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Huge performance regression between kernel 4.13 and 4.14

Hi!

I recently tried to upgrade a HP DL380 G8 server with 396G memory running mysql replication and a threaded java program from F25 to F27. After upgrade, the server was not able to do what it did before at all.

The java program creates a small Lucene index and send 140 000 pre-compiled queries to if threaded. This takes at least twice as long on 4.14 as on 4.13. The kernel was 4.14.8, so this was before the PTI fix.

Fortunately, booting into the 4.13.16 from F25 (without downgrading the rest of the system) fixed the problem completely.

Any ideas about what is causing this / how to proceed to figure that out? Could some of the memory management changes in 4.14 cause the regression?