From b77e6704c760532f6e3fd7990c937f413740aefa Mon Sep 17 00:00:00 2001
From: Adam Kupczyk <akupczyk@redhat.com>
Date: Mon, 25 Jan 2021 11:33:28 +0100
Subject: [PATCH] common/mempool: Add test for mempool shards

Add test that checks quality of mempool sharing.
It refuses to accept cases when variance is too large.

Signed-off-by: Adam Kupczyk <akupczyk@redhat.com>
(cherry picked from commit e4e07de8f36d0d44b341d5784a8bf56192201b95)
---
 src/include/mempool.h    |  9 +++++++--
 src/test/test_mempool.cc | 27 +++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/src/include/mempool.h b/src/include/mempool.h
index 2247cb15bc0aa..a18f6a502daad 100644
--- a/src/include/mempool.h
+++ b/src/include/mempool.h
@@ -255,11 +255,16 @@ public:
 
   void adjust_count(ssize_t items, ssize_t bytes);
 
-  shard_t* pick_a_shard() {
+  static size_t pick_a_shard_int() {
     // Dirt cheap, see:
-    //   http://fossies.org/dox/glibc-2.24/pthread__self_8c_source.html
+    //   https://fossies.org/dox/glibc-2.32/pthread__self_8c_source.html
     size_t me = (size_t)pthread_self();
     size_t i = (me >> 3) & ((1 << num_shard_bits) - 1);
+    return i;
+  }
+
+  shard_t* pick_a_shard() {
+    size_t i = pick_a_shard_int();
     return &shard[i];
   }
 
diff --git a/src/test/test_mempool.cc b/src/test/test_mempool.cc
index f4c2abb7d3b0b..99b87d11f265a 100644
--- a/src/test/test_mempool.cc
+++ b/src/test/test_mempool.cc
@@ -402,6 +402,33 @@ TEST(mempool, btree_map_test)
   ASSERT_EQ(0, mempool::osd::allocated_bytes());
 }
 
+TEST(mempool, check_shard_select)
+{
+  const size_t samples = 100;
+  std::atomic_int shards[mempool::num_shards] = {0};
+  std::vector<std::thread> workers;
+  for (size_t i = 0; i < samples; i++) {
+    workers.push_back(
+      std::thread([&](){
+          size_t i = mempool::pool_t::pick_a_shard_int();
+          shards[i]++;
+        }));
+  }
+  for (auto& t:workers) {
+    t.join();
+  }
+  workers.clear();
+
+  double EX = (double)samples / (double)mempool::num_shards;
+  double VarX = 0;
+  for (size_t i = 0; i < mempool::num_shards; i++) {
+    VarX += (EX - shards[i]) * (EX - shards[i]);
+  }
+  //random gives VarX below 200
+  //when half slots are 0, we get ~300
+  //when all samples go into one slot, we get ~9000
+  EXPECT_LT(VarX, 200);
+}
 
 
 int main(int argc, char **argv)
-- 
2.39.5