Add talos info.

Benchmarking-your-changes.md

@@ -1,3 +1,8 @@
+Two options for benchmarking:
+1) [Using the Test Runner](#using-the-test-runner)
+2) [Using Firefox Talos](#using-firefox-talos)
+
+## Using the Test Runner
 When working on issues related to performance, it is important to provide a performance benchmark for your changes to assess whether or not your change has a performance impact. PDF.js provides tools to do this easily. Normally you would create a simple manifest file has a couple of PDF files you're trying to optimize and run it multiple times, e.g. `benchmark.json`:
 
 ```
@@ -35,3 +40,49 @@ Finally, you can compare the measurements and see any performance differences:
 As a sanity check, you should do this twice with the same code and compare the results.
 
 For faster benchmarking during development, it could be useful to pass the `--noChrome` command line parameter to `test.js` to skip Chrome and only test with Firefox.
+
+
+## Using Firefox Talos
+
+### About
+Versions of PDF.js that are merged in to Firefox are automatically benchmarked as part of the talos tests to monitor performance regressions/improvements. The talos [pdfpaint](https://firefox-source-docs.mozilla.org/testing/perfdocs/talos.html#pdfpaint) test benchmarks how long it takes from a PDF being opened to the first page of the PDF being rendered. The advantage of using this system for benchmarking is that it is closer to a real world benchmark where we test the most important thing, which is how long it takes for the first page to render.
+
+### Running Locally
+
+#### Prerequisites
+This assumes you've setup a [local PDF.js repo](https://github.com/mozilla/pdf.js#getting-the-code) and you've setup a [build of Firefox locally](https://firefox-source-docs.mozilla.org/setup/index.html). We recommend using an non-debug artifact build of Firefox.
+
+#### Adding a Custom PDF
+_If you just want to benchmark your changes with the default PDFs you can skip this step._ 
+1. Copy the PDF file into `<firefox_src_dir>/testing/talos/talos/tests/pdfpaint/`
+2. Add the file to the manifest in `<firefox_src_dir>/testing/talos/talos/tests/pdfpaint/pdfpaint.manifest`
+3. (Optional) Remove any other PDF files in the manifest if you only want to benchmark your PDF
+
+#### Record the Baseline Data
+First, we want to run the test PDF without any changes to see how it will compare. In the `<firefox_src_dir>` run:
+
+`./mach talos-test -a pdfpaint`
+
+When the test finishes there will be a section in the terminal with `------- Summary: start -------` and `-------- Summary: end --------`. You'll want to copy that section down somewhere for comparison.
+
+#### Apply Your Changes & Test Again
+1. In the `<pdfjs_src_dir>`: 
+   1. make your changes
+   2. build `gulp mozcentral`
+   3. copy the pdf.js build to firefox `rsync -a -v <pdfjs_src_dir>/build/mozcentral/browser/extensions/pdfjs/ <firefox_src_dir>toolkit/components/pdfjs/`
+2. Back in `<firefox_src_dir>`:
+   1. Build firefox `./mach build faster`
+   1. Run the tests again `./mach talos-test -a pdfpaint`
+
+The talos tests will run and output a test summary. You can now compare these results to see how your changes affected performance.
+
+### Running pdfpaint on Mozilla's infrastructure
+Similar to above, you'll want to create a baseline result without your changes and another result with your changes. The pdfpaint test is part of the talos "other" group. An example of running the tests 6 times on windows:
+
+`./mach try fuzzy -q 'windows10-64-shippable opt-talos-other' --rebuild 6 --artifact`
+
+Once the tasks finish, you can then compare the results by using the two commit IDs and replacing `<baseline>` and `<changed>` below:
+
+```
+https://treeherder.mozilla.org/perfherder/compare?originalProject=try&originalRevision=<baseline>&newProject=try&newRevision=<changed>&framework=1&showOnlyImportant=1&page=1
+```