Increase allocation granularity dynamically with each MMAP call (#32)

* Increase allocation granularity dynamically with each MMAP call

* Fewer MMAP calls required when workload contains several objects.

* Delay hitting the per-process file descriptor constraint.

* Change type of GRANULARITY_MULTIPLIER

* Make granularity update more concise.

src/malloc.c

@@ -12,13 +12,15 @@
 void *fake_mmap(size_t);
 int fake_munmap(void *, size_t);
 
+size_t dlmalloc_granularity = ((size_t) 128U * 1024U);
+
 #define MMAP(s) fake_mmap(s)
 #define MUNMAP(a, s) fake_munmap(a, s)
 #define DIRECT_MMAP(s) fake_mmap(s)
 #define DIRECT_MUNMAP(a, s) fake_munmap(a, s)
 #define USE_DL_PREFIX
 #define HAVE_MORECORE 0
-#define DEFAULT_GRANULARITY ((size_t) 1U << 25)
+#define DEFAULT_GRANULARITY (dlmalloc_granularity)
 
 #include "thirdparty/dlmalloc.c"
 
@@ -42,6 +44,8 @@ struct mmap_record {
 struct mmap_record *records_by_fd = NULL;
 struct mmap_record *records_by_pointer = NULL;
 
+const int GRANULARITY_MULTIPLIER = 2;
+
 /* Create a buffer. This is creating a temporary file and then
  * immediately unlinking it so we do not leave traces in the system. */
 int create_buffer(int64_t size) {
@@ -79,6 +83,10 @@ void *fake_mmap(size_t size) {
     return pointer;
   }
 
+  /* Update dlmalloc's allocation granularity for future calls */
+  dlmalloc_granularity *= GRANULARITY_MULTIPLIER;
+  dlmallopt(M_GRANULARITY, dlmalloc_granularity);
+
   struct mmap_record *record = malloc(sizeof(struct mmap_record));
   record->fd = fd;
   record->pointer = pointer;