Parallel Chunks

new MultipleUpload(document.getElementById('demo-parallel-chunks'), {
 uploadUrl: '/api/upload',
 multiple: true,
 chunked: true,
 chunkSize: 2 * 1024 * 1024, // 2 MB chunks
 concurrency: 3,
 onTaskProgress: function(task) {
 console.log(task.fileName + ': ' + Math.round(task.progress) + '%');
 }
});

Chunks of one file, in flight together

chunkConcurrency defaults to 1, meaning chunks of a single file are sent strictly in order. Raising it sends several at once, which is where the throughput on a fast connection actually comes from — one request rarely saturates the link on its own.

Two different concurrencies

This is not concurrency. That one controls how many FILES upload at the same time; this controls how many CHUNKS of one file do. They multiply: concurrency: 3 with chunkConcurrency: 4 is up to twelve requests in flight, which is more than a browser will run per host and more than most servers enjoy. Raise one or the other, and measure rather than guess.