Multi-threaded dist_sink

[balaji99]

When using an async logger with a dist_sink_mt sink and multiple threads in the thread pool, will logging in the different sinks in dist_sink_mt be multi-threaded?

For example, I create a logger like so:

spdlog::init_thread_pool(8192 /* max items in queue */, 2 /* number of threads */);
 logger = spdlog::create_async<spdlog::sinks::dist_sink_mt>("async logger", std::vector<spdlog::sink_ptr>{file_sink, console_sink});

where file_sink and console_sink are sinks I created earlier.

Now, will logging to those two sinks take place in different threads from the thread pool?

OR, will the logging take place in the same thread since they are part of the same logger? If yes, there is no point in using two or more threads for the logging.

[tt4g]

All sink operations use one of the threads in the thread pool.

The sink passed to dist_sink can be a non-thread-safe sink (_st suffix) if it is not used in shared with other loggers.

spdlog::init_thread_pool(8192, 2);
auto logger = spdlog::create_async<spdlog::sinks::dist_sink_mt>("async logger", std::vector<spdlog::sink_ptr>{file_sink, console_sink});

// file_sink does not need to be a thread-safe.
// console_sink must be thread-safe.
auto logger = spdlog::create_async<spdlog::sinks::dist_sink_mt>("other logger", console_sink);

[balaji99]

What I meant is - given the instance of dist_sink_mt I specified in my previous post, when I log something to that sink, like so
logger.info("Sample log");
the log statement would reach the file_sink and the console_sink since they are part of the dist_sink_mt.

Now, would the writing to the file_sink and console_sink happen parallelly in two different threads or sequentially in the same thead?

In other words, which of the following would be true?
Scenario 1
Thread 1 -> writes to file_sink
Thread 2 -> writes to console_sink

OR

Scenario 2
Thread 1 -> writes to file_sink and then to console_sink.

[tt4g]

The behavior described in "Scenario 2."

[balaji99]

Thanks!

Any chance this behavior will change in the future? Would like to see multiple threads acting on the different sinks under one dist_sink.

[tt4g]

We have no plans.
The async logger was developed to prevent applications from being blocked by sink I/O operations.
It is not designed to perform sink in parallel at high speed.

If you want to run sinks in separate threads, develop a custom sink.
Implementing your own sink - Wiki: https://github.com/gabime/spdlog/wiki/4.-Sinks#implementing-your-own-sink

[gabime]

The sink passed to dist_sink can be a non-thread-safe sink (_st suffix) if it is not used in shared with other loggers.

If the spdlog::thread_pool size > 1, then the sinks must be _mt. because multiple threads might handle concurrently multiple log messages for same sink.

If on the other hand, only single thread is used for the pool (which is the default in spdlog), _st can be used (provided they are not shared between multiple loggers). If in doubt just use _mt.