What does syslog entry Memory pressure relief indicate

Well, looks like you ran out of available physical memory. adding swap would not help unless another process can be swapped out to make enough physical memory available.

I see the message being generated by liferea. It is being generated from webkit.

A comment in the webkit bugzilla shows the same message being logged https://bugs.webkit.org/show_bug.cgi?id=165533.

14:40:36.224304 -0800   Safari  Memory pressure relief: Total: res = 45481984/44896256/-585728, res+swap =  46080000/45494272/-585728

In the Webkit source MemoryPressureHandler::releaseMemory, a log message is generated with the 'Total' prefix.