fair_queue: Export the number of times class was activated

Activation of a class happens when it receives a request into its empty queue. When activated, class also updates its "accumulator" that's responsible for fair-selection of classes according to its shares. Each activation a class may lose some accumulator point, thus losing some portion of the disk capacity. Knowing the activation rate is very useful, right now we only have some side ways to get an idea of this counter. Signed-off-by: Pavel Emelyanov <[email protected]>
scylladb · Aug 25, 2024 · 83e6cdf · 83e6cdf
1 parent 3c70194
commit 83e6cdf
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diff --git a/apps/io_tester/io_tester.cc b/apps/io_tester/io_tester.cc
@@ -651,6 +651,7 @@ class io_class_data : public class_data {
         emit_one_metrics(out, "io_queue_starvation_time_sec");
         emit_one_metrics(out, "io_queue_consumption");
         emit_one_metrics(out, "io_queue_adjusted_consumption");
+        emit_one_metrics(out, "io_queue_activations");
     }
 
 public:

diff --git a/src/core/fair_queue.cc b/src/core/fair_queue.cc
@@ -140,6 +140,7 @@ class fair_queue::priority_class_data {
     fair_queue_entry::container_list_t _queue;
     bool _queued = false;
     bool _plugged = true;
+    uint32_t _activations = 0;
 
 public:
     explicit priority_class_data(uint32_t shares) noexcept : _shares(std::max(shares, 1u)) {}
@@ -190,6 +191,7 @@ void fair_queue::push_priority_class_from_idle(priority_class_data& pc) noexcept
         _handles.assert_enough_capacity();
         _handles.push(&pc);
         pc._queued = true;
+        pc._activations++;
     }
 }
 
@@ -396,6 +398,9 @@ std::vector<seastar::metrics::impl::metric_definition_impl> fair_queue::metrics(
             sm::make_counter("adjusted_consumption",
                     [&pc] { return fair_group::capacity_tokens(pc._accumulated); },
                     sm::description("Consumed disk capacity units adjusted for class shares and idling preemption")),
+            sm::make_counter("activations",
+                    [&pc] { return pc._activations; },
+                    sm::description("The number of times the class was woken up from idle")),
     });
 }