Quill

Asynchronous Low Latency C++ Logging Library

• Introduction
• Documentation
• Features
• Caveats
• Performance
• Basic Usage
• CMake Integration
• Design
• License

homebrew	vcpkg	conan
brew install quill	vcpkg install quill	quill/[>=1.2.3]

Introduction

Quill is a high-performance, cross-platform logging library designed for C++14 and onwards. It provides two versions of the library:

• v1.7: This version is based on C++14 and focuses on bug fixes and stability improvements.
• v2 and onwards: Starting from version 2, Quill is based on C++17 and includes new features and ongoing maintenance.

Quill is a production-ready logging library that has undergone extensive unit testing. It has been successfully utilized in production environments, including financial trading applications, providing high-performance and reliable logging capabilities.

Documentation

For detailed documentation and usage instructions, please visit the Quill Documentation on Read the Docs. It provides comprehensive information on how to integrate and utilize Quill in your C++ applications.

Additionally, you can explore the examples folder in the Quill repository on GitHub. These examples serve as valuable resources to understand different usage scenarios and demonstrate the capabilities of the library.

Features

• Low Latency Logging: Achieve fast logging performance with low latency. Refer to the Benchmarks for more details.
• Backend Logging Thread: Format logs outside the critical path in a backend logging thread. For non-built-in types, the backend logging thread invokes ostream::operator<<() on a copy of the object. Compile-time detection of unsafe copying for non-trivial user defined types is supported. To avoid formatting them on the critical path, such types can be tagged as safe-to-copy. See User Defined Types for more information.
• Custom Formatters: Customize log formatting based on user-defined patterns. Explore Formatters for further details.
• Flexible Timestamp Generation: Choose between rdtsc, chrono, or custom clocks (useful for simulations) for timestamp generation.
• Log Stack Traces: Store log messages in a ring buffer and display them later in response to a higher severity log statement or on demand. Refer to Backtrace Logging for more information.
• Multiple Logging Targets: Utilize various logging targets, including:
  • Console logging with color support.
  • File logging.
  • Rotating log files.
  • Time rotating log files.
  • JSON logging.
  • Custom handlers.
• Log Message Filtering: Apply filters to selectively process log messages. Learn more about Filters.
• Structured Logging: Generate JSON structured logs. See Structured-Log for details.
• Blocking or Dropping Message Modes: Choose between blocking or dropping message modes in the library. In blocking mode, the hot threads pause and wait when the queue is full until space becomes available, ensuring no message loss but introducing potential latency. In dropping mode, log messages beyond the queue's capacity may be dropped to prioritize low latency. The library provides reports on dropped messages, queue reallocations, and blocked hot threads for monitoring purposes.
• Queue Types: The library supports two types of queues for transferring logs from the hot path to the backend thread: bounded queues with a fixed capacity and unbounded queues that start small and can dynamically grow.
• Wide Character Support: Log messages and filenames with wide characters are supported (Windows and v1.7.x only).
• Ordered Log Statements: Log statements are ordered by timestamp even when produced by different threads, facilitating easier debugging of multithreaded applications.
• Compile-Time Log Level Stripping: Completely strip out log levels at compile time, reducing if branches.
• Clean and Warning-Free Codebase: Ensure a clean and warning-free codebase, even with high compiler warning levels.
• Crash-Safe Behavior: Benefit from crash-safe behavior with a built-in signal handler.
• Type-Safe Python-Style API: Utilize a type-safe API inspired by Python, with compile-time checks and built-in support for logging STL types/containers using the excellent {fmt} library.
• Huge Pages: Benefit from support for huge pages on the hot path. This feature allows for improved performance and efficiency.
• Printf-style format: The library offers support for the printf style format in addition to the existing libfmt style format. This feature is particularly useful when migrating from legacy codebases that rely on the printf style format.

Caveats

Quill may not work well with fork() since it spawns a background thread and fork() doesn't work well with multithreading.

If your application uses fork() and you want to log in the child processes as well, you should call quill::start() after the fork() call. Additionally, you should ensure that you write to different files in the parent and child processes to avoid conflicts.

For example :

#include "quill/Quill.h"
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

int main()
{
  // DO NOT CALL THIS BEFORE FORK
  // quill::start();

  if (fork() == 0)
  {
    // Get or create a handler to the file - Write to a different file
    std::shared_ptr<quill::Handler> file_handler =
      quill::file_handler("child_log.log", "w", quill::FilenameAppend::DateTime);

    quill::Config cfg;
    cfg.default_handlers.push_back(std::move(file_handler));
    quill::configure(cfg);

    quill::start();

    QUILL_LOG_INFO(quill::get_logger(), "Hello from Child {}", 123);
  }
  else
  {
    // Get or create a handler to the file - Write to a different file
    std::shared_ptr<quill::Handler> file_handler =
      quill::file_handler("parent_log.log", "w", quill::FilenameAppend::DateTime);

    quill::Config cfg;
    cfg.default_handlers.push_back(std::move(file_handler));
    quill::configure(cfg);

    quill::start();

    QUILL_LOG_INFO(quill::get_logger(), "Hello from Parent {}", 123);
  }
}

Performance

Latency

Logging Numbers

The following message is logged 100,000 times per thread: LOG_INFO(logger, "Logging int: {}, int: {}, double: {}", i, j, d).

