escape_json_string_in_html causes ~23 MB of allocations for 1 MB of JSON

[tv42]

Describe the bug

I was troubleshooting why my SvelteKit app SSR was consuming a lot of RAM. There's likely other reasons, but this jumped out of the profile:

escape_json_string_in_html (which is just a wrapper for escape,

kit/packages/kit/src/utils/escape.js

Line 53 in 59badb5

function escape(str, dict, unicode_encoder) {

) allocates 23 MB for a 1MB input.

Reproduction

From a Page's load function, make a fetch to a 1 MB JSON file, and return it as a prop.

The SSR HTML will end with something like

 <script type="application/json" data-type="svelte-data" data-url="/api/graphql" data-body="foo">...</script>

containing said HTML, but creating that HTML apparently takes about 23x the size of the JSON in allocations.

Logs

No response

System Info

System:
    OS: Linux 5.15 NixOS 21.11 (Porcupine) 21.11 (Porcupine)
    CPU: (8) x64 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
    Memory: 6.33 GB / 62.61 GB
    Container: Yes
    Shell: 5.1.8 - /nix/store/xdxv22yb7qnjfyf997r9fx804761z14c-bash-interactive-5.1-p8/bin/bash
  Binaries:
    Node: 16.13.0 - /nix/store/26lslfmvwyhspppbinznh111fyaxx62z-nodejs-16.13.0/bin/node
    npm: 8.1.0 - /nix/store/26lslfmvwyhspppbinznh111fyaxx62z-nodejs-16.13.0/bin/npm
  Browsers:
    Firefox: 96.0.2
  npmPackages:
    @sveltejs/adapter-cloudflare-workers: ^1.0.0-next.30 => 1.0.0-next.30
    @sveltejs/kit: ^1.0.0-next.239 => 1.0.0-next.239
    svelte: ^3.46.1 => 3.46.2

Severity

annoyance

Additional Information

No response

[bluwy]

@tv42 there has been changes to the function (renamed escape_json_in_html). Can you confirm that it still takes up large amount of memory with the latest version?

[benmccann]

@tv42 can you please confirm whether this issue is still present? Otherwise we will close this issue shortly

[tv42]

This seems fixed.

In @sveltejs/kit@1.0.0-next.294, the memory use looks to be roughly equal to size of the JSON response (~1 MB JSON, ~1.1 MB allocated in render_json_payload_script).

In older @sveltejs/kit@1.0.0-next.267, that was ~34 MB allocated in escape.

How to see SvelteKit SSR memory profiles

npm run build && node --inspect node_modules/.bin/svelte-kit preview

Follow instructions at https://nodejs.org/en/docs/guides/debugging-getting-started/ to open inspector in browser.
I'm assuming Chrome here for the instructions.

• Select "Memory" tab.
• Select "Allocation instrumentation on timeline".
• Click "Start".
• curl localhost:3000
• Click red circle to stop recording profile.
• In the "Summary" popdown, select "Allocation".
• Click "Size" header twice to sort by size descending.