compaction: avoid excessive reallocation and during input list format…

…ting

with off-strategy, input list size can be close to 1k, which will
lead to unneeded reallocations when formatting the list for
logging.

in the past, we faced stalls in this area, and excessive reallocation
(log2 ~1k = ~10) may have contributed to that.

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

Closes #13907

(cherry picked from commit 5544d12)

Fixes #14071

compaction/compaction.cc

@@ -414,9 +414,12 @@ struct compaction_read_monitor_generator final : public read_monitor_generator {
 
 class formatted_sstables_list {
     bool _include_origin = true;
-    std::vector<sstring> _ssts;
+    std::vector<std::string> _ssts;
 public:
     formatted_sstables_list() = default;
+    void reserve(size_t n) {
+        _ssts.reserve(n);
+    }
     explicit formatted_sstables_list(const std::vector<shared_sstable>& ssts, bool include_origin) : _include_origin(include_origin) {
         _ssts.reserve(ssts.size());
         for (const auto& sst : ssts) {
@@ -435,9 +438,7 @@ class formatted_sstables_list {
 };
 
 std::ostream& operator<<(std::ostream& os, const formatted_sstables_list& lst) {
-    os << "[";
-    os << boost::algorithm::join(lst._ssts, ",");
-    os << "]";
+    fmt::print(os, "[{}]", fmt::join(lst._ssts, ","));
     return os;
 }
 
@@ -641,6 +642,7 @@ class compaction {
     future<> setup() {
         auto ssts = make_lw_shared<sstables::sstable_set>(make_sstable_set_for_input());
         formatted_sstables_list formatted_msg;
+        formatted_msg.reserve(_sstables.size());
         auto fully_expired = _table_s.fully_expired_sstables(_sstables, gc_clock::now());
         min_max_tracker<api::timestamp_type> timestamp_tracker;