[DVFS] Change pstate selection for conservative gov

[simgrid.git] / src / surf / plugins / host_dvfs.cpp

diff --git a/src/surf/plugins/host_dvfs.cpp b/src/surf/plugins/host_dvfs.cpp
index 5ff2dcb..fd3a238 100644 (file)
--- a/src/surf/plugins/host_dvfs.cpp
+++ b/src/surf/plugins/host_dvfs.cpp
@@ -61,6 +61,16 @@ public:
   double samplingRate() { return sampling_rate; }
 };
 
+/**
+ * The linux kernel doc describes this governor as follows:
+ * https://www.kernel.org/doc/Documentation/cpu-freq/governors.txt
+ *
+ * > The CPUfreq governor "performance" sets the CPU statically to the
+ * > highest frequency within the borders of scaling_min_freq and
+ * > scaling_max_freq.
+ *
+ * We do not support scaling_min_freq/scaling_max_freq -- we just pick the lowest frequency.
+ */
 class Performance : public Governor {
 public:
   explicit Performance(simgrid::s4u::Host* ptr) : Governor(ptr) {}
@@ -69,6 +79,16 @@ public:
   std::string getName() override { return "Performance"; }
 };
 
+/**
+ * The linux kernel doc describes this governor as follows:
+ * https://www.kernel.org/doc/Documentation/cpu-freq/governors.txt
+ *
+ * > The CPUfreq governor "powersave" sets the CPU statically to the
+ * > lowest frequency within the borders of scaling_min_freq and
+ * > scaling_max_freq.
+ *
+ * We do not support scaling_min_freq/scaling_max_freq -- we just pick the lowest frequency.
+ */
 class Powersave : public Governor {
 public:
   explicit Powersave(simgrid::s4u::Host* ptr) : Governor(ptr) {}
@@ -77,8 +97,21 @@ public:
   std::string getName() override { return "Powersave"; }
 };
 
+/**
+ * The linux kernel doc describes this governor as follows:
+ * https://www.kernel.org/doc/Documentation/cpu-freq/governors.txt
+ *
+ * > The CPUfreq governor "ondemand" sets the CPU frequency depending on the
+ * > current system load. [...] when triggered, cpufreq checks
+ * > the CPU-usage statistics over the last period and the governor sets the
+ * > CPU accordingly.
+ */
 class OnDemand : public Governor {
-  double freq_up_threshold = 0.95;
+  /**
+   * See https://elixir.bootlin.com/linux/v4.15.4/source/drivers/cpufreq/cpufreq_ondemand.c 
+   * DEF_FREQUENCY_UP_THRESHOLD and od_update()
+   */
+  double freq_up_threshold = 0.80;
 
 public:
   explicit OnDemand(simgrid::s4u::Host* ptr) : Governor(ptr) {}
@@ -86,12 +119,12 @@ public:
   std::string getName() override { return "OnDemand"; }
   void update() override
   {
-    double load = sg_host_get_current_load(host);
+    double load = host->getCoreCount() * sg_host_get_avg_load(host);
+    sg_host_load_reset(host); // Only consider the period between two calls to this method!
 
-    // FIXME I don't like that we multiply with the getCoreCount() just here...
-    if (load*host->getCoreCount() > freq_up_threshold) {
+    if (load > freq_up_threshold) {
       host->setPstate(0); /* Run at max. performance! */
-      XBT_INFO("Load: %f > threshold: %f --> changed to pstate %i", load * host->getCoreCount(), freq_up_threshold, 0);
+      XBT_INFO("Load: %f > threshold: %f --> changed to pstate %i", load, freq_up_threshold, 0);
     } else {
       /* The actual implementation uses a formula here: (See Kernel file cpufreq_ondemand.c:158)
        *
@@ -101,14 +134,28 @@ public:
        * lowest_pstate - load*pstatesCount()
        */
       int max_pstate = host->getPstatesCount() - 1;
-      int new_pstate = max_pstate - load * max_pstate;
+      // Load is now < freq_up_threshold; exclude pstate 0 (the fastest)
+      // because pstate 0 can only be selected if load > freq_up_threshold
+      int new_pstate = max_pstate - load * (max_pstate + 1);
       host->setPstate(new_pstate);
 
-      XBT_DEBUG("Load: %f --> changed to pstate %i", load*host->getCoreCount(), new_pstate);
+      XBT_DEBUG("Load: %f < threshold: %f --> changed to pstate %i", load, freq_up_threshold, new_pstate);
     }
   }
 };
 
+/**
+ * This is the conservative governor, which is very similar to the
+ * OnDemand governor. The Linux Kernel Documentation describes it
+ * very well, see https://www.kernel.org/doc/Documentation/cpu-freq/governors.txt:
+ *
+ * > The CPUfreq governor "conservative", much like the "ondemand"
+ * > governor, sets the CPU frequency depending on the current usage.  It
+ * > differs in behaviour in that it gracefully increases and decreases the
+ * > CPU speed rather than jumping to max speed the moment there is any load
+ * > on the CPU. This behaviour is more suitable in a battery powered
+ * > environment.
+ */
 class Conservative : public Governor {
   double freq_up_threshold   = .8;
   double freq_down_threshold = .2;
@@ -119,8 +166,9 @@ public:
   virtual std::string getName() override { return "Conservative"; }
   virtual void update() override
   {
-    double load = sg_host_get_current_load(host)*host->getCoreCount();
+    double load = host->getCoreCount() * sg_host_get_avg_load(host);
     int pstate  = host->getPstate();
+    sg_host_load_reset(host); // Only consider the period between two calls to this method!
 
     if (load > freq_up_threshold) {
       if (pstate != 0) {