Slow performance when parsing file with no EJS code in the file.

[Anders Janmyr]

When parsing a file with no EJS in, performance degrades substantially. The performance improves the closer to the end of the file the EJS code is located.
For a 60k file (600 lines with 100 chars) with no EJS, performance is around 1400ms.
When I add <%= it %> at the end of the file (line 600), performance is1ms
When I add <%= it %> at the middle of the file (line 300), performance is360ms

To Reproduce

1. Create a large file with no EJS. (My test file is a 60k text file with 600 lines with 100 chars each.)
2. Eta.render(body, { dummy: "yes" });
3. Estimated time ~1400ms
4. Add <%= it %> at end of file.
5. Estimated time 1ms.

Package & Environment Details

• OSX 12.6.2
• Node v18.15.0

Test case

import assert from "assert";
import * as Eta from "eta";
import fs from "fs";

const body = fs.readFileSync("./dummy.txt", "utf8");
describe("metrics", function () {
  it("eta", function () {
    const start = Date.now();
    const text = Eta.render(body, { dummy: "yes" });
    const end = Date.now();
    console.log("eta " + (end - start));
    assert(text);
  });
});

[Ben Gubler]

@andersjanmyr wow, this is fascinating. Tbh, I have no idea what the cause of this bug is.

[Ben Gubler]

It seems that str.slice(0, str.length), which is called when there are no tags in the template, may just be significantly slower than RegExp capture. I'll look into this further.

[Anders Janmyr]

My hack to fix the performance is to append an <%= "" %> to the input.

[Ben Gubler]

@andersjanmyr thanks for the tip. I'll see if I can fix this in Eta v3.

[joshcancode]

It seems the RegExp that checks for opening tags is the culprit. The expression consistently takes ~1000ms to run though a 60k file with no tags.

I added a check in the parse function to return if the file has no opening or closing tags, which cut down execution time to ~1ms.

Right after the "pushString" function declaration:

if (!str.includes(config.tags[0]) || !str.includes(config.tags[1])) {
  pushString(str);
  return buffer;
}

[Ben Gubler]

@joshcancode you're right, it seems it was the RegExp. I just tried a change to the RegExp itself, and it seems to resolve the problem (should work if there's a tag in the middle of the file too.)

I pushed my code to https://github.com/eta-dev/eta/tree/fix-issue-227. Do you have any feedback?

[Anders Janmyr]

I can confirm that the fix works. I tried to add a test case for it but, somehow the timeout (10 ms) doesn't seem to have any effect, so it doesn't actually fail. :( jest.test docs

// test/parse.spec.ts
  const largeChunkWithNoTags = "A".repeat(64 * 1024);
  test("parses large chunk with no tags with good performance", () => {
    parse(largeChunkWithNoTags, config);
  }, 10);

[joshcancode]

I pushed my code to https://github.com/eta-dev/eta/tree/fix-issue-227. Do you have any feedback?

Looks good to me! Consistently fast processing times, only a couple of ms now.

I can confirm that the fix works. I tried to add a test case for it but, somehow the timeout (10 ms) doesn't seem to have any effect, so it doesn't actually fail. :( jest.test docs

I'm not too familiar with this library unfortunately :( I'm not sure what could be causing that.

[Ben Gubler]

@andersjanmyr @joshcancode just merged the fix and released in https://github.com/eta-dev/eta/releases/tag/v2.2.0!

If you come up with a way to test this, feel free to submit a PR.

Thanks for your help & feedback!