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#84 in Debugging

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Build Status License Latest Version ftlog

Logging is affected by the disk IO and pipe system call. Sequential log calls can be a bottleneck in scenarios where low latency is critical (e.g., high-frequency trading).

ftlog mitigates this bottleneck by sending messages to a dedicated logger thread and computing as little as possible in the main/worker thread.

ftlog can improve log performance in main/worker thread a few times over. See performance for details.


Add to your Cargo.toml:

ftlog = "0.2"

Configure and initialize ftlog at the start of your main function:

// ftlog re-export `log`'s macros, so no need to add `log` to dependencies
use ftlog::appender::FileAppender;
use ftlog::{debug, trace};
use log::{error, info, warn};

// minimal configuration with default setting

// When drops, the guard calls and waits `flush` to logger.
// With guard that share the lifetime of `main` fn, there is no need to manually call flush at the end of `main` fn.
let _guard = ftlog::builder().try_init().unwrap();

trace!("Hello world!");
debug!("Hello world!");
info!("Hello world!");
warn!("Hello world!");
error!("Hello world!");

A more complicated but feature rich usage:

use ftlog::{
    appender::{Duration, FileAppender, Period},
    FtLogFormatter, LevelFilter,

let time_format = time::format_description::parse_owned::<1>(
    "[year]-[month]-[day] [hour]:[minute]:[second].[subsecond digits:6]",
// configurate logger
let _guard = ftlog::builder()
    // global max log level
    // custom timestamp format
    // set global log formatter
    // use bounded channel to avoid large memory comsumption when overwhelmed with logs
    // Set `false` to tell ftlog to discard excessive logs.
    // Set `true` to block log call to wait for log thread.
    // here is the default settings
    .bounded(100_000, false) // .unbounded()
    // define root appender, pass anything that is Write and Send
    // omit `Builder::root` will write to stderr
    // timezone of log message timestamp, use local by default
    // .local_timezone()
    // or use fiexed timezone for better throughput, since retrieving timezone is a time consuming operation
    // this does not affect worker threads (that call log), but can boost log thread performance (higher throughput).
    // level filter for root appender
    // write logs in ftlog::appender to "./ftlog-appender.log" instead of "./current.log"
    .filter("ftlog::appender", "ftlog-appender", LevelFilter::Error)
    .appender("ftlog-appender", FileAppender::new("ftlog-appender.log"))
    .expect("logger build or set failed");

See ./examples for more (e.g. custom format).

Default Log Format

2022-04-08 19:20:48.190+08 298ms INFO main src/ftlog.rs:14 My log message

Here 298ms denotes the latency between the call of the log (e.g. log::info!("msg")) and the actual printing in log thread. Normally this is 0ms.

A large delay indicates that the log thread may be blocked by excessive log messages.

2022-04-10 21:27:15.996+08 0ms 2 INFO main src/main.rs:29 limit running3 !

The number 2 above indicates how many log messages were discarded. Only shown if the frequency of logging for a single log call is limited (e.g. log::info!(limit=3000i64;"msg")).

Randomly drop log

Use random_drop or drop to specify the probability of randomly discarding logs. No message is dropped by default.

log::info!(random_drop=0.1f32;"Random log 10% of log calls, keeps 90%");
log::info!(drop=0.99f32;"Random drop 99% of log calls, keeps 1%");

This can be helpful when formatting log message into string is too costly,

When both random_drop and limit is specified, ftlog will limit logs after messages are randomly dropped.

    "Drop 99% messages. The survived 1% messages are limit to at least 1000ms between adjacent log messages output"

Custom timestamp format

ftlog relies on the time crate for the formatting of timestamp. To use custom time format, first construct a valid time format description, and then pass it to ftlog builder by ftlog::time_format(&mut self).

