here - Stefan-Marr.de

4.4. RoarVM
is a problematic operation, too. Another detail that needs to be considered is
the approximately 10% space penalty imposed by the extra header word we
use as a backpointer from the object to the object table entry. In addition to
assertion-checking for debugging, this backpointer is required for the sweep
phase of the garbage collector.
As discussed in Sec. 8.1.3, the object table is disabled for the performance
evaluation in order to avoid the performance overhead.
4.4.3. Process-based Parallel VM
Ungar and Adams implemented the VM for the TILE64 and used libraries
that relied on operating system processes instead of threads to utilize the
64 processor cores. When we ported the RoarVM to commodity multicore
systems, we decided to use traditional threads instead of the process-based
variant. The main driver for this decision was the absence of sufficient debugging
tools and that the initially used libraries were only available for the
TILE64.
However, the thread-based implementation is significantly slower than the
process-based implementation. The performance cost for using thread-local
variables instead of static globals is significant. Therefore, the decision was
revisited and a version using processes instead of threads was also implemented
on classic Linux and Mac OS X systems. 8
For this dissertation, the ability to use multiple operating system processes
is relevant in the context of future work (cf. Sec. 9.5.3). It enables the VM to
use different memory protection settings as an implementation technique for
managed state (cf. Sec. 3.4). A similar approach has been used for instance
by Hoffman et al. [2011].
4.4.4. Final Remarks
While the RoarVM is an interpreter and performance evaluation yields results
that are not generalizable to high-performance VMs with JIT compilers, 9 the
RoarVM as a research platform has a number of relevant features that facilitate
experiments and opens opportunities for future research.
The support for arbitrary additional words in front of objects greatly simplifies
experiments that need to adapt the notion of objects and extend it with
8 https://github.com/smarr/RoarVM/tree/features/processes-on-x86
9 An addition of a JIT compiler to the RoarVM would be possible and it would improve the
generalizability of the results, but it is outside of the scope of this dissertation.
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