In trying to debug periodic gate instability of CentOS, I noticed that
it is using the prefork mpm, while Ubuntu is defaulting to the
multi-threaded worker mpm.
One of the problems seems related to 502 proxy errors from the TLS
proxy. We see out-of-sync timestamps in the centos TLS proxy access
logs, which might be innocent behaviour based on the prefork model or
indicate something else.
Before going too deep down this rabbit-hole, I think it is better for
consistency to use the same mpm model on all our platforms, and start
debugging from there.
Change-Id: I9881f2e7d51fdd9fc0f7fb3e37179aa53171b531
41e6e12318