On Yosemite only, I see some tests time out, apparently stalled waiting for Apache to respond. See e.g. <https://webkit-test-results.appspot.com/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=http%2Ftests%2Fxmlhttprequest%2Fredirect-cross-origin-tripmine.html>
Enabled Apache performance logging in r178798 to hopefully see where the stall occurs. There are a lot of "slow" CGI scripts being executed in parallel with this test when the problem occurs, so it could be that Apache somehow delays handling the resource. On the other hand, we have a high number of concurrent requests allowed, so this shouldn't be happening.
Created attachment 245213 [details] sample log <https://build.webkit.org/results/Apple%20Yosemite%20Debug%20WK2%20(Tests)/r178954%20(1621)/access_log.txt> has some interesting data. All the requests for xmlhttprequest/redirect-cross-origin-tripmine.html test show up as taking a small amount of time. However, there are long periods of time during which no requests are made at all, which is weird - with