Don't spawn a new thread pool for every sample

Summary:
below was creating and destroying a large number of very short lived
threads. This was creating enough threads that just the creation of
the threads was consuming 0.1% CPU and spawning ~60 threads a minute
on average.

Reviewed By: dschatzberg

Differential Revision: D45800206

fbshipit-source-id: c56c3eb224c1ea413fbd2e44d555872aea3ef368

below/model/src/collector.rs

@@ -59,6 +59,7 @@ impl Default for CollectorOptions {
 /// Collects data samples and maintains the latest data
 pub struct Collector {
     logger: slog::Logger,
+    proc_reader: procfs::ProcReader,
     prev_sample: Option<(Sample, Instant)>,
     collector_options: CollectorOptions,
 }
@@ -67,13 +68,14 @@ impl Collector {
     pub fn new(logger: slog::Logger, collector_options: CollectorOptions) -> Self {
         Self {
             logger,
+            proc_reader: procfs::ProcReader::new(),
             prev_sample: None,
             collector_options,
         }
     }
 
-    pub fn collect_sample(&self) -> Result<Sample> {
-        collect_sample(&self.logger, &self.collector_options)
+    pub fn collect_sample(&mut self) -> Result<Sample> {
+        collect_sample(&self.logger, &mut self.proc_reader, &self.collector_options)
     }
 
     /// Collect a new `Sample`, returning an updated Model
@@ -171,8 +173,11 @@ fn is_all_zero_disk_stats(disk_stats: &procfs::DiskStat) -> bool {
         && disk_stats.time_spend_discard_ms == Some(0)
 }
 
-fn collect_sample(logger: &slog::Logger, options: &CollectorOptions) -> Result<Sample> {
-    let mut reader = procfs::ProcReader::new();
+fn collect_sample(
+    logger: &slog::Logger,
+    reader: &mut procfs::ProcReader,
+    options: &CollectorOptions,
+) -> Result<Sample> {
     let btrfs_reader =
         btrfs::BtrfsReader::new(options.btrfs_samples, options.btrfs_min_pct, logger.clone());
     let ethtool_reader = ethtool::EthtoolReader::new();

below/src/main.rs

@@ -1002,7 +1002,7 @@ fn record(
         None
     };
 
-    let collector = model::Collector::new(
+    let mut collector = model::Collector::new(
         logger.clone(),
         model::CollectorOptions {
             cgroup_root: below_config.cgroup_root.clone(),