[perf] Improve ExportMap.for performance on larger codebases

[leipert]

While looking at a larger code base https://gitlab.com/gitlab-org/gitlab, ExportMap.for seems to take a significant amount of time. More than half of it is spend on calculating hashes with hashObject.

Digging a little deeper, it seems like we are calling it around 500 thousand times. Each iteration calculates the hash of a context object. This context object is created inside of the childContext function and consists of four parts:

• settings -> an Object itself
• parserOptions -> an Object itself
• parserPath -> a String
• path -> a String.

Interestingly settings, parserOptions and parserPath rarely do change for us, so calculating their hashes on every iteration seems unnecessary. hashObject recursively calculates the hashes of each key / value pair.

So instead of doing:

cacheKey = hashObject({settings, parserOptions, parserPath, path})

We could also do:

cacheKey = parserPath +
  hashObject(parserOptions) +
  hashObject(settings) +
  path

This would be just as stable as before, although resulting in longer cache keys. parserPath and path would not need to be hashed, because they are strings and a single character change in them would result in a different cache key.

Furthermore we can memoize the hashes of parserOptions and settings, in case they didn't change compared. The equality is checked with a simple JSON.stringify.

We move this cacheKey calculation to childContext, adding the cache key to the context object. This way, we can fall back to the old calculation inside of ExportMap.for, as it is a public interface which consumers might be using.

In our code base the results speak for itself:

• 51.59s spent in ExportMap.for, 0ms spent in childContext.
  • 16.89s is spent in node:crypto/hash update (overall)
• 41.02s spent in ExportMap.for, 1.91s spent in childContext.
  • Almost no time spent in hashObject, actually all calls in
    our flame graph come from other code paths
  • 7.86s is spent in node:crypto/hash update (overall)

So on this machine, and project, we are cutting the execution time of ExportMap.for in half. On machines, which are hashing slower, the effect might be more pronounced. Similarly machines with less memory, as the hashObject function creates a lot of tiny strings.

One side-effect here could be, that the memoization is in-efficient if the settings or parserOptions change often. (I cannot think of such a scenario, but I am not that versed in the use cases of this plugin.) But even then, the overhead should mainly be the equality checks with JSON.stringify.

[leipert]

This change is quite similar to #2755, but might be a little controversial, because it introduces a new dependency.

I had a look at fast-deep-equal: http://unpkg.com/browse/[email protected]/index.js and I think it's simplicity aligns well with our use case here.

[leipert]

Rebased and ready for review 👍 Thank you!

[ljharb]

The utils changes need to be in a separate commit from the main plugin changes, please.

[leipert]

@ljharb

The utils changes need to be in a separate commit from the main plugin changes, please.

Ha, I added the changes to the utils changelog by accident 🙈 Fixed.

[ljharb]

Seems good!