Improving startup time

Charles Oliver Nutter edited this page Mar 9, 2017 · 21 revisions
Clone this wiki locally

JRuby, being built atop an optimizing VM (the JVM), sometimes tends to start up more slowly than the C-based, non-optimizing standard Ruby. This can often be very frustrating when running lots of commands at the command line. This page offers some tips and tricks to improve startup time

Note that some of these tips are not recommended for general execution; for example, disabling JRuby's JIT will obviously reduce runtime performance. Mix and match to suit your needs.

Use the "--dev" flag

As of JRuby 1.7.12 and jruby-launcher 1.1.0, you can pass the --dev option to JRuby to choose the fastest-starting JVM and JRuby flags (at the expense of long-running code speed). Currently this enables the following settings:

  • client mode where applicable (generally older 32-bit JVMs)
  • TieredCompilation and TieredStopAtLevel=1, equivalent to client mode on newer Hotspot-based JVMs
  • compile.mode=OFF to disable JRuby's JVM bytecode compiler
  • jruby.compile.invokedynamic=false to disable the slow-to-warmup invokedynamic features of JRuby

The --dev flag combines several tips from below, as noted.

Ensure your system has adequate entropy

Via issues like jruby/jruby#1896, we have learned that some Linux users run into startup and execution bottlenecks due to exhausting the random number entropy pool.

When JRuby boots up, the JDK libraries responsible for random number generation go to /dev/random for (at least) initial entropy. After this point, more recent versions of JRuby will use a PRNG for subsequent random numbers, but older versions will continue to return to /dev/random. Unfortunately /dev/random can "run out" of "good" random numbers, providing a guarantee that reads from it will not return until the entropy pool is restored. On some systems -- especially virtualized -- the entropy pool can be small enough that this slows down JRuby's startup time or execution time significantly.

You may be able to solve these issues by supplying more entropy to the Linux kernel or by configuring it to maintain a larger entropy pool on its own. Here's a good article on setting up additional sources of entropy.

Use the "client" mode of the JVM

The most commonly-used JVM, Hotspot (aka Sun/Oracle's VM, OpenJDK), ships its 32-bit version with two execution modes: "client" and "server". The client mode is designed to start up quickly and not optimize as much. Server mode takes longer to start and warm up code, but eventually optimizes code much more than client mode.

On 64-bit Hotspot, client mode is not available, but you can configure the "tiered" compiler mode to behave similarly.

Enabling client mode (32-bit)

All 32-bit versions of Hotspot ship with client mode. You can turn it on by passing -client to the java command, or through JRuby you can pass --client or -J-client. This is the most effective way to improve startup time.

Provided by the --dev flag.

Client mode on OS X

On OS X, where the Hotspot JVM is sometimes (Java 1.6 and lower) shipped in a "universal" 32/64-bit binary, you may also have to pass -d32 to the java command, or -J-d32 to JRuby, to force the JVM to start up in 32-bit mode. Without this flag, the JVM may start in 64-bit mode, which does not have a "client" mode.

Provided by the --dev flag.

Tiered compilation (64-bit)

When running a 64-bit-only Hotspot JVM, there is no client mode, and the -client flag will not change execution in any way. In these environments, you can get the same effect as client mode by passing two flags: -XX:+TieredCompilation -XX:TieredStopAtLevel=1 (prefixed with -J if using the jruby command, as usual). This forces the "tiered" compiler into a mode that starts up fast and does not optimize beyond what "client" would. These flags have been tested on Java 6 (OpenJDK 1.6) through Java 8 (OpenJDK 1.8) and like "client" mode they appear to provide the best startup times.

Note that limiting the compiler to tier one has the same effect on straight-line execution performance as setting -client mode...i.e. it's worse, where worse can mean a few times slower to an order of magnitude slower. If you need to run code as fast as possible, you don't want this option. If you don't need code to be fast but you want the application to start up quickly, this option may be good for you.

Provided by the --dev flag.

Disable JRuby's JIT

An interesting side effect of JRuby's JIT is that it sometimes actually slows execution for really short runs. The compilation isn't free, obviously, nor is the cost of loading, verifying, and linking the resulting JVM bytecode. If you have a very short command that touches a lot of code, you might want to try disabling or delaying the JIT.

Disabling is easy: pass the -X-C flag to JRuby or set the jruby.compile.mode property to "OFF" by passing -Djruby.compile.mode=OFF to the java command.

Provided by the --dev flag.

Disable invokedynamic

Similar to the JIT invokedynamic is an optimization that can speed up steady state operation but slows down startup, especially on some early JVM implementations of that feature. Disable with -Xcompile.invokedynamic=false as cli argument/in JRUBY_OPTS.

Provided by the --dev flag.

Regenerate the JVM's shared class-data archive

Starting with Java 5, the HotSpot JVM has included a feature known as Class Data Sharing (CDS). Originally created by Apple for their OS X version of HotSpot, this feature loads all the common JDK classes as a single archive into a shared memory location. Subsequent JVM startups then simply reuse this read-only shared memory rather than reloading the same data again. It's a large reason why startup times on Windows and OS X have been so much better in recent years, and users of those systems may be able to ignore this tip.

On Linux, however, the shared archive is often never generated, since installers mostly just unpack the JVM into its final location and never run it. In order to force your system to generate the shared archive, run the following command (as a user with write permissions to Java's install directory): java -Xshare:dump

Many Linux systems will install Java but not generate or include the CDS archive. Forcing it to regenerate can improve startup quite a bit.

