## Runtime {#runtime}

### Container-aware `GOMAXPROCS`

<!-- go.dev/issue/73193 -->

The default behavior of the `GOMAXPROCS` has changed. In prior versions of Go,
`GOMAXPROCS` defaults to the number of logical CPUs available at startup
([runtime.NumCPU]). Go 1.25 introduces two changes:

1. On Linux, the runtime considers the CPU bandwidth limit of the cgroup
   containing the process, if any. If the CPU bandwidth limit is lower than the
   number of logical CPUs available, `GOMAXPROCS` will default to the lower
   limit. In container runtime systems like Kubernetes, cgroup CPU bandwidth
   limits generally correspond to the "CPU limit" option. The Go runtime does
   not consider the "CPU requests" option.

2. On all OSes, the runtime periodically updates `GOMAXPROCS` if the number
   of logical CPUs available or the cgroup CPU bandwidth limit change.

Both of these behaviors are automatically disabled if `GOMAXPROCS` is set
manually via the `GOMAXPROCS` environment variable or a call to
[runtime.GOMAXPROCS]. They can also be disabled explicitly with the [GODEBUG
settings](/doc/godebug) `containermaxprocs=0` and `updatemaxprocs=0`,
respectively.

In order to support reading updated cgroup limits, the runtime will keep cached
file descriptors for the cgroup files for the duration of the process lifetime.

### New experimental garbage collector

<!-- go.dev/issue/73581 -->

A new garbage collector is now available as an experiment. This garbage
collector's design improves the performance of marking and scanning small objects
through better locality and CPU scalability. Benchmark result vary, but we expect
somewhere between a 10—40% reduction in garbage collection overhead in real-world
programs that heavily use the garbage collector.

The new garbage collector may be enabled by setting `GOEXPERIMENT=greenteagc`
at build time. We expect the design to continue to evolve and improve. To that
end, we encourage Go developers to try it out and report back their experiences.
See the [GitHub issue](/issue/73581) for more details on the design and
instructions for sharing feedback.

### Change to unhandled panic output

<!-- go.dev/issue/71517 -->

The message printed when a program exits due to an unhandled panic
that was recovered and repanicked no longer repeats the text of
the panic value.

Previously, a program which panicked with `panic("PANIC")`,
recovered the panic, and then repanicked with the original
value would print:

    panic: PANIC [recovered]
      panic: PANIC

This program will now print:

    panic: PANIC [recovered, repanicked]

### VMA names on Linux

<!-- go.dev/issue/71546 -->

On Linux systems with kernel support for anonymous VMA names
(`CONFIG_ANON_VMA_NAME`), the Go runtime will annotate anonymous memory
mappings with context about their purpose. e.g., `[anon: Go: heap]` for heap
memory. This can be disabled with the [GODEBUG setting](/doc/godebug)
`decoratemappings=0`.