Tunable parameters

Obsolete virtual memory (VM) parameters

Obsolete kernel virtual address space parameters

PIOSEGSZ: Formerly, the amount of virtual address space in clicks available for programmed I/O.
PIOMAP: Formerly, determined the size of the map entry array used by the kernel programmed I/O (PIO) breakup routine. This routine allows device drivers to do programmed I/O of large data blocks at interrupt level by breaking the data blocks into smaller data units.
The buf breakup routine and async I/O both work a different way now, and have no need for the PIO segment.
PIOMAXSZ: Formerly, maximum number of pages to use at one time for programmed I/O.
SEGMAPSZ: Formerly, the amount of virtual address space in clicks available for kernel mapping of user read/write requests.
Has been replaced by SEGKMEM_BYTES and SEGKMEM_PERCENT. segkmap is no longer restricted to 16 MB of kernel virtual.
SYSSEGSZ: Formerly, the amount of virtual address space in clicks (pages) available for allocation. This was used for either kma, where the virtual addresses are backed by physical memory, or for device memory mapped into the kernel.
Has been replaced by the combination of SEGKMEM_BYTES and SEGKMEM_PERCENT. KMA is no longer restricted to 16MB.

NOTPGOVERFLOW: Formerly, this parameter was similar to PGOVERFLOW, but applied to all other conditions (non-overflow).
PGOVERFLOW: Formerly, this parameter was the number of buffer headers preserved for page overflow conditions. These buffers were used by pageout or other processes when a buffer head could not be allocated from kma. This is a deadlock prevention mechanism.

PSE_PHYSMEM: Formerly, this parameter specified an amount of physical memory to reserve for use by large shared memory segments. Setting aside memory for this purpose provided a performance benefit to applications that create large shared memory segments using shared memory primitives. The physical memory was reserved for use in shared memory segments and could not be used for anything else, so PSE_PHYSMEM was set to ``0'' unless applications using large segments (greater than 4MB of memory) were going to be run, and there was enough memory on the system remaining for other uses. Functions performed by PSE_PHYSMEM were replaced by the DEDICATED_MEMORY parameter.
PSE_VMEMSIZE: Formerly, specified the amount of device memory that can be mapped by drivers using page size extension. Device drivers that mapped more than 1MB of memory obtained a performance benefit. PSE_VMEMSIZE could be set by taking all device mappings that were greater than 1MB, rounding each such mapping up to the next 4MB boundary, and taking the sum of all these roundings.