Avoid spawning "sub-rubies"

It's a fairly common idiom for Rubyists to spawn a Ruby subprocess using Kernel#system, Kernel#exec, or backquotes. For example, you may want to prepare a clean environment for a test run. That sort of scenario is perfectly understandable, but spawning many sub-rubies can take a tremendous toll on overall runtime.

Running sub-rubies in the same JVM

If you need to run simple scripts or utilities as JRuby subprocesses (or simply as embedded JRuby instances in any JVM application), you can avoid the cost of shelling out and launching an entirely new JVM and JRuby by simply using JRuby's embedding API, RedBridge to run it as an isolated JRuby on the same JVM.

container =

See the article above for more detailed information. RedBridge is a Java API, but of course you can call it from Ruby code as in this example.

bundle exec

Bundler's "exec" command causes a second JRuby instance to be launched for the sole purpose of booting only your Gemfile gems. You can avoid the second process by passing -G to JRuby, which will do the Bundler pre-booting before starting JRuby and loading RubyGems.

bundle exec foo.rb
# is equivalent to
jruby -G foo.rb

Do less at startup

This is a difficult tip to follow, since often it's not your code doing so much at startup (and usually it's RubyGems itself--avoid if possible). One of the sad truths of JRuby is that because we're based on the JVM, and the JVM takes a while to warm up, code executed early in a process runs a lot slower than code executed later. Add to this the fact that JRuby doesn't JIT Ruby code into JVM bytecode until it's been executed a few times, and you can see why cold performance is not one of JRuby's strong areas.

It may seem like delaying the inevitable, but doing less at startup can have surprisingly good results for your application. If you are able to eliminate most of the heavy processing until an application window starts up or a server starts listening, you may avoid (or spread out) the cold performance hit. Smart use of on-disk caches and better boot-time algorithms can help a lot, like saving a cache of mostly-read-only data rather than reloading and reprocessing it on every boot.

Try using Theine for Rails apps

theine is a Rails application pre-loader designed for JRuby. It spawns processes in the background that load your Rails application, and when you need to run a supported command (rails console, server, rspec, rake), you can run it in one of these background process. In combination with allowing you to use a different Ruby for the Theine client (such as Ruby 2.0), this can decrease the startup time to around half a second. Theine will automatically manage a pool of these processes, and there are configuration settings for the pool size. Generally, it works similarly to pry-remote and Spork.

Try using Drip

drip is a command line tool that can be used to lower perceived JVM startup time. It does this by preloading an entirely new JVM process\instance and allowing you to simply use the preloaded environment. A few resources to get you started using drip with JRuby:

Using Drip with JRuby

Try using Nailgun

In JRuby 1.3, we officially shipped support for Nailgun. Nailgun is a small library and client-side tool that reuses a single JVM for multiple invocations. With Nailgun, small JRuby command-line invocations can be orders of magnitude faster.

To use Nailgun, you need to first start a server instance (probably in the background) by passing --ng-server to JRuby. Subsequent commands can use that server instance by passing --ng to JRuby.

Nailgun seems like a magic bullet, but unfortunately it does little to help certain common cases like booting RubyGems or starting up Rails (such as when running tests). It also can't help cases where you are causing lots of sub-rubies to be launched, and if you have long-running commands the usual Control-C may not be able to shut them down on the server. Your best bet is to give it a try and let us know if it helps.

Help us find bottlenecks

The biggest advances in startup-time performance have come from users like you investigating the load process to see where all that time is going. If you do a little poking around and find that particular libraries take unreasonably long to start (or just do too much at startup) or if you find that startup time seems to be limited by something other than CPU (like if your hard drive starts thrashing madly or your memory bus is being saturated) there may be improvements possible in JRuby or in the libraries and code you're loading. Dig a may be surprised what you find.

Here's a few JRuby flags that might help you investigate:

  • --profile and --profile.graph turn on JRuby's built-in profiler in "flat" and "graph" modes. Often there will be a slow Ruby-level operation during startup that can be fixed or omitted.
  • --sample turns on the JVM's sampling profiler. It's not super accurate, but if there's some egregious bottleneck it should rise to the top. This can be useful to find especially slow code in JRuby itself.
  • -J-Xrunhprof:cpu=times turns on the JVM's instrumented profiler, saving profile results to java.hprof.txt. This slows down execution tremendously, but can give you more accurate low-level timings for JRuby and JDK code.
  • -J-Djruby.debug.loadService.timing=true turns on timing of all requires, showing fairly accurately where boot-time load costs are heaviest. If there are files that take especially long, there may be a good reason for it.
  • On Windows, where you may not have a "time" command, pass -b to JRuby (as in jruby -b ...) to print out a timing of your command's runtime execution (excluding JVM boot time).

Use a splashscreen

If yours is an end user app with a GUI, you can instruct java to display a splash screen while it loads the JVM. This can help alleviate the pain (to end users) of the startup time. The jruby parameter is something like -J-splash=XXX

When all else fails

If none of these tips gets your startup time to a manageable level, you should connect with JRuby devs and users by posting to the JRuby Users mailing list. They may be able to help with tips specific to your case, or help you investigate if there's a startup-impacting bug that can be fixed.