HP-UX 11i Version 1.5 Release Notes, May 2001

HP-UX Features
32-bit DMA Pool
Chapter 346
32-bit DMA Pool
To support I/O devices and drivers that are limited to 32-bit DMA
addresses in HP-UX 11i Version 1.5 (B.11.20), a special area of reserved
memory accessible by those devices may be allocated. This area is
allocated only on systems where the highest physical memory address is
greater than 4GB.
A new system tunable, dma32_pool_size, is used to specify the amount
of memory to be allocated to support 32-bit I/O cards and devices. The
default value of 256MB is expected to handle most configurations. For
systems configured with a large number of I/O cards, a larger pool can be
reserved, using these memory requirements for each supported 32-bit
DMA card:
Although dma32_pool_size is specified in bytes, the actual allocation is
done in MB units. The specified size, if not an integral multiple of 1MB,
will be rounded up to the next higher MB.
DMA Pool Effect on Performance
While this tunable allows support for I/O cards that do not use 64-bit
addressing, the need to copy data to or from the 32-bit DMA pool incurs
an obvious performance penalty. For those systems where this is
unacceptable, the use of 64-bit cards exclusively is recommended.
Table 3-1 32-bit DMA Card Memory Pool Size
32-bit DMA Card Memory Pool Size
SCSI Adapters 33MB
LAN Adapters 1MB
FC - Tachlite 512KB
Internal IDE 256KB