From 40b6616d3d75e784ac2f7a7091700f9e5e09f38e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Botond=20D=C3=A9nes?= <bdenes@scylladb.com>
Date: Tue, 10 Sep 2024 10:32:23 -0400
Subject: [PATCH] reader_concurrency_semaphore: add bottleneck self-diagnosis
 to diagnosis dump

There are a few typical cases of bottlenecks, which can be easily
identified when dumping the semaphore diagnostics. Identify and print
these to fast-track investigations.
---
 reader_concurrency_semaphore.cc | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/reader_concurrency_semaphore.cc b/reader_concurrency_semaphore.cc
index 0d6e755a11..d28ab1db99 100644
--- a/reader_concurrency_semaphore.cc
+++ b/reader_concurrency_semaphore.cc
@@ -809,6 +809,21 @@ static void do_dump_reader_permit_diagnostics(std::ostream& os, const reader_con
             permit->get_state());
     }
 
+    std::vector<sstring> bottlenecks;
+    if (semaphore.get_stats().need_cpu_permits > 0) {
+        bottlenecks.push_back("CPU");
+    }
+    if (semaphore.available_resources().memory <= 0) {
+        bottlenecks.push_back("memory");
+    }
+    if (semaphore.available_resources().count <= 0) {
+        bottlenecks.push_back("disk");
+    }
+
+    if (!bottlenecks.empty()) {
+        fmt::print(os, "Identified bottleneck(s): {}\n", fmt::join(bottlenecks, ", "));
+    }
+
     fmt::print(os, "\n");
     total += do_dump_reader_permit_diagnostics(os, permits, max_lines);
     fmt::print(os, "\n");