Patchwork [2/4] Free large chunks in ggc v2

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Submitter Andi Kleen
Date Oct. 22, 2011, 5:54 a.m.
Message ID <1319262853-32370-3-git-send-email-andi@firstfloor.org>
Download mbox | patch
Permalink /patch/121158/
State New
Headers show

Comments

Andi Kleen - Oct. 22, 2011, 5:54 a.m.
From: Andi Kleen <ak@linux.intel.com>

This implements the freeing back of large chunks in the ggc madvise path
Richard Guenther asked for.  This way on systems with limited
address space malloc() and other allocators still have
a chance to get back at some of the memory ggc freed. The
fragmented pages are still just given back, but the address space
stays allocated.

I tried freeing only aligned 2MB areas to optimize for 2MB huge
pages, but the hit rate was quite low, so I switched to 1MB+
unaligned areas.

Passed bootstrap and testing on x86_64-linux

v2: Hardcode free unit size instead of param

gcc/:
2011-10-18  Andi Kleen  <ak@linux.intel.com>

	* ggc-page (release_pages): First free large continuous
	chunks in the madvise path.
---
 gcc/ggc-page.c |   48 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 48 insertions(+), 0 deletions(-)

Patch

diff --git a/gcc/ggc-page.c b/gcc/ggc-page.c
index ba88e3f..99bf2df 100644
--- a/gcc/ggc-page.c
+++ b/gcc/ggc-page.c
@@ -972,6 +972,54 @@  release_pages (void)
   page_entry *p, *start_p;
   char *start;
   size_t len;
+  size_t mapped_len;
+  page_entry *next, *prev, *newprev;
+  size_t free_unit = (GGC_QUIRE_SIZE/2) * G.pagesize;
+
+  /* First free larger continuous areas to the OS.
+     This allows other allocators to grab these areas if needed.
+     This is only done on larger chunks to avoid fragmentation. 
+     This does not always work because the free_pages list is only
+     sorted over a single GC cycle. */
+
+  p = G.free_pages;
+  prev = NULL;
+  while (p)
+    {
+      start = p->page;
+      start_p = p;
+      len = 0;
+      mapped_len = 0;
+      newprev = prev;
+      while (p && p->page == start + len)
+        {
+          len += p->bytes;
+	  if (!p->discarded)
+	      mapped_len += p->bytes;
+	  newprev = p;
+          p = p->next;
+        }
+      if (len >= free_unit)
+        {
+          while (start_p != p)
+            {
+              next = start_p->next;
+              free (start_p);
+              start_p = next;
+            }
+          munmap (start, len);
+	  if (prev)
+	    prev->next = p;
+          else
+            G.free_pages = p;
+          G.bytes_mapped -= mapped_len;
+	  continue;
+        }
+      prev = newprev;
+   }
+
+  /* Now give back the fragmented pages to the OS, but keep the address 
+     space to reuse it next time. */
 
   for (p = G.free_pages; p; )
     {