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Optimizing canisters

Overview​

The Motoko compiler produces small binaries with reasonably efficient code, but is not a highly optimized compiler. It is possible to further optimize Motoko binaries, both for code size and cycle usage, using additional tools such as wasm-opt.

Using wasm-opt​

Wasm-opt is a general purpose Wasm optimizer that is now available in dfx, versions 0.14.0 and newer.

Wasm-opt can be used to enable canister optimizations through a configuration option in the project's dfx.json file, such as:

{
  "canisters": {
    "my_canister": {
      "optimize": "cycles"
    }
  }
}

Optimization levels for cycle usage​

Using the "optimize": "cycles" option, you can expect a rough estimate of decreased cycles usage for Motoko canisters by around 10%.

The "optimize": "cycles" option is the recommended default, as it maps to optimization level 3 in the wasm-opt package.

The optimization levels for cycles usage are as follows:

O4
O3 (equivalent to “cycles”)
O2
O1
O0 (performs no optimizations)

Optimization levels for binary size​

To optimize the binary size instead, you can use the "optimize": "size" option. By using the size option, binary sizes can be reduced by roughly 16%.

The optimization levels for binary size are as follows:

Oz (equivalent to “size”)
Os

Each optimization preserves the BigFile specific metadata sections of each canister.

Note that in certain cases the optimizations can increase the complexity of certain functions in your Wasm module such that they are rejected by the replica. If you run into this issue, it is recommended to use a less aggressive optimization level such that you do not exceed the complexity limit.

More information on canister optimization and information on wasm-opt benchmark testing can be found on this forum post.