Kernel Memory Allocator

The Solaris kernel memory allocator distributes chunks of memory for use by clients inside the kernel. The allocator creates a number of caches of varying size for use by its clients. Clients can also request the allocator to create a cache for use by that client (for example, to allocate structures of a particular size). Statistics about each cache that the allocator manages can be seen by using the kstat -c kmem_cache command.

Occasionally, systems might panic because of memory corruption. The kernel memory allocator supports a debugging interface (a set of flags), that performs various integrity checks on the buffers. The kernel memory allocator also collects information on the allocators. The integrity checks provide the opportunity to detect errors closer to where they actually occurred. The collected information provides additional data for support people when they try to ascertain the reason for the panic.

Use of the flags incurs additional overhead and memory usage during system operations. The flags should only be used when a memory corruption problem is suspected.

kmem_flags

Description

The Solaris kernel memory allocator has various debugging and test options that were extensively used during the internal development cycle of the Solaris OS. Starting in the Solaris 2.5 release, a subset of these options became available. They are controlled by the kmem_flags variable, which was set with a kernel debugger, and then rebooting the system. Because of issues with the timing of the instantiation of the kernel memory allocator and the parsing of the /etc/system file, it was not possible to set these flags in the /etc/system file until the Solaris 8 release.

Five supported flag settings are described here.

Flag Setting Description AUDIT 0x1 The allocator maintains a log that contains recent history of its activity. The number of items logged depends on whether CONTENTS is also set. The log is a fixed size. When space is exhausted, earlier records are reclaimed. TEST 0x2 The allocator writes a pattern into freed memory and checks that the pattern is unchanged when the buffer is next allocated. If some portion of the buffer is changed, then the memory was probably used by a client that had previously allocated and freed the buffer. If an overwrite is identified, the system panics. REDZONE 0x4 The allocator provides extra memory at the end of the requested buffer and inserts a special pattern into that memory. When the buffer is freed, the pattern is checked to see if data was written past the end of the buffer. If an overwrite is identified, the kernel panics. CONTENTS 0x8 The allocator logs up to 256 bytes of buffer contents when the buffer is freed. This flag requires that AUDIT also be set. The numeric value of these flags can be logically added together and set by the /etc/system file, starting in the Solaris 8 release, or for previous releases, by booting kadb and setting the flags before starting the kernel. LITE 0x100 Does minimal integrity checking when a buffer is allocated and freed. When enabled, the allocator checks that the redzone has not been written into, that a freed buffer is not being freed again, and that the buffer being freed is the size that was allocated. This flag is available as of the Solaris 7 3/99 release. Do not combine this flag with any other flags.

Data Type

Signed integer

Default

0 (disabled)

Range

0 (disabled) or 1 - 15 or 256 (0x100)

Dynamic?

Yes. Changes made during runtime only affect new kernel memory caches. After system initialization, the creation of new caches is rare.

Validation

None

When to Change

When memory corruption is suspected

Commitment Level

Unstable

kmem_stackinfo

Description

If the kmem_stackinfo variable is enabled in the /etc/system file at kernel thread creation time, the kernel thread stack is filled with a specific pattern instead of filled with zeros. During kernel thread execution, this kernel thread stack pattern is progressively overwritten. A simple count from the stack top until the pattern is not found gives a high watermark value, which is the maximum kernel stack space used by a kernel thread. This mechanism allows the following features:

• Compute the percentage of kernel thread stack really used (a high watermark) for current kernel threads in the system

• When a kernel thread ends, the system logs the last kernel threads that have used the most of their kernel thread stacks before dying to a small circular memory buffer

Data Type

Unsigned integer

Default

0 (disabled)

Range

0 (disabled) or 1 (enabled)

Dynamic?

Yes

Validation

None

When to Change

When you want to monitor kernel thread stack usage. Keep in mind that when kmem_stackinfo is enabled, the performance of creating and deleting kthreads is decreased. For more information, see the Chapter 5, Built-In Commands, in Oracle Solaris Modular Debugger Guide.

Zone Configuration

This parameter must be set in the global zone.

Commitment Level

Unstable