From 2941ce89c1cdfd81e26c3de21bfaa368f18a9ea6 Mon Sep 17 00:00:00 2001
From: Chris Blume <ProgramMax@gmail.com>
Date: Thu, 25 Sep 2025 15:48:56 -0400
Subject: [PATCH] Use unbuffered IO

---
 Code/EntryPoint.cpp           | 10 ++++++++--
 Code/OverlappedIOFileRead.cpp | 19 ++++++++++++++++---
 Code/OverlappedIOFileRead.hpp | 14 ++++++++++++--
 3 files changed, 36 insertions(+), 7 deletions(-)

diff --git a/Code/EntryPoint.cpp b/Code/EntryPoint.cpp
index bb3957e..4ffbb46 100644
--- a/Code/EntryPoint.cpp
+++ b/Code/EntryPoint.cpp
@@ -2,10 +2,16 @@
 
 #include "OverlappedIOFileRead.hpp"
 
-int main() {
+int main(int argc, const char * argv[]) {
+	if (argc != 2) {
+		std::cerr << "Error: No file provided.\n";
+		std::cerr << "Use the form: FileReadSpeedTest.exe C:\\my\\file.txt" << std::endl;
+		return -1;
+	}
+
 	const DWORD worker_thread_count = 6;
 	// TODO: Take the file as part of the command line parameters.
-	auto overlapped_io_file_read = PrepareToReadFile(R"(../../Test Data/crystaldiskmark nvme ssd.png)", worker_thread_count);
+	auto overlapped_io_file_read = PrepareToReadFile(argv[1], worker_thread_count);
 	if (!overlapped_io_file_read.has_value()) {
 		std::cerr << "Error";
 		return -1;
diff --git a/Code/OverlappedIOFileRead.cpp b/Code/OverlappedIOFileRead.cpp
index ee98158..092f598 100644
--- a/Code/OverlappedIOFileRead.cpp
+++ b/Code/OverlappedIOFileRead.cpp
@@ -14,10 +14,14 @@
 
 namespace {
 
+	std::chrono::time_point<std::chrono::high_resolution_clock> pre_open_time_;
+
+
 	std::optional<OverlappedIOFile> CreateOverlappedIOFile(LPCSTR file_name) noexcept {
-		// TODO: Add FILE_FLAG_NO_BUFFERING flag
-		// and FILE_FLAG_RANDOM_ACCESS or FILE_FLAG_SEQUENTIAL_SCAN
-		HANDLE file_handle = CreateFileA(file_name, GENERIC_READ, FILE_SHARE_READ, nullptr, OPEN_EXISTING, FILE_FLAG_OVERLAPPED, NULL);
+		pre_open_time_ = std::chrono::high_resolution_clock::now();
+		// Use FILE_FLAG_RANDOM_ACCESS instead of FILE_FLAG_SEQUENTIAL_SCAN for file formats where that is a better fit
+		// FILE_FLAG_NO_BUFFERING
+		HANDLE file_handle = CreateFileA(file_name, GENERIC_READ, FILE_SHARE_READ, nullptr, OPEN_EXISTING, FILE_FLAG_OVERLAPPED | FILE_FLAG_SEQUENTIAL_SCAN | FILE_FLAG_NO_BUFFERING, NULL);
 		if (file_handle == INVALID_HANDLE_VALUE) {
 			// GetLastError
 			return std::nullopt;
@@ -183,6 +187,11 @@ void OverlappedIOFileRead::WaitForThreadsToFinish() noexcept {
 	// Wait for worker threads to finish
 	WaitForMultipleObjects(static_cast<DWORD>(thread_pool_.threads_.size()), (const HANDLE*)thread_pool_.threads_.data(), TRUE, INFINITE);
 
+	auto open_delay = std::chrono::duration_cast<std::chrono::nanoseconds>(overlapped_io_file_.file_open_time_ - pre_open_time_);
+	auto open_to_read_delay = std::chrono::duration_cast<std::chrono::nanoseconds>(read_issue_time_ - overlapped_io_file_.file_open_time_);
+	std::cout << "Open delay: " << open_delay << std::endl;
+	std::cout << "Open to read delay: " << open_to_read_delay << std::endl;
+
 	size_t i = 0;
 	for (auto& context : contexts_) {
 		auto read_issue_delay = std::chrono::duration_cast<std::chrono::nanoseconds>(context.request_start_time_ - read_issue_time_);
@@ -231,6 +240,10 @@ std::expected<OverlappedIOFileRead, PrepareToReadFileError> PrepareToReadFile(LP
 		return std::unexpected{PrepareToReadFileError::CouldNotCreateCompletionPort};
 	}
 
+	// TODO: Creating the threads takes a significant amount of time.
+	// This happens after the file is opened and the completion port is created.
+	// The OS *could* begin prefetching the data. I don't know if it does or not.
+	// But if it does, this gives it ample opportunity to impact the measurements.
 	auto thread_pool = CreateThreadPool(WorkerThread, completion_port->handle_, worker_thread_count);
 	if (!thread_pool.has_value()) {
 		return std::unexpected{PrepareToReadFileError::CouldNotCreateThread};
diff --git a/Code/OverlappedIOFileRead.hpp b/Code/OverlappedIOFileRead.hpp
index 46f5952..65bfb85 100644
--- a/Code/OverlappedIOFileRead.hpp
+++ b/Code/OverlappedIOFileRead.hpp
@@ -14,10 +14,21 @@
 #define NOMINMAX
 #include <Windows.h>
 
-using OverlappedIOFile = SpecificHandleObject;
+//using OverlappedIOFile = SpecificHandleObject;
 using CompletionPort = SpecificHandleObject;
 using Thread = SpecificHandleObject;
 
+struct OverlappedIOFile : public SpecificHandleObject {
+
+	explicit OverlappedIOFile(HANDLE handle) noexcept
+		: SpecificHandleObject(std::move(handle))
+		, file_open_time_(std::chrono::high_resolution_clock::now())
+	{}
+
+	std::chrono::time_point<std::chrono::high_resolution_clock> file_open_time_;
+
+};
+
 class ThreadPool {
 public:
 
@@ -40,7 +51,6 @@ struct IOContext {
 	std::chrono::time_point<std::chrono::high_resolution_clock> request_start_time_;
 	std::chrono::time_point<std::chrono::high_resolution_clock> request_complete_time_;
 
-
 };
 
 class OverlappedIOFileRead {