Quill

Asynchronous Low Latency C++ Logging Library

• Introduction
• Documentation
• Features
• Caveats
• Performance
• Quick Start
• Usage
• Design
• License

Package Manager	Installation Command
vcpkg	vcpkg install quill
Conan	conan install quill
Homebrew	brew install quill
Meson WrapDB	meson wrap install quill
Conda	conda install -c conda-forge quill
Bzlmod	bazel_dep(name = "quill", version = "x.y.z")
xmake	xrepo install quill

Introduction

Quill is a high-performance, cross-platform logging library designed for C++17 and onwards. 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.
• Asynchronous logging: Log arguments and messages are formatted in a backend logging thread, effectively offloading the formatting overhead from the critical path.
• 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 log message 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 Sinks: Utilize various logging targets, including:
  • Console logging with color support.
  • File logging.
  • Rotating log files based on time or size.
  • JSON logging.
  • Custom sinks.
• 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 different 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: Wide strings compatible with ASCII encoding are supported, applicable to Windows only. Additionally, there is support for logging STL containers consisting of wide strings. Note that chaining STL types, such as std::vector<std::vector<std::wstring>> is not supported for wide strings.
• 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 API: Type safe api 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.

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/Backend.h"
#include "quill/Frontend.h"
#include "quill/LogMacros.h"
#include "quill/Logger.h"
#include "quill/sinks/FileSink.h"

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

  if (fork() == 0)
  {
    quill::Backend::start();
        
    // Get or create a handler to the file - Write to a different file
    auto file_sink = quill::Frontend::create_or_get_sink<quill::FileSink>(
      "child.log");
    
    quill::Logger* logger = quill::Frontend::create_or_get_logger("root", std::move(file_sink));

    QUILL_LOG_INFO(logger, "Hello from Child {}", 123);
  }
  else
  {
    quill::Backend::start();
          
    // Get or create a handler to the file - Write to a different file
    auto file_sink = quill::Frontend::create_or_get_sink<quill::FileSink>(
      "parent.log");
    
    quill::Logger* logger = quill::Frontend::create_or_get_logger("root", std::move(file_sink));
    
    QUILL_LOG_INFO(logger, "Hello from Parent {}", 123);
  }
}

Performance

Latency

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

Logging Numbers

LOG_INFO(logger, "Logging int: {}, int: {}, double: {}", i, j, d).

1 Thread Logging

Library	50th	75th	90th	95th	99th	99.9th
Quill v4.4 Bounded Dropping Queue	8	9	9	9	10	12
fmtlog	8	9	10	10	11	13
Quill v4.4 Unbounded Queue	11	11	11	12	13	17
PlatformLab NanoLog	12	13	16	17	20	25
MS BinLog	19	19	19	20	56	83
Reckless	25	27	29	31	33	39
XTR	6	6	39	42	47	59
Iyengar NanoLog	89	102	124	132	231	380
spdlog	147	151	155	158	166	174
g3log	1167	1240	1311	1369	1593	1769

4 Threads Logging Simultaneously

Library	50th	75th	90th	95th	99th	99.9th
XTR	6	6	8	9	40	48
Quill v4.4 Bounded Dropping Queue	8	9	9	10	11	13
fmtlog	8	9	9	10	12	14
Quill v4.4 Unbounded Queue	12	12	13	13	14	18
PlatformLab NanoLog	13	15	18	21	25	28
Reckless	17	21	24	25	28	47
MS BinLog	19	19	20	21	58	88
Iyengar NanoLog	94	105	135	144	228	314
spdlog	209	248	297	330	423	738
g3log	1253	1332	1393	1437	1623	2063

Logging Large Strings

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 Logging

Library	50th	75th	90th	95th	99th	99.9th
Quill v4.4 Bounded Dropping Queue	10	11	12	13	13	16
fmtlog	10	12	13	14	16	17
Quill v4.4 Unbounded Queue	13	13	14	15	16	19
PlatformLab NanoLog	15	18	22	25	29	34
MS BinLog	20	21	22	23	58	86
XTR	8	8	29	30	33	49
Reckless	89	108	115	117	123	141
Iyengar NanoLog	94	106	125	133	240	388
spdlog	123	126	130	133	140	148
g3log	890	966	1028	1119	1260	1463

4 Threads Logging Simultaneously

Library	50th	75th	90th	95th	99th	99.9th
Quill v4.4 Bounded Dropping Queue	11	11	13	13	14	17
XTR	9	11	13	14	31	39
Quill v4.4 Unbounded Queue	13	14	15	16	17	20
fmtlog	12	13	16	16	19	21
MS BinLog	21	22	23	25	60	90
PlatformLab NanoLog	19	24	33	36	42	49
Reckless	82	96	104	108	118	145
Iyengar NanoLog	57	96	123	137	172	302
spdlog	185	207	237	257	362	669
g3log	983	1046	1112	1171	1376	1774

Logging Complex Types

LOG_INFO(logger, "Logging int: {}, int: {}, vector: {}", i, j, v).

Logging std::vector<std::string> v containing 16 large strings, each ranging from 50 to 60 characters. The strings used in the log message are over 35 characters to prevent the short string optimization of std::string.

