Re: Heads up: ubc_direct enabled by default

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

On Thu, Apr 23, 2020 at 10:10:56PM +0000, Andrew Doran wrote:
> This affects amd64, alpha and aarch64, but only 1 and 2 CPU systems so far. 
> Any more and it's still off by default.  Only the default has changed so the
> sysctl (vm.ubc_direct) still works for turning it on and off manually.
> 
> This works great for me on amd64 but needs some tweaks to handle many CPUs.
> I have some ideas on that one and hopefully will have something to try soon.


What are the risks of switching it on for > 2 CPUs?


(With a LOCKDEBUG kernel on a 8-core ryzen 1700 for PR kern/55362, after
days I'm at

# dd if=/dev/zero obs=64k | progress -l 12884901888b dd of=/dev/rdk6 ibs=64k
  8% |**                                 |   544 GiB    2.18 MiB/s    --:-- ETA

which I think could be helped with ubc_direct. I don't wnat to increase
the problem domain though.)


Cheers,

Patrick