The results presented in the tables below are measured in nanoseconds (ns).

1 Thread

Library	50th	75th	90th	95th	99th	99.9th
Quill v3.0.0 Unbounded Queue	20	21	24	25	27	34
Quill v3.0.0 Bounded Queue	17	19	21	22	26	36
fmtlog	16	19	21	22	27	40
MS BinLog	41	43	44	46	66	118
PlatformLab NanoLog	53	66	75	80	92	106
Reckless	62	75	79	84	94	103
Iyengar NanoLog	164	186	213	232	305	389
spdlog	694	761	838	887	996	1143
g3log	5398	5639	5875	6025	6327	6691

4 Threads

Library	50th	75th	90th	95th	99th	99.9th
Quill v3.0.0 Unbounded Queue	20	22	24	26	28	35
Quill v3.0.0 Bounded Queue	17	19	21	22	26	36
fmtlog	16	19	21	23	26	35
MS BinLog	42	44	46	48	76	118
PlatformLab NanoLog	56	67	77	82	95	159
Reckless	46	62	78	92	113	155
Iyengar NanoLog	150	168	247	289	355	456
spdlog	728	828	907	959	1140	1424
g3log	5103	5318	5525	5657	5927	6279

Logging Numbers and Large Strings

The following message is logged 100,000 times per thread: LOG_INFO(logger, "Logging int: {}, int: {}, string: {}", i, j, large_string). The large string used in the log message is over 35 characters to prevent the short string optimization of std::string.

1 Thread

Library	50th	75th	90th	95th	99th	99.9th
Quill v3.0.0 Unbounded Queue	31	33	35	36	39	48
Quill v3.0.0 Bounded Queue	30	32	33	35	43	51
fmtlog	29	31	34	37	44	53
MS BinLog	50	51	53	56	77	127
PlatformLab NanoLog	71	86	105	117	136	158
Reckless	215	242	268	284	314	517
Iyengar NanoLog	172	191	218	238	312	401
spdlog	653	708	770	831	950	1083
g3log	4802	4998	5182	5299	5535	5825

4 Threads

Library	50th	75th	90th	95th	99th	99.9th
Quill v3.0.0 Unbounded Queue	31	33	35	37	40	48
Quill v3.0.0 Bounded Queue	29	31	33	35	41	49
fmtlog	29	31	35	37	44	54
MS BinLog	50	52	54	58	86	130
PlatformLab NanoLog	69	82	99	111	134	194
Reckless	187	209	232	247	291	562
Iyengar NanoLog	159	173	242	282	351	472
spdlog	679	751	839	906	1132	1478
g3log	4739	4955	5157	5284	5545	5898

The benchmark was conducted on an Ubuntu system with an Intel Xeon Gold 6254 CPU running at 3.10GHz. The benchmark used GCC 12.2 as the compiler.

To ensure accurate performance measurement, each thread was pinned to a different CPU. The cores are also isolated on this particular system.

The benchmark methodology involved logging 20 messages in a loop, calculating and storing the average latency for those messages, waiting between 1-2 milliseconds, and repeating this process for a specified number of iterations.

The benchmark was executed four times for each logging library, and the reported latencies represent the second-best result obtained.

You can find the benchmark code on the logger_benchmarks repository.

Throughput

While the primary focus of the library is not on throughput, it does provide efficient handling of log messages across multiple threads. The backend logging thread, responsible for formatting and ordering log messages from hot threads, ensures that all queues are emptied on a high priority basis. This approach prevents the need for allocating new queues or dropping messages on the hot path. Instead, the backend thread internally buffers log messages and writes them when the hot thread queues are empty or when a predefined limit, backend_thread_transit_events_soft_limit, is reached.

Comparing throughput with other logging libraries in an asynchronous logging scenario has proven challenging. Some libraries may drop log messages, resulting in smaller log files than expected, while others only offer asynchronous flush, making it difficult to determine when the logging thread has finished processing all messages.

In contrast, Quill provides a blocking flush() guarantee, ensuring that every log message from the hot threads up to that point is flushed to the file. The maximum throughput is measured by determining the maximum number of log messages the backend logging thread can write to the file per second.

For benchmarking purposes, you can find the code here. When measured on the same system as the latency benchmarks mentioned earlier, logging 4 million messages resulted in a log file size of 476 MB. The average throughput achieved was 1.76 million messages per second, with a total elapsed time of 2266 ms.

Please note that while Quill performs well in terms of throughput, its primary strength lies in its efficient handling of log messages across threads.

