3 research outputs found
An Analysis of the Search Spaces for Generate and Validate Patch Generation Systems
We present the first systematic analysis of the characteristics of patch
search spaces for automatic patch generation systems. We analyze the search
spaces of two current state-of-the-art systems, SPR and Prophet, with 16
different search space configurations. Our results are derived from an analysis
of 1104 different search spaces and 768 patch generation executions. Together
these experiments consumed over 9000 hours of CPU time on Amazon EC2.
The analysis shows that 1) correct patches are sparse in the search spaces
(typically at most one correct patch per search space per defect), 2) incorrect
patches that nevertheless pass all of the test cases in the validation test
suite are typically orders of magnitude more abundant, and 3) leveraging
information other than the test suite is therefore critical for enabling the
system to successfully isolate correct patches.
We also characterize a key tradeoff in the structure of the search spaces.
Larger and richer search spaces that contain correct patches for more defects
can actually cause systems to find fewer, not more, correct patches. We
identify two reasons for this phenomenon: 1) increased validation times because
of the presence of more candidate patches and 2) more incorrect patches that
pass the test suite and block the discovery of correct patches. These
fundamental properties, which are all characterized for the first time in this
paper, help explain why past systems often fail to generate correct patches and
help identify challenges, opportunities, and productive future directions for
the field
An Analysis of the Search Spaces for Generate and Validate Patch Generation Systems
We present the first systematic analysis of the characteristics of patch search spaces for automatic patch generation systems. We analyze the search spaces of two current state-of- the-art systems, SPR and Prophet, with 16 different search space configurations. Our results are derived from an analysis of 1104 different search spaces and 768 patch generation executions. Together these experiments consumed over 9000 hours of CPU time on Amazon EC2.The analysis shows that 1) correct patches are sparse in the search spaces (typically at most one correct patch per search space per defect), 2) incorrect patches that nevertheless pass all of the test cases in the validation test suite are typically orders of magnitude more abundant, and 3) leveraging information other than the test suite is therefore critical for enabling the system to successfully isolate correct patches.We also characterize a key tradeoff in the structure of the search spaces. Larger and richer search spaces that contain correct patches for more defects can actually cause systems to find fewer, not more, correct patches. We identify two reasons for this phenomenon: 1) increased validation times because of the presence of more candidate patches and 2) more incorrect patches that pass the test suite and block the discovery of correct patches. These fundamental properties, which are all characterized for the first time in this paper, help explain why past systems often fail to generate correct patches and help identify challenges, opportunities, and productive future directions for the field
High Performance Reference Counting and Conservative Garbage Collection
Garbage collection is an integral part of modern programming languages. It automatically
reclaims memory occupied by objects that are no longer in use. Garbage
collection began in 1960 with two algorithmic branches — tracing and reference counting.
Tracing identifies live objects by performing a transitive closure over the object
graph starting with the stacks, registers, and global variables as roots. Objects not
reached by the trace are implicitly dead, so the collector reclaims them. In contrast,
reference counting explicitly identifies dead objects by counting the number of incoming
references to each object. When an object’s count goes to zero, it is unreachable
and the collector may reclaim it.
Garbage collectors require knowledge of every reference to each object, whether
the reference is from another object or from within the runtime. The runtime provides
this knowledge either by continuously keeping track of every change to each reference
or by periodically enumerating all references. The collector implementation faces two
broad choices — exact and conservative. In exact garbage collection, the compiler and
runtime system precisely identify all references held within the runtime including
those held within stacks, registers, and objects. To exactly identify references, the
runtime must introspect these references during execution, which requires support
from the compiler and significant engineering effort. On the contrary, conservative
garbage collection does not require introspection of these references, but instead
treats each value ambiguously as a potential reference.
Highly engineered, high performance systems conventionally use tracing and
exact garbage collection. However, other well-established but less performant systems
use either reference counting or conservative garbage collection. Reference counting has
some advantages over tracing such as: a) it is easier implement, b) it reclaims memory
immediately, and c) it has a local scope of operation. Conservative garbage collection
is easier to implement compared to exact garbage collection because it does not
require compiler cooperation. Because of these advantages, both reference counting
and conservative garbage collection are widely used in practice. Because both suffer
significant performance overheads, they are generally not used in performance critical
settings. This dissertation carefully examines reference counting and conservative
garbage collection to understand their behavior and improve their performance.
My thesis is that reference counting and conservative garbage collection can perform
as well or better than the best performing garbage collectors.
The key contributions of my thesis are: 1) An in-depth analysis of the key design
choices for reference counting. 2) Novel optimizations guided by that analysis that
significantly improve reference counting performance and make it competitive with
a well tuned tracing garbage collector. 3) A new collector, RCImmix, that replaces
the traditional free-list heap organization of reference counting with a line and block heap structure, which improves locality, and adds copying to mitigate fragmentation.
The result is a collector that outperforms a highly tuned production generational
collector. 4) A conservative garbage collector based on RCImmix that matches the
performance of a highly tuned production generational collector.
Reference counting and conservative garbage collection have lived under the
shadow of tracing and exact garbage collection for a long time. My thesis focuses
on bringing these somewhat neglected branches of garbage collection back to life
in a high performance setting and leads to two very surprising results: 1) a new
garbage collector based on reference counting that outperforms a highly tuned production
generational tracing collector, and 2) a variant that delivers high performance
conservative garbage collection