Decommits when coallescing spans instead of committing.

third_party/tcmalloc/page_heap.cc

 // Copyright (c) 2008, Google Inc.
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
@@ skipping 139 matching lines @@
   return leftover;
 }
 
 void PageHeap::CommitSpan(Span* span) {
   TCMalloc_SystemCommit(
       reinterpret_cast<void*>(span->start << kPageShift),
       static_cast<size_t>(span->length << kPageShift)
   );
 }
 
+void PageHeap::DecommitSpan(Span* span) {
+  TCMalloc_SystemRelease(reinterpret_cast<void*>(span->start << kPageShift),
+                         static_cast<size_t>(span->length << kPageShift));
+}
+
 Span* PageHeap::Carve(Span* span, Length n) {
   ASSERT(n > 0);
   ASSERT(span->location != Span::IN_USE);
   const int old_location = span->location;
   DLL_Remove(span);
   span->location = Span::IN_USE;
   Event(span, 'A', n);
 
   const int extra = span->length - n;
   ASSERT(extra >= 0);
@@ skipping 24 matching lines @@
 }
 
 void PageHeap::Delete(Span* span) {
   ASSERT(Check());
   ASSERT(span->location == Span::IN_USE);
   ASSERT(span->length > 0);
   ASSERT(GetDescriptor(span->start) == span);
   ASSERT(GetDescriptor(span->start + span->length - 1) == span);
   span->sizeclass = 0;
   span->sample = 0;
+  DecommitSpan(span);
 
   // Coalesce -- we guarantee that "p" != 0, so no bounds checking
   // necessary.  We do not bother resetting the stale pagemap
   // entries for the pieces we are merging together because we only
   // care about the pagemap entries for the boundaries.
   //
   // Note that the spans we merge into "span" may come out of
-  // a "returned" list.  We move those into "normal" list
-  // as for 'immediate' operations we favour committing over
-  // decommitting (decommitting is performed offline).
+  // a "normal" or "returned" list.  We move those into the
+  // "returned" list in order to favor decommitting over committing.
   const PageID p = span->start;
   const Length n = span->length;
   Span* prev = GetDescriptor(p-1);
   if (prev != NULL && prev->location != Span::IN_USE) {
     // Merge preceding span into this span
     ASSERT(prev->start + prev->length == p);
     const Length len = prev->length;
-    if (prev->location == Span::ON_RETURNED_FREELIST) {
-      CommitSpan(prev);
+    if (prev->location == Span::ON_NORMAL_FREELIST) {
+      DecommitSpan(prev);
     }
     DLL_Remove(prev);
     DeleteSpan(prev);
     span->start -= len;
     span->length += len;
     pagemap_.set(span->start, span);
     Event(span, 'L', len);
   }
   Span* next = GetDescriptor(p+n);
   if (next != NULL && next->location != Span::IN_USE) {
     // Merge next span into this span
     ASSERT(next->start == p+n);
     const Length len = next->length;
-    if (next->location == Span::ON_RETURNED_FREELIST) {
-      CommitSpan(next);
+    if (next->location == Span::ON_NORMAL_FREELIST) {
+      DecommitSpan(next);
     }
     DLL_Remove(next);
     DeleteSpan(next);
     span->length += len;
     pagemap_.set(span->start + span->length - 1, span);
     Event(span, 'R', len);
   }
 
   Event(span, 'D', span->length);

[antonm, 2009/10/01 11:59:27]

why you don't decommit the span itself?  is it by intent or just an oversight?

BTW, I won't be surprise that if you change that, it could have performance
implications (if committing is cheaper for already committed pages)

-  span->location = Span::ON_NORMAL_FREELIST;
+  span->location = Span::ON_RETURNED_FREELIST;
   if (span->length < kMaxPages) {
     DLL_Prepend(&free_[span->length].normal, span);

[Mike Belshe, 2009/10/01 02:39:48]

I think this is a bug -

DLL_Prepend(&free_[span->length].returned, span);

?

[antonm, 2009/10/01 11:59:27]

That's definitely a bug and I would expect immediate crash under real load---you
put into normal list spans in mixed state (some pages are committed and some are
not).

   } else {
     DLL_Prepend(&large_.normal, span);

[Mike Belshe, 2009/10/01 02:39:48]

ditto

   }
   free_pages_ += n;
 
   IncrementalScavenge(n);
   ASSERT(Check());
 }
 
 void PageHeap::IncrementalScavenge(Length n) {
   // Fast path; not yet time to release memory
   scavenge_counter_ -= n;
@@ skipping 235 matching lines @@
 
 void PageHeap::ReleaseFreePages() {
   for (Length s = 0; s < kMaxPages; s++) {
     ReleaseFreeList(&free_[s].normal, &free_[s].returned);
   }
   ReleaseFreeList(&large_.normal, &large_.returned);
   ASSERT(Check());
 }
 
 }  // namespace tcmalloc