On 5/5/23 6:38 AM, Ed Maste wrote:
FreeBSD supports up to 256 CPU cores in the default kernel configuration
(on Tier-1 architectures). Systems with more than 256 cores are
available now, and will become increasingly common over FreeBSD 14’s
lifetime. The FreeBSD Foundation is supporting the effort to increase
MAXCPU, and PR269572[1] is open to track tasks and changes.
As a project we have scalability work ahead of us to make best use of
high core count machines, but at a minimum we should be able to boot a
GENERIC kernel on such systems, and have an ABI for the FreeBSD 14
release that supports such a configuration.
Some changes have already been committed in support of increased MAXCPU,
including increasing MAX_APIC_ID (commit c8113dad7ed4) and a number of
changes to reduce bloat (such as commits 42f722e721cd, e72f7ed43eef,
78cfa762ebf2 and 74ac712f72cf).
The next step is to increase the maximum cpuset size for userland.
I have this change open in review D39941[2] and an exp-run request in
PR271213[3]. Following that the kernel change for increasing MAXCPU is
in D36838[4].
Additional work on bloat reduction will continue after this change, and
looking forward FreeBSD is going to need ongoing effort from the
community and the FreeBSD Foundation to continue improving scalability.
[1] https://bugs.freebsd.org/269572
[2] https://reviews.freebsd.org/D39941
[3] https://bugs.freebsd.org/271213
[4] https://reviews.freebsd.org/D36838
FWIW, I think it will be useful for main to run with a larger userspace
MAXCPU than kernel for at least a while so that we have better testing
of that configuration and to give headroom for bumping MAXCPU in the
kernel during the 14.x branch.
The only other viable path I think which would be more work would be to
rework cpuset_t in userspace to always use a dynamically sized mask.
This could perhaps be done in an API-preserving manner by making cpuset_t
an opaque wrapper type in userland and requiring CPU_* to indirect to
functions in libc, etc. That's a fair bit more work however.
--
John Baldwin
