Friday, June 20, 2008

Exokernel Concept

I'm doing the module Advanced Operating Systems and while I'm reading on that I found some interesting topic called Exo-kernel Operating systems.
This was initially proposed by a research group at MIT in 1995. This is a concept which is very useful when it comes to arena of developing high performing applications and this concept is really controversial when we concern about the idea people had about operating systems.Previously people thought about operating systems as a piece of software which hide all the tricky information from application developer and user of the system.But when we concentrate about the performance this abstract layer leads to a huge performance hit due to this fixed abstraction.
As an example if we concern about a Webserver it will keep on increasing it's cache although it's exceed the main memory since the operating system itself handle the memory using the virtual memory so application doesn't have any idea about what's going on inside.So this leads to reduce the performance of the web server.
Likewise using the high level abstraction is always reduce the flexibility of the application development and limit the performance and functionality of the system.So what they suggest with this exokernel concept is kernel (exokernel) will handle the hardware multiplexing and resource protection while application itself will handle all the resource management using their own applicaton specific policies. So what happen is application developer is suppose to implement a piece of software called library operating system which do all the resource management and the kernel will take care of multiplexing the hardware. But the main challenge of this concept is every application has to develop it's own library operating system and this leads to a real overhead.Exokernel researchers develop their own library operating system called ExoS and an Exo kernel called Aegis and they did some benchmark with their resources and click here to see that...

And they have developed a web server called cheetah which use it's own library operating system which is specific for a web server and they used infomations like knowledge about HTTP to bind with IO and they are not copying data but they send data directly to the client from it's cache. So this is the performance difference they had with that cheetah .. click here

So Exokernel principle is something which seperate the hardware protection from resource management from a typical operating system.With exokernel it's not trying to give a virtual machine to each and every application but it's tying to expose hardware to the application while protecting hardware resources from untrusted application.Inorder to implement the protection with exokernels they use this techniques
  • Secure Binding
  • Visiblel Revocation
  • Abort Protocol
Secure binding is a way of protecting resource by binding them with the library operating systems in order to impelement it they have use several techniques like hardware protection, software protection and download code.Visible revocation is revocating resources with keep in contact with the library operating system, with normal applications revocation happens invisibly as an example when kernel wants to deallocate a particular page it doesn't go and ask from application but what it does is simple deallocate the resource but in here kernel deallocate memory by asking from the library operating system. And at last Abort Protocol is a way of break binding when library operating system is unable to respond to the kernel.So here what happen is when libOS is unable to respond to particular request it breaks appropriate resources which bind with library operating system and let the application to know that.

Those are the information I got about exokernels and more commanets will be very useful.

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