1. Characterizing the Microarchitectural Side Effects of Operating System Calls
How much does OS operation impact your code’s performance?
As your program executes, data is stored within the CPU to improve instruction execution time
Branch Predictor
TLB
Cache
Addison Mayberry, Matt Laquidara, Charles Weems
Architecture and Language Implementation Lab
University of Massachusetts Amherst
When user code makes call to the operating system, these structures are altered
When your code resumes, it will be slowed until this data is replaced
How do we measure this impact?
Features
Challenges
Within a simulator, we back up all of the data in these structures before a system call
We make use of the MARSSx86 simulator, which is cycle-accurate and validated against actual hardware
The performance of MARSSx86 varies based on external factors such as clock events and I/O, multiple trials must be run for accuracy
We then reset the structures, clearing the original data
We are able to describe in detail the low-level effects of OS interference on microarchitectural state
This profiling must be done for each OS version, the results vary between operating systems
The system call is then allowed to run and put data into the structures
Our system can clearly demonstrate the performance decrease that is caused by OS calls
The results are sensitive to perturbing factors such as the link order of the compiled kernel
When the call concludes, we record the amount of data that has been stored – this is the call’s impact
Before resuming user code, the original data is placed back into the structure, allowing user code to proceed at its optimal speed with little or no performance impact
The results we generate could be used to improve the accuracy of user-mode simulators that currently ignore these effects on performance
There may be constructive interference from the OS, such as data being loaded into cache for user code, which our system removes
We are able to perfectly save system state for easily repeatable trials
Performing this backup / restore procedure increases the hit rate in each structure, indicating that most of the impact is destructive