Persistent workers

Persistent workers (often called workers for short) is an execution strategy that decreases start-up overhead, allows more JIT compilation, and enables caching of for example the abstract syntax trees in the action execution. This strategy achieves these improvements by sending multiple requests to a long-running process. Persistent workers are available for Java (built-in), TypeScript, Scala, and more.

Using persistent workers

Bazel 0.27 and higher by default uses persistent workers when executing builds, though remote execution takes precedence. For actions that do not support persistent workers, Bazel falls back to regular workers instead. You can explicitly set your build to use persistent workers by setting the worker strategy for the applicable tool mnemonics. As a best practice, this example includes specifying local as a fallback to the worker strategy:

bazel build //my:target --strategy=Javac=worker,local

Using the workers strategy instead of the local strategy can boost compilation speed significantly, depending on implementation. For Java, we typically see 2–4 times faster builds, sometimes more for incremental compilation. Compiling Bazel is about 2.5 times as fast with workers. For more details, see the “Choosing number of workers” section.

If you also have a remote build environment that matches your local build environment, you can use the experimental dynamic strategy, which races a remote execution and a worker execution. To enable the dynamic strategy, pass the –experimental_spawn_scheduler flag. This strategy automatically enables workers, so there is no need to specify the worker strategy, but you can still use local or sandboxed as fallbacks.

Choosing number of workers

The default number of worker instances per mnemonic is 4, but can be adjusted with the worker_max_instances flag. There is a trade-off between making good use of the available CPUs and the amount of JIT compilation and cache hits you get. With more workers, more targets will pay start-up costs of running non-JITted code and hitting cold caches. If you have a small number of targets to build, a single worker may give the best trade-off between compilation speed and resource usage (for example, see issue #8586. The worker_max_instances flag sets the maximum number of worker instances per mnemonic and flag set (see below), so in a mixed system you could end up using quite a lot of memory if you keep the default value. For incremental builds the benefit of multiple worker instances is even smaller.

This graph shows the from-scratch compilation times for Bazel (target //src:bazel) on a 6-core hyper-threaded Intel Xeon 3.5 GHz Linux workstation with 64 GB of RAM. For each worker configuration, we ran 5 clean builds and took the average of the last 4.

Graph of performance improvements of clean builds

For this configuration, two workers give the fastest compile, though at only 14% improvement compared to one worker. One worker is a good option if you want to use less memory.

Incremental compilation typically benefits even more. Clean builds are relatively rare, but changing a single file between compiles is common, in particular in test-driven development. The above example also has some non-Java packaging actions to it that can overshadow the incremental compile time. Recompiling the Java sources only (//src/main/java/com/google/devtools/build/lib/bazel:BazelServer_deploy.jar) after changing an internal string constant in gives a 3x speed-up (average of 20 incremental builds with one warmup build discarded):

Graph of performance improvements of incremental builds

The speed-up depends on the change being made. We have measured a speed-up of a factor 6 in the above situation when a commonly used constant was changed.

Modifying persistent workers

You can pass the --worker_extra_flag flag to specify start-up flags to workers, keyed by mnemonic. For instance, passing --worker_extra_flag=javac=--debug turns on debugging for Javac only. Only one worker flag can be set per use of this flag, and only for one mnemonic. Workers are not just created separately for each mnemonic, but also for variations in their start-up flags. Each combination of mnemonic and start-up flags is combined into a WorkerKey, and for each WorkerKey up to worker_max_instances workers may be created. See the next section for how the action configuration can also specify set-up flags.

You can use the --high_priority_workers flag to specify a mnemonic that should be run in preference to normal-priority mnemonics. This can help prioritize actions that are always in the critical path. If there are two or more high priority workers executing requests, all other workers are prevented from running. This flag can be used multiple times.

Passing the --worker_sandboxing flag makes each worker request use a separate sandbox directory for all its inputs. Setting up the sandbox takes some extra time, especially on MacOS, but gives a better correctness guarantee.

You can use the --experimental_worker_allow_json_protocol flag to allow workers to communicate with Bazel through JSON instead of protocol buffers (protobuf). The worker and the rule that consumes it can then be modified to support JSON.

The --worker_quit_after_build flag is mainly useful for debugging and profiling. This flag forces all workers to quit once a build is done. You can also pass --worker_verbose to get more output about what the workers are doing.

Workers store their logs in the <outputBase>/bazel-workers directory, for example /tmp/_bazel_larsrc/191013354bebe14fdddae77f2679c3ef/bazel-workers/worker-1-Javac.log. The file name includes the worker id and the mnemonic. Since there can be more than one WorkerKey per mnemonic, you may see more than worker_max_instances log files for a given mnemonic.

For Android builds, see details at the Android Build Performance page.

Implementing persistent workers

Persistent workers are implemented for multiple languages, including Java, TypeScript, Scala, Kotlin, and more. You can implement persistent workers in other languages and for other tools, as well.

See the creating persistent workers page for information on how to make a worker.

