In some situations font code is now forced to iterate over all fonts in the system, to find a font for a given character, or to look up a localized font name. The chromehang patch (bug 429592) appears to be seeing hangs in GetFontData, one of the routines called when system font fallback occurs (bug 705258). Essentially this is inferring that iterating over fonts takes >30 secs for some users. To verify this, we should instrument the system font fallback code and the other names code with telemetry probes to tell us what the histogram of response times are.
Created attachment 577513 [details] [diff] [review] patch, add in timing of system font fallback This adds two histograms for measuring system font fallback timing. The first time is separated out because the first time cmaps are pulled in if not already cached.
Landed on mozilla-inbound https://hg.mozilla.org/integration/mozilla-inbound/rev/a36ee10d4f5d