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.