How to optimize canvas circle for retina devices?

Question

Please tell me how to make canvas look sharp for retina devices? I would be grateful if you show in my example - fiddle

var canvas = document.getElementById('timer');
var context = canvas.getContext('2d');
var x = canvas.width / 2;
var y = canvas.height / 2;
var radius = 105;
var endPercent = 100;
var curPerc = 0;
var counterClockwise = false;
var circ = Math.PI * 2;
var quart = Math.PI / 2;