Add JK to update concurrency levels

Summary: We've been experiencing CPU spikes during start up of the bookmark service, and tried to fix it by adjusting the concurrency level during repo construction. Since there is a tradeoff between time to start and CPU level, having this value in an JK will make it easier to modify in the future.

Reviewed By: mzr

Differential Revision: D90111305

fbshipit-source-id: a5ce591a0e034dd8ab8fc91fd12fd86d808544cd

Author: Jose García Gimeno

eden/mononoke/cmdlib/mononoke_app/src/repos_manager.rs

@@ -43,6 +43,11 @@ use repo_factory::RepoFactoryBuilder;
 use stats::prelude::*;
 use tracing::info;
 
+fn repos_manager_concurrency() -> Result<usize> {
+    justknobs::get_as::<usize>("scm/mononoke:repos_manager_concurrency", None)
+        .context("Failed to read scm/mononoke:repos_manager_concurrency JustKnob")
+}
+
 define_stats! {
     prefix = "mononoke.app";
     initialization_time_millisecs: dynamic_timeseries(
@@ -219,8 +224,8 @@ impl<Repo> MononokeReposManager<Repo> {
                     anyhow::Ok((repo_id, repo_name, repo))
                 })
             })
-            // Repo construction can be heavy, 10 at a time is sufficient.
-            .buffer_unordered(10)
+            // Repo construction can be heavy, limit concurrency via JK.
+            .buffer_unordered(repos_manager_concurrency()?)
             .map(|r| anyhow::Ok(r??))
             .try_collect::<Vec<_>>()
             .await?;
@@ -358,8 +363,8 @@ where
                     anyhow::Ok((repo_id, repo_name, repo))
                 })
             })
-            // Repo construction can be heavy, 10 at a time is sufficient.
-            .buffer_unordered(10)
+            // Repo construction can be heavy, limit concurrency via JK.
+            .buffer_unordered(repos_manager_concurrency()?)
             .map(|r| anyhow::Ok(r??))
             .try_collect::<Vec<_>>()
             .await?;

eden/mononoke/common/mononoke_macros/just_knobs_defaults/just_knobs.json

@@ -108,6 +108,7 @@
     "scm/mononoke:retry_query_from_replica_with_consistency_check_interval_ms": 200,
     "scm/mononoke:retry_query_from_replica_with_consistency_check_hlc_drift_tolerance_ns": 0,
     "scm/mononoke:cgdm_reloading_interval_secs": 5,
-    "scm/mononoke:shardmanager_shutdown_timeout_secs": 10
+    "scm/mononoke:shardmanager_shutdown_timeout_secs": 10,
+    "scm/mononoke:repos_manager_concurrency": 30
   }
 }