From 8a5c1f52a77bc92320c90ecbe5424e841874296e Mon Sep 17 00:00:00 2001
From: Moustafa Maher <m.maher@10gen.com>
Date: Thu, 26 Aug 2021 19:23:39 +0000
Subject: SERVER-34938 Secondary slowdown or hang due to content pinned in
 cache by single oplog batch

---
 src/mongo/db/repl/SConscript        | 1 +
 src/mongo/db/repl/oplog_applier.cpp | 8 +++++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/mongo/db/repl/SConscript b/src/mongo/db/repl/SConscript
index 39ecc27ce01..41040f145ba 100644
--- a/src/mongo/db/repl/SConscript
+++ b/src/mongo/db/repl/SConscript
@@ -677,6 +677,7 @@ env.Library(
         '$BUILD_DIR/mongo/db/namespace_string',
         '$BUILD_DIR/mongo/executor/task_executor_interface',
         '$BUILD_DIR/mongo/util/concurrency/thread_pool',
+        '$BUILD_DIR/mongo/util/processinfo',
         'oplog_entry',
     ],
     LIBDEPS_PRIVATE=[
diff --git a/src/mongo/db/repl/oplog_applier.cpp b/src/mongo/db/repl/oplog_applier.cpp
index 34be41fda71..92b2f84db27 100644
--- a/src/mongo/db/repl/oplog_applier.cpp
+++ b/src/mongo/db/repl/oplog_applier.cpp
@@ -40,6 +40,7 @@
 #include "mongo/db/repl/repl_server_parameters_gen.h"
 #include "mongo/db/repl/sync_tail.h"
 #include "mongo/util/log.h"
+#include "mongo/util/processinfo.h"
 #include "mongo/util/time_support.h"
 
 namespace mongo {
@@ -53,7 +54,12 @@ std::unique_ptr<ThreadPool> OplogApplier::makeWriterPool() {
         severe() << "replWriterMinThreadCount must be less than or equal to replWriterThreadCount.";
         fassertFailedNoTrace(5605400);
     }
-    return makeWriterPool(replWriterThreadCount);
+
+    // Reduce content pinned in cache by single oplog batch on small machines by reducing the number
+    // of threads of ReplWriter to reduce the number of concurrent open WT transactions.
+    auto numberOfThreads =
+        std::min(replWriterThreadCount, 2 * static_cast<int>(ProcessInfo::getNumAvailableCores()));
+    return makeWriterPool(numberOfThreads);
 }
 
 // static
-- 
cgit v1.2.1