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Dynamic execution is a feature in Bazel where local and remote execution of the same action are started in parallel, using the output from the first branch that finishes, cancelling the other branch. It combines the execution power and/or large shared cache of a remote build system with the low latency of local execution, providing the best of both worlds for clean and incremental builds alike.
This page describes how to enable, tune, and debug dynamic execution. If you have both local and remote execution set up and are trying to adjust Bazel settings for better performance, this page is for you. If you don't already have remote execution set up, go to the Bazel Remote Execution Overview first.
The dynamic execution module is part of Bazel, but to make use of dynamic execution, you must already be able to compile both locally and remotely from the same Bazel setup.
To enable the dynamic execution module, pass the --internal_spawn_scheduler flag to Bazel. This adds a new execution strategy called dynamic. You can now use this as your strategy for the mnemonics you want to run dynamically, such as --strategy=Javac=dynamic. See the next section for how to pick which mnemonics to enable dynamic execution for.
For any mnemonic using the dynamic strategy, the remote execution strategies are taken from the --dynamic_remote_strategy flag, and local strategies from the --dynamic_local_strategy flag. Passing --dynamic_local_strategy=worker,sandboxed sets the default for the local branch of dynamic execution to try with workers or sandboxed execution in that order. Passing --dynamic_local_strategy=Javac=worker overrides the default for the Javac mnemonic only. The remote version works the same way. Both flags can be specified multiple times. If an action cannot be executed locally, it is executed remotely as normal, and vice-versa.
If your remote system has a cache, the --dynamic_local_execution_delay flag adds a delay in milliseconds to the local execution after the remote system has indicated a cache hit. This avoids running local execution when more cache hits are likely. The default value is 1000ms, but should be tuned to being just a bit longer than cache hits usually take. The actual time depends both on the remote system and on how long a round-trip takes. Usually, the value will be the same for all users of a given remote system, unless some of them are far enough away to add roundtrip latency. You can use the Bazel profiling features to look at how long typical cache hits take.
Dynamic execution can be used with local sandboxed strategy as well as with persistent workers. Persistent workers will automatically run with sandboxing when used with dynamic execution, and cannot use multiplex workers. On Darwin and Windows systems, the sandboxed strategy can be slow; you can pass --reuse_sandbox_directories to reduce overhead of creating sandboxes on these systems.
Dynamic execution can also run with the standalone strategy, though since the standalone strategy must take the output lock when it starts executing, it effectively blocks the remote strategy from finishing first. The --experimental_local_lockfree_output flag enables a way around this problem by allowing the local execution to write directly to the output, but be aborted by the remote execution, should that finish first.
If one of the branches of dynamic execution finishes first but is a failure, the entire action fails. This is an intentional choice to prevent differences between local and remote execution from going unnoticed.
For more background on how dynamic execution and its locking works, see Julio Merino's excellent blog posts{: .external}
Dynamic execution requires some form of remote execution system. It is not currently possible to use a cache-only remote system, as a cache miss would be considered a failed action.
Not all types of actions are well suited for remote execution. The best candidates are those that are inherently faster locally, for instance through the use of persistent workers, or those that run fast enough that the overhead of remote execution dominates execution time. Since each locally executed action locks some amount of CPU and memory resources, running actions that don't fall into those categories merely delays execution for those that do.
As of release 5.0.0-pre.20210708.4{: .external}, performance profiling contains data about worker execution, including time spent finishing a work request after losing a dynamic execution race. If you see dynamic execution worker threads spending significant time acquiring resources, or a lot of time in the async-worker-finish, you may have some slow local actions delaying the worker threads.
In the profile above, which uses 8 Javac workers, we see many Javac workers having lost the races and finishing their work on the async-worker-finish threads. This was caused by a non-worker mnemonic taking enough resources to delay the workers.
When only Javac is run with dynamic execution, only about half of the started workers end up losing the race after starting their work.
The previously recommended --experimental_spawn_scheduler flag is deprecated. It turns on dynamic execution and sets dynamic as the default strategy for all mnemonics, which would often lead to these kinds of problems.
The dynamic execution approach assumes there are enough resources available locally and remotely that it's worth spending some extra resources to improve overall performance. But excessive resource usage may slow down Bazel itself or the machine it runs on, or put unexpected pressure on a remote system. There are several options for changing the behaviour of dynamic execution:
--dynamic_local_execution_delay delays the start of a local branch by a number of milliseconds after the remote branch has started, but only if there has been a remote cache hit during the current build. This makes builds that benefit from remote caching not waste local resources when it is likely that most outputs can be found in the cache. Depending on the quality of the cache, reducing this might improve build speeds, at the cost of using more local resources.
--experimental_dynamic_local_load_factor is an experimental advanced resource management option. It takes a value from 0 to 1, 0 turning off this feature. When set to a value above 0, Bazel adjusts the number of locally scheduled actions when many actions waiting to be scheduled. Setting it to 1 allows as many actions to be scheduled as there are CPUs available (as per --local_cpu_resources). Lower values set the number of actions scheduled to correspondingly fewer as higher numbers of actions are available to run. This may sound counter-intuitive, but with a good remote system, local execution does not help much when many actions are being run, and the local CPU is better spent managing remote actions.
--experimental_dynamic_slow_remote_time prioritizes starting local branches when the remote branch has been running for at least this long. Normally the most recently scheduled action gets priority, as it has the greatest chance of winning the race, but if the remote system sometimes hangs or takes extra long, this can get a build to move along. This is not enabled by default, because it could hide issues with the remote system that should rather be fixed. Make sure to monitor your remote system performance if you enable this option.
--experimental_dynamic_ignore_local_signals can be used to let the remote branch take over when a local spawn exits due to a given signal. This is is mainly useful together with worker resource limits (see --experimental_worker_memory_limit_mb, --experimental_worker_sandbox_hardening, and --experimental_sandbox_memory_limit_mb)), where worker processes may be killed when they use too many resources.
The JSON trace profile contains a number of performance-related graphs that can help identify ways to improve the trade-off of performance and resource usage.
Problems with dynamic execution can be subtle and hard to debug, as they can manifest only under some specific combinations of local and remote execution. The --debug_spawn_scheduler adds extra output from the dynamic execution system that can help debug these problems. You can also adjust the --dynamic_local_execution_delay flag and number of remote vs. local jobs to make it easier to reproduce the problems.
If you are experiencing problems with dynamic execution using the standalone strategy, try running without --experimental_local_lockfree_output, or run your local actions sandboxed. This may slow down your build a bit (see above if you're on Mac or Windows), but removes some possible causes for failures.