New York: If you want to know why Facebook never hangs or slows down on your old smartphone, this is because a team of engineers is working to improve the functioning of the social media app 24/7. According to The Verge, Facebook's Mobile Device Lab tests Facebook's software on older phones to discover whether any bit of a new code, no matter how minor, results in a dip in performance or poorer battery life. The Mobile Device Lab has a series of about 60 server racks and each one houses 32 smartphones, all of which are running a version of one of Facebook's many mobile apps. "In total, Facebook has almost 2,000 handsets used to tell developers when they've screwed something up, and whether that degradation is only noticeable on an older phone," the report quoted Antoine Reversat, part of Facebook's production engineering team, as saying. "When a developer makes a change, we build a new version of the app. We then install it here on one of the devices and check that the change didn't introduce regressions," Reversat added. Facebook engineers worry that even the tiniest change in code could have unpredictable results in something as major as battery consumption.