Custom BUFSIZ support

[randomizedthinking]

The default BUFSIZ parameter on most Linux systems is simply 8K. On my box, a dd command shows a buffer size of 512K would have much better performance. I wonder whether there is any way I can use a custom buffer size when creating a logger.

In my case, spdlog is used to stream data to multiple (about 100) files. I suspect the default flush policy is hitting the disk too frequently, and would like to see whether there is any way to improve the performance.

[gabime]

Maybe calling https://en.cppreference.com/w/cpp/io/c/setvbuf somewhere in the file sink impl would do the trick?

[randomizedthinking]

It should work, and spdlog can add a parameter to expose it to end users.

[gabime]

Would be nice if you could try and report the actual performance gain.

[randomizedthinking]

Here is a simple test of using different buffer size, and the results are significant.

For simplicity, I only tested single-thread since the buffer is managed by myself. The bench_single_threaded function is used to evaluation the performance.

[info] **************************************************************                                                                                                                                                                         
[info] Single threaded: 2500000 messages; BufSize: 8192                                                                                                                                                                                       
[info] **************************************************************                                                                                                                                                                         
[info] basic_st                       Elapsed: 1.20 secs        2,089,389/sec                                                                                                                                                                 
[info] basic_st/backtrace-on          Elapsed: 1.29 secs        1,931,904/sec                                                                                                                                                                 
[info] **************************************************************                                                                                                                                                                         
[info] Single threaded: 2500000 messages; BufSize: 262144                                                                                                                                                                                      
[info] **************************************************************                                                                                                                                                                         
[info] basic_st                       Elapsed: 0.62 secs        4,021,943/sec                                                                                                                                                                 
[info] basic_st/backtrace-on          Elapsed: 0.64 secs        3,896,717/sec