Allow to unmap memory block right in GC cycle where block is freed

This commit changes how GC_unmap_threshold is interpreted:
GC_unmap_threshold==N now means block is returned to OS if the block
is freed N-1 collections ago (previously was: N collections ago).

* allchblk.c [USE_MUNMAP && !MUNMAP_THRESHOLD] (MUNMAP_THRESHOLD):
Change value from 6 to 7.
* configure.ac [$enable_munmap!=no && ($MUNMAP_THRESHOLD=""
|| $MUNMAP_THRESHOLD=yes)] (MUNMAP_THRESHOLD): Likewise.
* allchblk.c [USE_MUNMAP] (GC_unmap_old): Go on with unmapping also if
GC_gc_no==hb_last_reclaimed+GC_unmap_threshold; update comment.
* configure.ac (munmap): Update help string (including change of the
default value from 6 to 7).
* configure.ac [$enable_munmap!=no] (MUNMAP_THRESHOLD): Likewise.
* doc/README.environment (GC_UNMAP_THRESHOLD): Update documentation.
* doc/README.macros (MUNMAP_THRESHOLD): Likewise.
* include/private/gcconfig.h [USE_MUNMAP && !MUNMAP_THRESHOLD
&& (SN_TARGET_PS3 || SN_TARGET_PSP2 || MSWIN_XBOX1)]
(MUNMAP_THRESHOLD): Change value from 2 to 3.

5 files changed, 19 insertions, 17 deletions

diff --git a/allchblk.c b/allchblk.c
index 37cbdada..d03ef174 100644
--- a/allchblk.c
+++ b/allchblk.c
@@ -402,7 +402,7 @@ STATIC void GC_add_to_fl(struct hblk *h, hdr *hhdr)
 #ifdef USE_MUNMAP
 
 #   ifndef MUNMAP_THRESHOLD
-#     define MUNMAP_THRESHOLD 6
+#     define MUNMAP_THRESHOLD 7
 #   endif
 
 GC_INNER int GC_unmap_threshold = MUNMAP_THRESHOLD;
@@ -484,10 +484,10 @@ GC_INNER void GC_unmap_old(void)
         hhdr = HDR(h);
         if (!IS_MAPPED(hhdr)) continue;
 
-        /* Check that the interval is larger than the threshold (the    */
-        /* truncated counter value wrapping is handled correctly).      */
-        if ((unsigned short)(GC_gc_no - hhdr->hb_last_reclaimed) >
-                (unsigned short)GC_unmap_threshold) {
+        /* Check that the interval is not smaller than the threshold.   */
+        /* The truncated counter value wrapping is handled correctly.   */
+        if ((unsigned short)(GC_gc_no - hhdr->hb_last_reclaimed)
+            >= (unsigned short)GC_unmap_threshold) {
 #         ifdef COUNT_UNMAPPED_REGIONS
             /* Continue with unmapping the block only if it will not    */
             /* create too many unmapped regions, or if unmapping        */
diff --git a/configure.ac b/configure.ac
index 0962f51f..094b5b16 100644
--- a/configure.ac
+++ b/configure.ac
@@ -922,8 +922,8 @@ AC_ARG_ENABLE(mmap,
 
 AC_ARG_ENABLE(munmap,
     [AS_HELP_STRING([--enable-munmap=N],
-                    [return page to the OS if empty for N collections
-                     (default: 6)])],
+                    [return page to the OS if page is marked as
+                     empty during N collections (default: 7)])],
     MUNMAP_THRESHOLD=$enableval)
 if test x$enable_munmap != xno; then
     AC_DEFINE([USE_MMAP], 1,
@@ -935,10 +935,11 @@ if test x$enable_munmap != xno; then
               [Define to return memory to OS with munmap calls
                (see doc/README.macros).])
     if test x$MUNMAP_THRESHOLD = x -o x$MUNMAP_THRESHOLD = xyes; then
-      MUNMAP_THRESHOLD=6
+      MUNMAP_THRESHOLD=7
     fi
     AC_DEFINE_UNQUOTED([MUNMAP_THRESHOLD], [${MUNMAP_THRESHOLD}],
-        [Number of GC cycles to wait before unmapping an unused block.])
+        [Number of sequential garbage collections during those a candidate
+         block for unmapping should be marked as free.])
 else
     if test "${gc_use_mmap}" = "yes"; then
       AC_DEFINE([USE_MMAP], 1,
diff --git a/doc/README.environment b/doc/README.environment
index 16e527d4..d6423ad2 100644
--- a/doc/README.environment
+++ b/doc/README.environment
@@ -147,10 +147,10 @@ GC_FREE_SPACE_DIVISOR - Set GC_free_space_divisor to the indicated value.
                       Setting it to larger values decreases space consumption
                       and increases GC frequency.
 
-GC_UNMAP_THRESHOLD - Set the desired memory blocks unmapping threshold (the
-                   number of sequential garbage collections for which
-                   a candidate block for unmapping should remain free).  The
-                   special value "0" completely disables unmapping.
+GC_UNMAP_THRESHOLD - Set the desired threshold of memory blocks unmapping
+                   (the number of sequential garbage collections during those
+                   a candidate block for unmapping should be marked as free).
+                   The special value "0" completely disables unmapping.
 
 GC_FORCE_UNMAP_ON_GCOLLECT - Turn "unmap as much as possible on explicit GC"
                 mode on (overrides the default value).  Has no effect on
diff --git a/doc/README.macros b/doc/README.macros
index e013566c..887b3240 100644
--- a/doc/README.macros
+++ b/doc/README.macros
@@ -312,9 +312,10 @@ USE_MUNMAP      Causes memory to be returned to the OS under the right
 USE_WINALLOC (Cygwin only)   Use Win32 VirtualAlloc (instead of sbrk or mmap)
   to get new memory.  Useful if memory unmapping (USE_MUNMAP) is enabled.
 
-MUNMAP_THRESHOLD=<value>        Set the desired memory blocks unmapping
-  threshold (the number of sequential garbage collections for which
-  a candidate block for unmapping should remain free).
+MUNMAP_THRESHOLD=<value>        Set the default threshold of memory blocks
+  unmapping (the number of sequential garbage collections during those
+  a candidate block for unmapping should be marked as free).  The special
+  value "0" completely disables unmapping.
 
 GC_FORCE_UNMAP_ON_GCOLLECT      Set "unmap as much as possible on explicit GC"
   mode on by default.  The mode could be changed at run-time.  Has no effect
diff --git a/include/private/gcconfig.h b/include/private/gcconfig.h
index 694b66d6..ec849a56 100644
--- a/include/private/gcconfig.h
+++ b/include/private/gcconfig.h
@@ -2755,7 +2755,7 @@ EXTERN_C_BEGIN
 #if defined(USE_MUNMAP) && !defined(MUNMAP_THRESHOLD) \
     && (defined(SN_TARGET_PS3) \
         || defined(SN_TARGET_PSP2) || defined(MSWIN_XBOX1))
-# define MUNMAP_THRESHOLD 2
+# define MUNMAP_THRESHOLD 3
 #endif
 
 #if defined(USE_MUNMAP) && defined(COUNT_UNMAPPED_REGIONS) \