Asynchronous/double-buffering support? #16
Description
This has come up a few times in the past, but figured I'd re-raise it after the transition from google code to github:
I'm seeing a case in my application where doing LOG() calls from an event loop thread is causing the entire event loop to stall for hundreds of milliseconds. Eventually I captured stacks showing that it's blocking trying to acquire log_mutex while another thread is in the middle of a disk flush. The disk flush can be very slow in the case that the logging disk is in the middle of an ext4 checkpoint (the buffered IO calls file_update_time to update the inode mtime, but the inode is locked for writeback)
A partial solution here would be to add double-buffering within the glog process (instead of relying on fwrite/fflush buffering). Writes would be appended into a buffer, and at "flush" time, we swap the buffer and do a non-buffered write of the old buffer to the file, while not holding the lock. Meanwhile other threads can continue to append to the new buffer without blocking.
A fuller solution would involve deferring the actual logging work to a background thread (eg by a MPSC queue). Either a bounded implementation (which drops messages or blocks when the queue is full) or an unbounded one (which uses a lot of RAM if you can't keep up) could be reasonable choices depending on the workload.
Does anyone have a fork of glog that does this? Would the glog maintainers be willing to accept a pull request if we decided to implement it ourselves?