Switching to MQ by default may generate some bug reports

[Mel Gorman]

Hi Christoph,

I know the reasons for switching to MQ by default but just be aware that it's
not without hazards albeit it the biggest issues I've seen are switching
CFQ to BFQ. On my home grid, there is some experimental automatic testing
running every few weeks searching for regressions. Yesterday, it noticed
that creating some work files for a postgres simulator called pgioperf
was 38.33% slower and it auto-bisected to the switch to MQ. This is just
linearly writing two files for testing on another benchmark and is not
remarkable. The relevant part of the report is

Last good/First bad commit
==========================
Last good commit: 6d311fa7d2c18659d040b9beba5e41fe24c2a6f5
First bad commit: 5c279bd9e40624f4ab6e688671026d6005b066fa