationId id);
 
   ZPage* alloc_page(ZPageType type, size_t size, ZAllocationFlags flags, ZPageAge age);
+  ZPage* prepare_to_recycle(ZPage* page, bool allow_defragment);
   void recycle_page(ZPage* page);
   void safe_destroy_page(ZPage* page);
-  void free_page(ZPage* page);
+  void free_page(ZPage* page, bool allow_defragment);
   void free_pages(const ZArray<ZPage*>* pages);
 
   void enable_safe_destroy() const;

src/hotspot/share/gc/z/zRelocate.cpp

@@ -411,7 +411,7 @@ static void retire_target_page(ZGeneration* generation, ZPage* page) {
   // relocate the remaining objects, leaving the target page empty when
   // relocation completed.
   if (page->used() == 0) {
-    ZHeap::heap()->free_page(page);
+    ZHeap::heap()->free_page(page, true /* allow_defragment */);
   }
 }
 
@@ -1012,7 +1012,7 @@ public:
       page->log_msg(" (relocate page done normal)");
 
       // Free page
-      ZHeap::heap()->free_page(page);
+      ZHeap::heap()->free_page(page, true /* allow_defragment */);
     }
   }
 };