Free sort indices and task arrays before repartitioning

Freeing the sort indices and tasks arrays before repartitioning makes that memory available so that we have more room for accepting particles from other nodes.

Part of #98 (closed).

Also adds a useful debug function that reports the current memory usage of the process tree (i.e. call from main thread to get all usage). Same numbers you get out of the top program.

Free the task arrays and cells are refactored into simple functions to cut down on cut and paste.

src/debug.c

@@ -26,6 +26,7 @@
 /* Some standard headers. */
 #include <float.h>
 #include <stdio.h>
+#include <unistd.h>
 
 /* This object's header. */
 #include "debug.h"
@@ -450,3 +451,69 @@ void dumpCellRanks(const char *prefix, struct cell *cells_top, int nr_cells) {
 }
 
 #endif /* HAVE_MPI */
+
+/**
+ * @brief parse the process /proc/self/statm file to get the process
+ *        memory use (in KB). Top field in ().
+ *
+ * @param size     total virtual memory (VIRT)
+ * @param resident resident non-swapped memory (RES)
+ * @param share    shared (mmap'd) memory  (SHR)
+ * @param trs      text (exe) resident set (CODE)
+ * @param lrs      library resident set
+ * @param drs      data+stack resident set (DATA)
+ * @param dt       dirty pages (nDRT)
+ */
+void getProcMemUse(long *size, long *resident, long *share, long *trs,
+                   long *lrs, long *drs, long *dt) {
+
+  /* Open the file. */
+  FILE *file = fopen("/proc/self/statm", "r");
+  if (file != NULL) {
+    int nscan = fscanf(file, "%ld %ld %ld %ld %ld %ld %ld", size, resident,
+                       share, trs, lrs, drs, dt);
+
+    if (nscan == 7) {
+      /* Convert pages into bytes. Usually 4096, but could be 512 on some
+       * systems so take care in conversion to KB. */
+      long sz = sysconf(_SC_PAGESIZE);
+      *size *= sz;
+      *resident *= sz;
+      *share *= sz;
+      *trs *= sz;
+      *lrs *= sz;
+      *drs *= sz;
+      *dt *= sz;
+
+      *size /= 1024;
+      *resident /= 1024;
+      *share /= 1024;
+      *trs /= 1024;
+      *lrs /= 1024;
+      *drs /= 1024;
+      *dt /= 1024;
+    } else {
+      error("Failed to read sufficient fields from /proc/self/statm");
+    }
+    fclose(file);
+  } else {
+    error("Failed to open /proc/self/statm");
+  }
+}
+
+/**
+ * @brief Print the current memory use of the process. A la "top".
+ */
+void printProcMemUse() {
+  long size;
+  long resident;
+  long share;
+  long trs;
+  long lrs;
+  long drs;
+  long dt;
+  getProcMemUse(&size, &resident, &share, &trs, &lrs, &drs, &dt);
+  printf("## VIRT = %ld , RES = %ld , SHR = %ld , CODE = %ld, DATA = %ld\n",
+          size, resident, share, trs, drs);
+  fflush(stdout);
+}