Streaming ffmpeg from fifo file starts only when i close the fifo file

Question

Im starting an ffmpeg process, where the input is a FIFO file i created. Im writing some data in a loop to the FIFO file, but the ffmpeg process doesn't start streaming until one of the two happens:

1. i'm closing the file
2. iv'e written a certain amount of data. after a while of writing, the ffmpeg process starts streaming. The more data i write, the faster it starts running. (im writing a chunk of data on each loop, if i just duplicate those chunks times 100, it starts much faster).

What can be the reason for that? Is there a minimum of data required for the ffmpeg process to start streaming? How can i "force" it to start, without closing the FIFO file after writing?