COMP 3000 Essay 2 2010 Question 3

3.FlexSC: Flexible System Call Scheduling with Exception-Less System Calls

Paper

The Title of the paper we will be analyzing is named "FlexSC: Flexible System Call Scheduling with Exception-Less System Calls". The authors of this paper consist of Livio Stores and Michael Stumm, both of which are from the University of Toronto. The paper can be viewed here, [1] for further details on specifics of the essay.

Background Concepts:

In order to fully understand the FlexSC paper, it is essential to understand the key concepts that are discussed within the paper. Here listed below, are the main concepts required to fully comprehend the paper.

System Call

A System Call is the gateway between the User Space and the Kernel Space. The User Space is not given direct access to the Kernel's services, for several reasons (one being security), hence System calls are the messengers between the User and Kernel Space.[1][4]

Mode Switch

Mode Switches speak of moving from one medium to another. Specifically moving from the User Space mode to the Kernel mode or Kernel mode to User Space. It does not matter which direction or which modes we are swtiching from, this is simply a general term.[1]

Synchronous System Call

Synchronous Execution Model(System call Interface) refers to the structure in which system calls specifically are managed in a serialized manner. Moreover, the synchronous model completes one system call at a time, and does not move onto the next system call until the previous system call is finished executing. This form of system call is blocking, meaning the process which initiates the system call is blocked until the system call returns.[1][2]

Asynchronous System Call

An asynchronous system call is a system call which does not block upon invocation; control of execution is returned to the calling process immediately. Asynchronous system calls do not necessarily execute in order and can be compared to event driven programming.[2][3]

System Call Pollution

System Call Pollution is a more sophisticated manner of referring to wasteful or un-necessary delay in the system caused by system calls. This pollution is in direct correlation with the fact that the system call invokes a mode swtich which is not a costless task. The "pollution" involved takes the form of data over-written in critical processor structures like the TLB (translation lookaside buffer - table which reduces the frequency of main memory access for page table entries), branch prediction tables, and the cache (L1, L2, L3).[1][3]

Processor Exceptions

System Call Batching is the concept of batching (i.e. grouping) system calls together. This idea is very similar to the counterpart of system calls in a sequential sequence. Groups are formed of system calls after the idea of batching has occurred instead of the initial individual system calls in a sequence.

References:

[1] Soares, Livio and Michael Stumm, FlexSC: Flexible System Call Scheduling with Exception-Less System Calls, University of Toronto, 2010.PDF

[2] Tanenbaum, Andrew S., Modern Operating Systems: 3rd Edition, Pearson/Prentice Hall, New Jersey, 2008.

[3] Stallings, William, Operating Systems: Internals and Design Principles - 6th Edition, Pearson/Prentice Hall, New Jersey, 2009.

[4] Garfinkel, Tim, Traps and Pitfalls: Practical Problems in System Call Interposition Based Security Tools, Computer Science Department - Stanford University.PDF