Buffer Allocation

The first step in taking a kernel trace is to allocate a kernel trace buffer. The trace buffer is circular, which means that newer data overwrites older data after the buffer fills up.

The default size for the buffer is 384 kilobytes, but you can override this when you allocate the buffer. When deciding on the buffer size, keep in mind that the buffer occupies both physical memory and kernel virtual memory, so it will impact the system you are tracing--the more RAM you have, the less the impact of measurement on your experiment, and the bigger the buffer you can allocate.

Use the buffer command in prex to allocate the buffer. For example, you could allocate a 512 kilobyte buffer as follows:

prex> buffer			      # do we have a buffer?
No trace buffer allocated 	
prex> buffer alloc 512k# allocate 512KB buffer
Buffer of size 524288 bytes allocated 	
prex> 

The minimum buffer size is 128 kilobytes; prex allocates a buffer of this size if you request anything smaller.