[PATCH 2/3] implement dynamic mmap threshold

[Eric Wong <e@80x24.org>]

mmap is an expensive operation, so follow glibc's lead and
adjust the mmap threshold dynamically.  Unlike glibc, there's no
way to disable this at the moment.  I don't think disabling is
worth the effort since avoiding mmap is the default in glibc and
few users change it.  Furthermore, avoiding mmap lets us test
the effectiveness of the prior size classes commit at reducing
fragmentation.
---
 dlmalloc_c.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/dlmalloc_c.h b/dlmalloc_c.h
index e842a53..348c194 100644
--- a/dlmalloc_c.h
+++ b/dlmalloc_c.h
@@ -5623,6 +5623,8 @@ void* mspace_malloc(mspace msp, size_t bytes) {
 }
 
 void mspace_free(mspace msp, void* mem) {
+  const size_t mm_thresh_max = 1 << (sizeof(void *) == 4 ? 19U : 25U);
+
   if (mem != 0) {
     mchunkptr p  = mem2chunk(mem);
 #if FOOTERS && 0 /* mwrap called get_mstate_for */
@@ -5643,7 +5645,15 @@ void mspace_free(mspace msp, void* mem) {
         if (!pinuse(p)) {
           size_t prevsize = p->prev_foot;
           if (is_mmapped(p)) {
+	    size_t asize = psize;
+
             psize += prevsize + MMAP_FOOT_PAD;
+
+            /* adjust dynamic mmap threshold, similar to glibc */
+            if (asize > mparams.mmap_threshold && asize <= mm_thresh_max) {
+              mparams.mmap_threshold = asize;
+              mparams.trim_threshold = asize * 2;
+            }
             if (CALL_MUNMAP((char*)p - prevsize, psize) == 0)
               fm->footprint -= psize;
             goto postaction;