In case an error occurs when formatting timestamp, ftlog will fallback to RFC3339 time format.


let format = time::format_description::parse_owned::<1>(
    "[year]/[month]/[day] [hour]:[minute]:[second].[subsecond digits:6]",
let _guard = ftlog::builder().time_format(format).try_init().unwrap();
log::info!("Log with custom timestamp format");
// Output:
// 2023/06/14 11:13:26.160840 0ms INFO main [main.rs:3] Log with custom timestamp format

Log with interval

ftlog allows to limit the write frequency for individual log calls.

If the above line is called multiple times within 3000ms, then it is logged only once, with an added number reflecting the number of discarded log messages.

Each log call ha an independent interval, so we can set different intervals for different log calls. Internally, ftlog records the last print time by a combination of (module name, file name, code line).


info!(limit=3000i64; "limit running {}s !", 3);

The minimal interval of the the specific log call above is 3000ms.

2022-04-10 21:27:10.996+08 0ms 0 INFO main [src/main.rs:29] limit running 3s !
2022-04-10 21:27:15.996+08 0ms 2 INFO main [src/main.rs:29] limit running 3s !

The number 2 above shows how many log messages is discarded since last log.

Log rotation

ftlog supports log rotation in local timezone. The available rotation periods are:

  • minute Period::Minute
  • hour Period::Hour
  • day Period::Day
  • month Period::Month
  • year Period::Year

Log rotation is configured in FileAppender, and the timestamp is appended to the end of the filename:

use ftlog::appender::{FileAppender, Period};

let logger = ftlog::builder()
let _guard = logger.init().unwrap();

If the log file is configured to be split by minutes, the log file name has the format mylog-{MMMM}{YY}{DD}T{hh}{mm}.log. When divided by days, the log file name is something like mylog-{MMMM}{YY}{DD}.log.

Log filename examples:

$ ls
# by minute
# by hour
# by day
# by month
# by year
# omitting extension (e.g. "./log") will add datetime to the end of log filename

Clean outdated logs

With log rotation enabled, it is possible to clean outdated logs to free up disk space with FileAppender::rotate_with_expire method or set expire(Duration) when using builder.

ftlog first finds files generated by ftlog and cleans outdated logs by last modified time. ftlog find generated logs by filename matched by file stem and added datetime.

ATTENTION: Any files that matchs the pattern will be deleted.

use ftlog::{appender::{Period, FileAppender, Duration}};

// clean files named like `current-\d{8}T\d{4}.log`.
// files like `another-\d{8}T\d{4}.log` or `current-\d{8}T\d{4}` will not be deleted, since the filenames' stem do not match.
// files like `current-\d{8}.log` will remains either, since the rotation durations do not match.

// Rotate every day, clean stale logs that were modified 7 days ago on each rotation
let appender = FileAppender::rotate_with_expire("./current.log", Period::Day, Duration::days(7));
let logger = ftlog::builder()
let _guard = logger.init().unwrap();


  • tsc Use TSC for clock source for higher performance without accuracy loss.

    TSC offers the most accurate and cheapest way to access current time under certain condition:

    1. the CPU frequency must be constant
    2. must with CPU of x86/x86_64 architecture, since TSC is an x86/x86_64 specific register.
    3. never suspend

    The current feature further requires that the build target MUST BE LINUX. Otherwise it will fall back to a fast but much less accurate implementation.


For performance, timezone is detected once at logger buildup, and use it later in every log message. This is partly due to timezone detetion is expensive, and partly to the unsafe nature of underlying system call in multi-thread program in Linux.

It's also recommended to use UTC instead to further avoid timestamp convertion to timezone for every log message.



message type Apple M1 Pro, 3.2GHz AMD EPYC 7T83, 3.2GHz
ftlog static string 75 ns/iter 385 ns/iter
ftlog with i32 106 ns/iter 491 ns/iter
output to file
static string 1,674 ns/iter 1,142 ns/iter
output to file
with i32 1,681 ns/iter 1,179 ns/iter
output to file with BufWriter
static string 279 ns/iter 550 ns/iter
output to file with BufWriter
with i32 278 ns/iter 565 ns/iter

License: MIT OR Apache-2.0


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