runtime: TestPhysicalMemoryUtilization failures

[ianlancetaylor]

We've recently started seeing some failures of TestMemoryPhysicalUtilization.

CC @mknyszek

2019-11-08T18:00:54-a2cd2bd/linux-mips-rtrk
2019-11-08T23:22:06-11da2b2/linux-mips64le-rtrk
2019-11-11T17:53:51-70be481/linux-mips64-rtrk
2019-11-11T20:17:03-eb55a0c/windows-386-2008
2019-11-11T23:28:26-eb68c4a/plan9-arm
2019-11-12T01:09:31-0ee22d9/plan9-arm
2019-11-12T18:52:52-1fe33e3/illumos-amd64
2019-11-13T08:07:51-bf5f641/windows-386-2008
2019-11-13T14:18:45-c075115/plan9-arm
2019-11-13T18:03:37-e762378/linux-mips-rtrk

[mknyszek]

I suspect the problem here is that the scavenger's scavAddr may already be too low (for some reason, and only by a little bit) to adequately achieve the desired physical memory utilization. I have some ideas on how to fix it and will come back to this bug shortly.

[gopherbot]

Change https://golang.org/cl/207999 mentions this issue: runtime: make the scavenger loop around the address space

[gopherbot]

Change https://golang.org/cl/207998 mentions this issue: runtime: wake scavenger and update address on sweep done

[gopherbot]

Change https://golang.org/cl/208377 mentions this issue: runtime: increase TestPhysicalMemoryUtilization threshold

[mknyszek]

Found the problem. It's that the page cache was hiding memory from the scavenger (which is intentional). Just uploaded a fix to the test to account for that.