Caution: This article may provide out-of-date or incorrect information.

The intention of this page is to enumerate debugging knobs that are enabled (by default) in HEAD, with instructions on what do they do, their impact on performance and how to turn them off. All of these options are turned off by default in RELENG_x branches.

The canonical documentation for kernel debugging options is .


Scope: kernel config file

Performance impact: LOW

INVARIANT_SUPPORT brings a set of debugging functions to the kernel. INVARIANTS enables the use of this options for various parts of the kernel. The reason for this distinction is because kernel modules might be built with INVARIANTS but the kernel proper only needs INVARIANT_SUPPORT, thus reducing performance impact for the bulk of it. INVARIANTS_SUPPORT is what makes KASSERT work.

How to disable: Don't include INVARIANTS and INVARIANT_SUPPORT in the kernel config file


Scope: kernel config file

Performance impact: HIGH

WITNESS enables advanced and computationally intensive checks of kernel locks. All aspects of the kernel take a big performance hit if this option is enabled, because the kernel is multi-threaded.

How to disable: Don't include WITNESS option in the kernel config file

Malloc debug flags

Scope: runtime

Performance impact: MEDIUM

The userland memory allocator has several debugging flags that can always be enabled or disabled during system operation. They are useful mostly to developers and for tracking certain memory allocation problems. See malloc(3) for the list of available flags.

How to disable: See malloc(3)


Scope: libc

Performance impact: VERY HIGH (< 128MB RAM), otherwise MEDIUM

The userland memory allocator can do several checks for consistency of the allocations and its internal structures. This is an "inverse knob" - the existence of MALLOC_PRODUCTION disables these checks. This should be fixed to not be an invisible knob.

How to disable: edit lib/libc/stdlib/malloc.c or define it in /etc/src.conf


DefaultDebuggingKnobs (last edited 2015-12-22 07:22:44 by KubilayKocak)