Why do some servers have multiple CPUs, is it for performance or reliability?

it's pure performance. purpose of servers serve. provide service normal computer users. let's take simple example: web server.

yahoo uses web servers can visit website. server sends webpages destination, laptop in case.

imagine 100.000 people visiting website @ exact same time. server has immense ammount of work in short time. it's unaccaptable webpage take 5 seconds load these days.

solution problem adding more processing power. done adding more processors , in yahoo's case multiple servers. 100.000 page requests divided on multiple servers, multiple processors, multiple cores , threads, that's different story. dividing lot of work in small pieces, websites yahoo manage provide great performance, when lot of people visiting website.

same idea applies other situations, online game lot of players, cloud services dropbox , mroe.

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