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Pardon boundaries of my knowledge, but I wonder why still only one instance of any package manger can be run on a system? Why multiple installations are not possible? If there's a way to achieve this it'll be really helpful..

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Not in parallel, but you can install multiple packages with a single command apt-get install package1 package2 package3 ... packageN, similarly with yum or pacman or whichever package manager you use. – terdon Jul 24 '13 at 15:16
yeah i know, that just installs packages one after another.. – mahesh Jul 24 '13 at 15:17

Mostly because it's possible for packages to make changes to shared system files, and currently there's no method for keeping track of which shared system files the packages' contents, pre-installation or removal scripts, or post-installation or removal scripts make changes to.

If two packages try to modify the same system file at the same time, there is a good chance that the file will be mangled and broken, leading to very bad behavior of the system. Updating the graphics driver and the X Server package at the same time may render the system unable to display a graphical login / interface for the user! This would be very bad.

In theory, you could keep track of this information, and run everything in parallel except for the packages that have to be run one-at-a-time, but this adds a considerable amount of complexity to the package management software and dependency resolution, and generally isn't worth it for the benefits. Usually, the bottleneck is somewhere else - downloading the packages, or the harddrive. If your harddrive is slow, installing packages in parallel can actually slow down package installation.

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