Each worker is a long-running process started by the Bazel server, which functions as a wrapper around the actual tool (typically a compiler). In order to benefit from persistent workers, the tool must support doing a sequence of compilations, and the wrapper needs to translate between the tool’s API and the request/response format described below. The wrapper must recognize the --persistent_worker command-line flag and only make itself persistent if that flag is passed, otherwise it must do a one-shot compilation and exit. The same worker program might be called with and without the --persistent_worker flag in the same build, and is responsible for appropriately spawning and talking to the tool, as well as shutting down workers on exit. Each worker instance is assigned (but not chrooted to) a separate working directory under <outputBase>/bazel-workers.

The Bazel server communicates with the worker using stdin/stdout. It supports the use of protocol buffers or JSON strings. Support for JSON is experimental and thus subject to change. It is guarded behind the --experimental_worker_allow_json_protocol flag.

When using protobuf, the compilation requests are sent as WorkerRequest protocol buffers in standard binary format, and responses are similarly returned as WorkerResponse protocol buffers. Each protocol buffer is preceded by its length in varint format (see MessageLite.writeDelimitedTo().

JSON requests uphold the same structure as the protobuf, but uses standard JSON. Bazel stores the requests as protobufs and converts them to JSON using protobuf’s JSON format Responses are parsed by a JSON parser into the same structure as the WorkResponse protobuf, then converted to proto manually. JSON requests and responses are not preceded by a size indicator.

The request’s args field should contain a list of strings that describe the action to be done. The inputs field may contain input file names and their hash digests, allowing the caching of intermediate results without having to recompute the digest.

Because responses are sent on stdout, neither the worker nor the underlying tool should write other messages into that stream.

Writing other things to stdout crashes the worker. Any output that should be shown to the user can be put in the output field of the response, and output that should be logged should go to stderr. The wrapper should make sure that what the tool writes on stdout is appropriately redirected.

To enable the worker strategy for an action, the execution_requirements for that action must include {"supports-workers": "1"}. It can also include a requires-worker-protocol requirement specifying whether Bazel should communicate with that worker using json or proto. This is required for JSON but is optional for proto since proto is the default. You can also add a worker-key-mnemonic to the execution_requirements section, allowing the mnemonic for workers to be separate from the mnemonic for the action. This can be useful when the same executable is used for several mnemonics, though it limits how much the user can control when to use workers.

The action definition must also contain an arguments definition with a flag-file (@-preceded) argument at the end. Any non-flag-file arguments are startup flags that will be passed to the worker on startup, allowing configuration common to all requests. The flag-file argument is used to read arguments for each request, including possible non-startup flags. To pass an argument starting with a literal @, start the argument with @@ instead. If an argument is also an external repository label, it will not be considered a flag-file argument.

This example shows a Starlark configuration for a worker that uses JSON:

args_file = ctx.actions.declare_file( + "_args_file")
    output = args_file,
    content = "\n".join(["-g", "-source", "1.5"] + ctx.files.srcs),
    mnemonic = "SomeCompiler",
    executable = "bin/some_compiler_wrapper",
    inputs = inputs,
    outputs = outputs,
    arguments = [ "-max_mem=4G",  "@%s" % args_file.path],
    execution_requirements = {
        "supports-workers" : "1", "requires-worker-protocol" : "json" }

With this definition, the first use of this action would start with executing the command line /bin/some_compiler -max_mem=4G --persistent_worker. A request to compile would then look like:

arguments: [ "-g", "-source", "1.5", "" ]
inputs: [
  {path: "symlinkfarm/input1" digest: "d49a..." },
  {path: "symlinkfarm/input2", digest: "093d..."},

The worker receives this on stdin in JSON format (because “requires-worker-protocol” is set to JSON, and --experimental_worker_allow_json_protocol is passed to the build to enable this option). To communicate with the associated worker using binary-encoded protobuf instead of json, requires-worker-protocol would be set to proto, like this:

  execution_requirements = {
    "supports-workers" : "1" ,
    "requires-worker-protocol" : "proto"

If you do not include requires-worker-protocol in the execution requirements, Bazel will default the worker communication to use protobuf.

Bazel derives the WorkerKey from the mnemonic and the shared flags, so if this configuration allowed changing the max_mem parameter, a separate worker would be spawned for each value used. This can lead to excessive memory consumption if too many variations are used.

Each worker can currently only process one request at a time. The experimental multiplex workers feature allows using multiple threads, if the underlying tool is multithreaded and the wrapper is set up to understand this.

In this GitHub repo, you can see example worker wrappers written in Java as well as in Python. If you are working in JavaScript or TypeScript, the @bazel/worker package and nodejs worker example might be helpful.

How do workers affect sandboxing?

Using the worker strategy by default does not run the action in a sandbox, similar to the local strategy. You can set the --worker_sandboxing flag to run all workers inside sandboxes, making sure each execution of the tool only sees the input files it’s supposed to have. The tool may still leak information between requests internally, for instance through a cache. Using dynamic strategy requires workers to be sandboxed.

To allow correct use of compiler caches with workers, a digest is passed along with each input file. Thus the compiler or the wrapper can check if the input is still valid without having to read the file.

Even when using the input digests to guard against unwanted caching, sandboxed workers offer less strict sandboxing than a pure sandbox, because the tool may keep other internal state that has been affected by previous requests.

Further reading

For more information on persistent workers, see: