compaction_manager: periodically submit cfs for compaction

This is useful for a column family which isn't generating new content
and will have lots of expired data later on that can be purged.
Compaction submission is NO-OP if there's nothing to do, so I think
it's reasonable to do it at an interval of 1 hour.

Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>

sstables/compaction_manager.cc

@@ -344,6 +344,15 @@ void compaction_manager::register_metrics() {
 void compaction_manager::start() {
     _stopped = false;
     register_metrics();
+    _compaction_submission_timer.arm(periodic_compaction_submission_interval);
+}
+
+std::function<void()> compaction_manager::compaction_submission_callback() {
+    return [this] () mutable {
+        for (auto& e: _compaction_locks) {
+            submit(e.first);
+        }
+    };
 }
 
 future<> compaction_manager::stop() {
@@ -367,6 +376,7 @@ future<> compaction_manager::stop() {
         });
     }).then([this] {
         _weight_tracker.clear();
+        _compaction_submission_timer.cancel();
         cmlog.info("Stopped");
         return make_ready_future<>();
     });

sstables/compaction_manager.hh

@@ -82,6 +82,12 @@ private:
     semaphore _major_compaction_sem{1};
     // Prevents column family from running major and minor compaction at same time.
     std::unordered_map<column_family*, rwlock> _compaction_locks;
+
+    std::function<void()> compaction_submission_callback();
+    // all registered column families are submitted for compaction at a constant interval.
+    // Submission is a NO-OP when there's nothing to do, so it's fine to call it regularly.
+    timer<lowres_clock> _compaction_submission_timer = timer<lowres_clock>(compaction_submission_callback());
+    static constexpr std::chrono::seconds periodic_compaction_submission_interval = std::chrono::seconds(3600);
 private:
     future<> task_stop(lw_shared_ptr<task> task);