SERVER-56054: Change minThreads value for replication writer thread pool to 0

(cherry picked from commit 136fa52193c342038b3fa35152fa1ed3dee4ee87)
(cherry picked from commit 4935fe56743814ffc39e9f7d9eebb3648dc2846f)

3 files changed, 24 insertions, 6 deletions

diff --git a/src/mongo/db/repl/oplog_applier.cpp b/src/mongo/db/repl/oplog_applier.cpp
index f72639fdeba..34be41fda71 100644
--- a/src/mongo/db/repl/oplog_applier.cpp
+++ b/src/mongo/db/repl/oplog_applier.cpp
@@ -49,6 +49,10 @@ using CallbackArgs = executor::TaskExecutor::CallbackArgs;
 
 // static
 std::unique_ptr<ThreadPool> OplogApplier::makeWriterPool() {
+    if (replWriterThreadCount < replWriterMinThreadCount) {
+        severe() << "replWriterMinThreadCount must be less than or equal to replWriterThreadCount.";
+        fassertFailedNoTrace(5605400);
+    }
     return makeWriterPool(replWriterThreadCount);
 }
 
@@ -57,7 +61,9 @@ std::unique_ptr<ThreadPool> OplogApplier::makeWriterPool(int threadCount) {
     ThreadPool::Options options;
     options.threadNamePrefix = "repl-writer-worker-";
     options.poolName = "repl writer worker Pool";
-    options.maxThreads = options.minThreads = static_cast<size_t>(threadCount);
+    options.minThreads =
+        replWriterMinThreadCount < threadCount ? replWriterMinThreadCount : threadCount;
+    options.maxThreads = static_cast<size_t>(threadCount);
     options.onCreateThread = [](const std::string&) {
         Client::initThread(getThreadName());
         AuthorizationSession::get(cc())->grantInternalAuthorization(&cc());
diff --git a/src/mongo/db/repl/repl_server_parameters.idl b/src/mongo/db/repl/repl_server_parameters.idl
index 1dd3b2e4143..c97babde6ae 100644
--- a/src/mongo/db/repl/repl_server_parameters.idl
+++ b/src/mongo/db/repl/repl_server_parameters.idl
@@ -227,6 +227,16 @@ server_parameters:
             gte: 1
             lte: 256
 
+    replWriterMinThreadCount:
+        description: The minimum number of threads in the thread pool used to apply the oplog
+        set_at: startup
+        cpp_vartype: int
+        cpp_varname: replWriterMinThreadCount
+        default: 0
+        validator:
+            gte: 0
+            lte: 256
+
     replBatchLimitOperations:
         description: The maximum number of operations to apply in a single batch
         set_at: [ startup, runtime ]
diff --git a/src/mongo/db/repl/sync_tail.cpp b/src/mongo/db/repl/sync_tail.cpp
index 1b9e71a0c82..922f3c232d3 100644
--- a/src/mongo/db/repl/sync_tail.cpp
+++ b/src/mongo/db/repl/sync_tail.cpp
@@ -456,7 +456,7 @@ void scheduleWritesToOplog(OperationContext* opCtx,
     // setup/teardown overhead across many writes.
     const size_t kMinOplogEntriesPerThread = 16;
     const bool enoughToMultiThread =
-        ops.size() >= kMinOplogEntriesPerThread * threadPool->getStats().numThreads;
+        ops.size() >= kMinOplogEntriesPerThread * threadPool->getStats().options.maxThreads;
 
     // Only doc-locking engines support parallel writes to the oplog because they are required to
     // ensure that oplog entries are ordered correctly, even if inserted out-of-order. Additionally,
@@ -470,7 +470,7 @@ void scheduleWritesToOplog(OperationContext* opCtx,
     }
 
 
-    const size_t numOplogThreads = threadPool->getStats().numThreads;
+    const size_t numOplogThreads = threadPool->getStats().options.maxThreads;
     const size_t numOpsPerThread = ops.size() / numOplogThreads;
     for (size_t thread = 0; thread < numOplogThreads; thread++) {
         size_t begin = thread * numOpsPerThread;
@@ -1354,7 +1354,7 @@ StatusWith<OpTime> SyncTail::multiApply(OperationContext* opCtx, MultiApplier::O
     // Increment the batch size stat.
     oplogApplicationBatchSize.increment(ops.size());
 
-    std::vector<WorkerMultikeyPathInfo> multikeyVector(_writerPool->getStats().numThreads);
+    std::vector<WorkerMultikeyPathInfo> multikeyVector(_writerPool->getStats().options.maxThreads);
     {
         // Each node records cumulative batch application stats for itself using this timer.
         TimerHolder timer(&applyBatchStats);
@@ -1378,7 +1378,8 @@ StatusWith<OpTime> SyncTail::multiApply(OperationContext* opCtx, MultiApplier::O
         //   and create a pseudo oplog.
         std::vector<MultiApplier::Operations> derivedOps;
 
-        std::vector<MultiApplier::OperationPtrs> writerVectors(_writerPool->getStats().numThreads);
+        std::vector<MultiApplier::OperationPtrs> writerVectors(
+            _writerPool->getStats().options.maxThreads);
         fillWriterVectors(opCtx, &ops, &writerVectors, &derivedOps);
 
         // Wait for writes to finish before applying ops.
@@ -1404,7 +1405,8 @@ StatusWith<OpTime> SyncTail::multiApply(OperationContext* opCtx, MultiApplier::O
         }
 
         {
-            std::vector<Status> statusVector(_writerPool->getStats().numThreads, Status::OK());
+            std::vector<Status> statusVector(_writerPool->getStats().options.maxThreads,
+                                             Status::OK());
             _applyOps(writerVectors, &statusVector, &multikeyVector, isDataConsistent);
             _writerPool->waitForIdle();