[Bug 277476] graphics/drm-515-kmod: amdgpu periodic hangs due to phys contig allocations

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=277476

--- Comment #47 from Olivier Certner <[email protected]> ---
With all the fixes above, I'm still seeing slowdowns once all the machine's
memory is fragmented, but I had to wait for a much longer time for the
situation to degrade.

Another fix is needed:
https://github.com/freebsd/drm-kmod/pull/377
(It's basically what sigsys@ mentioned above, except I don't find it optional.
:-))

With this I've yet to experience slowdowns with DRM 6.1.  Observing free memory
and its fragmentation shows the latter has diminished tremendously after this
last fix even after a while, giving confidence that nothing is trying to
allocate non-trivial amount of memory (> PAGE_SIZE) in physically contiguous
chunks in the fast path.

So we are perhaps at the bottom of this now.  FreeBSD apparently does not
defragment enough to sustain frequent contiguous allocations (probably Linux
does, assuming nobody complains about slowdowns there?), but as a matter of
fact these are not really needed.  In new DRM code, we'll have to watch out for
new allocations of such kind, which would degrade performance again until
patched.

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