This is part of a series of periodic updates of development in Shadow. This work is sponsored by the NSF. Previous update: 2021-10.

We've merged 78 pull requests and closed 17 issues since our previous update.

Release status

We have released Shadow 2.0! Give it a try, and let us know if you run into any issues!

We have begun work on Shadow 2.1. Notable planned features are support for signals (which is needed to reliably run golang programs under Shadow), and improved Unix socket support.

Notable change since last update

Emulation accuracy

• Add unnamed unix sockets created with socketpair()
• Add support for the 'O_DIRECT' flag to pipes (packet mode)
• poll and epoll: wait for timeout even with no fd's

Performance improvements

• Set default min runahead to 1ms. This fixes a performance regression in many-host simulations using multiple CPUs.

Stability

• Detect and handle death by signals
• Handle process exit while it has events scheduled

UI improvements

• Move host heartbeat options to the experimental options, and disable by default

Shadow at Tor

We have been collaborating with The Tor Project to use Shadow in its development and testing.

We have been running Shadow simulations inside a Gitlab CI pipeline to help develop and tune improved congestion control algorithms in the upcoming 0.4.7 release. This is the first major application of Shadow inside the Tor Project itself, and we plan to use the pipeline we've developed for further testing and profiling.

We have also been making progress on running Arti (the experimental new Rust implemenation of tor) under Shadow: https://gitlab.torproject.org/tpo/core/arti/-/issues/174. In the course of this work we have fixed some bugs in Arti, fixed an upstream bug in the async-io crate, and found and identified and fixed several emulation accuracy bugs in Shadow.

Happy simulating!
The Shadow team