Add --cpus to specify max cpus to be used

Sometimes the default behaviour of using all CPUs can
be a bit hostile when the dataset is large.
Let's allow to reduce that number with a new parameter --cpus / -m

src/pcp2pdf/__main__.py

@@ -44,6 +44,8 @@ def __init__(self):
         self.labels = {}
         self.dpi = None
         self.histogram = False
+        # max nr of CPUs to use. None == All available CPUs
+        self.max_cpus = None
         self.opts = self.setup()
         configfiles = []
         path = os.path.join(pmapi.pmContext.pmGetConfig('PCP_SYSCONF_DIR'),
@@ -101,6 +103,7 @@ def setup(self):
         opts.pmSetLongOption("dpi", 1, 'd', '', "Sets the DPI used to create the images for the graph. Default is 200")
         opts.pmSetLongOptionText(t + "unless overridden in the configuration. The lower the value, the less memory the process will need")
         opts.pmSetLongOptionText(t + "and the less quality the graphs will have.")
+        opts.pmSetLongOption("cpus", 1, 'm', '', "Maximum nr of CPUs to use when rendering (default: all available CPUs)")
         opts.pmSetLongOptionStart()
         opts.pmSetLongOptionFinish()
         opts.pmSetLongOptionInterval()
@@ -135,6 +138,11 @@ def option_callback(self, opt, optarg, index):
                 sys.exit(1)
         elif opt == "t":
             self.interval = optarg
+        elif opt == "m":
+            if int(optarg) == 0:
+                self.max_cpus = None
+            else:
+                self.max_cpus = int(optarg)
         elif opt == "e":
             self.exclude.append(optarg)
         elif opt == "c":

src/pcp2pdf/stats.py

@@ -853,7 +853,7 @@ def output(self):
         progress_total = len(self.all_graphs)
         # Set this to False to disable multiprocessing
         if True:
-            pool = multiprocessing.Pool(None)
+            pool = multiprocessing.Pool(self.opts.max_cpus)
             l = zip(itertools.repeat(self), self.all_graphs)
             metrics_rets = pool.map(graph_wrapper, l)
             (metrics, rets) = zip(*metrics_rets)