1 Thread Logging

Library	50th	75th	90th	95th	99th	99.9th
Quill v4.4 Bounded Dropping Queue	50	52	54	56	59	74
Quill v4.4 Unbounded Queue	53	55	56	58	61	67
MS BinLog	64	66	70	80	89	271
XTR	282	290	338	343	350	575
fmtlog	721	750	779	793	821	847
spdlog	5881	5952	6026	6082	6342	6900

4 Threads Logging Simultaneously

Library	50th	75th	90th	95th	99th	99.9th
Quill v4.4 Bounded Dropping Queue	53	55	57	59	62	80
MS BinLog	66	68	71	74	87	295
Quill v4.4 Unbounded Queue	88	95	103	108	119	135
XTR	535	730	786	819	885	971
fmtlog	788	811	831	844	872	906
spdlog	6090	6165	6246	6337	7351	9322

The benchmark was conducted on Linux RHEL 9 with an Intel Core i5-12600 at 4.8 GHz. The cpus are isolated on this system and each thread was pinned to a different CPU. GCC 13.1 was used as the compiler.

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

In the Quill Bounded Dropping benchmarks, the dropping queue size is set to 262,144 bytes, which is double the default size of 131,072 bytes.

You can find the benchmark code on the logger_benchmarks repository.

Throughput

The maximum throughput is measured by determining the maximum number of log messages the backend logging thread can write to the log file per second.

When measured on the same system as the latency benchmarks mentioned above the average throughput of the backend logging thread when formatting a log message consisting of an int and a double is ~4.40 million msgs/sec

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. The backend thread internally buffers the log messages and then writes them later when the caller thread queues are empty or when a predefined limit, backend_thread_transit_events_soft_limit, is reached. This approach prevents the need for allocating new queues or dropping messages on the hot path.

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 log guarantee, ensuring that every log message from the hot threads up to that point is flushed to the file.

For benchmarking purposes, you can find the code here.

Compilation Time

Compile times are measured using clang 15 and for Release build.

Below, you can find the additional headers that the library will include when you need to log, following the recommended_usage example

There is also a compile-time benchmark measuring the compilation time of 2000 auto-generated log statements with various arguments. You can find it here. It takes approximately 30 seconds to compile.

Quick Start

#include "quill/Backend.h"
#include "quill/Frontend.h"
#include "quill/LogMacros.h"
#include "quill/Logger.h"
#include "quill/sinks/FileSink.h"

int main()
{
  // Start the backend thread
  quill::Backend::start();

  // Log to file
  auto file_sink = quill::Frontend::create_or_get_sink<quill::FileSink>(
    "example_file_logging.log");

  quill::Logger* logger =
    quill::Frontend::create_or_get_logger("root", std::move(file_sink));

  // set the log level of the logger to trace_l3 (default is info)
  logger->set_log_level(quill::LogLevel::TraceL3);
  
  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/Backend.h"
#include "quill/Frontend.h"
#include "quill/LogMacros.h"
#include "quill/Logger.h"
#include "quill/sinks/ConsoleSink.h"

int main()
{
  // Start the backend thread
  quill::Backend::start();

  // Frontend
  auto console_sink = quill::Frontend::create_or_get_sink<quill::ConsoleSink>("sink_id_1");
  quill::Logger* logger = quill::Frontend::create_or_get_logger("root", std::move(console_sink));

  // Change the LogLevel to print everything
  logger->set_log_level(quill::LogLevel::TraceL3);

  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

Usage

External CMake

Building and Installing Quill

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

Note: To install in custom directory invoke cmake with -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 PUBLIC quill::quill)

Embedded CMake

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 17)

add_subdirectory(quill)

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

Building Quill for Android NDK

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

-DQUILL_NO_THREAD_NAME_SUPPORT:BOOL=ON

Meson

Using WrapDB

Meson's wrapdb includes a quill package, which repackages quill to be built by meson as a subproject.

• Install quill subproject from the wrapdb by running from the root of your project

  meson wrap install quill

Manual Integration

If you prefer not to use WrapDB, you can manually integrate Quill into your project by following these steps:

• Copy the contents of this repository under the subprojects directory in your project.

Integration Steps

Once the library is integrated into your Meson project, follow these steps to ensure proper usage:

• In your project’s meson.build file, add an entry for the new subproject

  quill = subproject('quill')
  quill_dep = quill.get_variable('quill_dep')

• Include the new dependency object to link with quill

  my_build_target = executable('name', 'main.cpp', dependencies : [quill_dep], install : true)

Bazel

Using Blzmod

The library is available on BLZMOD, allowing for easy integration into your project.

Manual Integration

If you prefer manual integration, you can add the library as a dependency in your BUILD.bazel file. Below is a sample cc_binary rule demonstrating how to include the library. Ensure to replace //quill_path with the actual path to the directory containing the BUILD.bazel file for the quill library within your project structure.

cc_binary(name = "app", srcs = ["main.cpp"], deps = ["//quill_path:quill"])

Design

Frontend (caller-thread)

When invoking a LOG_ macro:

1. Creates a static constexpr metadata object to store Metadata such as the format string and source location.

2. Pushes the data SPSC lock-free queue. For each log message, the following variables are pushed

Variable	Description
timestamp	Current timestamp
Metadata*	Pointer to metadata information
Logger*	Pointer to the logger instance
DecodeFunc	A pointer to a templated function containing all the log message argument types, used for decoding the message
Args...	A serialized binary copy of each log message argument that was passed to the LOG_ macro

Backend

Consumes each message from the SPSC queue, retrieves all the necessary information and then formats the message. Subsequently, forwards the log message to all Sinks associated with the Logger.

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)