Basic usage

#include "quill/Quill.h"

int main()
{
  quill::configure(
    []()
    {
      // See Config.h and Tweaks.h for run time and compile time configuration options
      quill::Config cfg;
      return cfg;
    }());

  // Starts the logging backend thread
  quill::start();

  // Create a file logger
  quill::Logger* logger = quill::create_logger(
    "file_logger",
    quill::file_handler("example.log",
                        []()
                        {
                          quill::FileHandlerConfig cfg;
                          cfg.set_open_mode('w');
                          cfg.set_pattern(
                            "[%(ascii_time)] [%(thread)] [%(filename):%(lineno)] [%(logger_name)] "
                            "[%(level_name)] - %(message)",
                            "%H:%M:%S.%Qms");
                          return cfg;
                        }()));

  logger->set_log_level(quill::LogLevel::TraceL3);

  // enable a backtrace that will get flushed when we log CRITICAL
  logger->init_backtrace(2u, quill::LogLevel::Critical);

  LOG_BACKTRACE(logger, "Backtrace log {}", 1);
  LOG_BACKTRACE(logger, "Backtrace log {}", 2);

  LOG_INFO(logger, "Welcome to Quill!");
  LOG_ERROR(logger, "An error message. error code {}", 123);
  LOG_WARNING(logger, "A warning message.");
  LOG_CRITICAL(logger, "A critical error.");
  LOG_DEBUG(logger, "Debugging foo {}", 1234);
  LOG_TRACE_L1(logger, "{:>30}", "right aligned");
  LOG_TRACE_L2(logger, "Positional arguments are {1} {0} ", "too", "supported");
  LOG_TRACE_L3(logger, "Support for floats {:03.2f}", 1.23456);
}

#include "quill/Quill.h"

int main()
{
  quill::Config cfg;
  cfg.enable_console_colours = true;
  quill::configure(cfg);
  
  quill::start();

  // Default root logger to stdout
  quill::Logger* logger = quill::get_logger();
  logger->set_log_level(quill::LogLevel::TraceL3);

  // enable a backtrace that will get flushed when we log CRITICAL
  logger->init_backtrace(2u, quill::LogLevel::Critical);

  LOG_BACKTRACE(logger, "Backtrace log {}", 1);
  LOG_BACKTRACE(logger, "Backtrace log {}", 2);

  LOG_INFO(logger, "Welcome to Quill!");
  LOG_ERROR(logger, "An error message. error code {}", 123);
  LOG_WARNING(logger, "A warning message.");
  LOG_CRITICAL(logger, "A critical error.");
  LOG_DEBUG(logger, "Debugging foo {}", 1234);
  LOG_TRACE_L1(logger, "{:>30}", "right aligned");
  LOG_TRACE_L2(logger, "Positional arguments are {1} {0} ", "too", "supported");
  LOG_TRACE_L3(logger, "Support for floats {:03.2f}", 1.23456);
}

Output

CMake-Integration

External

Building and Installing Quill as Static Library

git clone http://github.com/odygrd/quill.git
mkdir cmake_build
cd cmake_build
make install

Note: To install in custom directory invoke cmake with -DCMAKE_INSTALL_PREFIX=/quill/install-dir/

Building and Installing Quill as Static Library With External libfmt

cmake -DCMAKE_PREFIX_PATH=/my/fmt/fmt-config.cmake-directory/ -DQUILL_FMT_EXTERNAL=ON -DCMAKE_INSTALL_PREFIX=/quill/install-dir/'

Then use the library from a CMake project, you can locate it directly with find_package()

Directory Structure

my_project/
├── CMakeLists.txt
├── main.cpp

CMakeLists.txt

# Set only if needed - quill was installed under a custom non-standard directory
set(CMAKE_PREFIX_PATH /test_quill/usr/local/)

find_package(quill REQUIRED)

# Linking your project against quill
add_executable(example main.cpp)
target_link_libraries(example PRIVATE quill::quill)

main.cpp

See basic usage

Embedded

To embed the library directly, copy the source folder to your project and call add_subdirectory() in your CMakeLists.txt file

Directory Structure

my_project/
├── quill/            (source folder)
├── CMakeLists.txt
├── main.cpp

CMakeLists.txt

cmake_minimum_required(VERSION 3.1.0)
project(my_project)

set(CMAKE_CXX_STANDARD 14)

add_subdirectory(quill)

add_executable(my_project main.cpp)
target_link_libraries(my_project PRIVATE quill::quill)

main.cpp

See basic usage

Building Quill as a Shared Library (DLL) on Windows

To build Quill as a shared library (DLL) on Windows, follow these steps:

1. Add the following CMake flags when configuring the build:

   -DCMAKE_WINDOWS_EXPORT_ALL_SYMBOLS=TRUE -DBUILD_SHARED_LIBS=ON
   

2. Additionally, you need to define QUILL_BUILD_SHARED either in your code before including Quill.h or as a compiler flag when building outside of CMake.

Building Quill for Android NDK

To build Quill for Android NDK add the following CMake flags when configuring the build:

-DQUILL_NO_THREAD_NAME_SUPPORT:BOOL=ON

Design

License

Quill is licensed under the MIT License

Quill depends on third party libraries with separate copyright notices and license terms. Your use of the source code for these subcomponents is subject to the terms and conditions of the following licenses.

• (MIT License) {fmt} (http://github.com/fmtlib/fmt/blob/master/LICENSE.rst)
• (MIT License) doctest (http://github.com/onqtam/doctest/blob/master/LICENSE.txt)