Adjusting kernel virtual segments for large mappings

The ZBM_LGMAP_PERCENT tunable can be used to control the percentage of the kpgseg (segkvn) kernel virtual segment reserved for most large mappings, but DRV_SEGKMEM_BYTES is the preferred tunable when setting aside kernel virtual memory for large driver mappings.

• Note that ZBM_LGMAP_PERCENT effectively replaces PSE_VMEMSIZE as a way to reserve kernel virtual space for large mappings. However, ZBM_LGMAP_PERCENT does not use PSE mappings, and does not require that the PSE driver be configured in.

• The ZBM_LGMAP_PERCENT tunable, in conjunction with SEGKMEM_BYTES, can be used to satisfy large requirements for kernel virtual space originating from physmap(D3), devmem_mapin(D3), kmem_alloc(D3), kmem_zalloc(D3), kmem_alloc_physreq(D3), and kmem_zalloc_physreq(D3). When you know the specific amount of space you need, increase SEGKMEM_BYTES by that quantity (in bytes).

• When used in conjunction with SEGKVN_BYTES, the ZBM_LGMAP_PERCENT tunable can be used to provide contiguous kernel virtual space for loadable modules. When you know the specific amount of space you need, increase SEGKVN_BYTES by that quantity (in bytes).

• In order to obtain the best overall system performance, you want to keep ZBM_LGMAP_PERCENT as small as possible, but still large enough to obtain the kernel virtual space you need. In order to estimate a new value for ZBM_LGMAP_PERCENT, you need to obtain the actual value for the size of segkmem. You can do this using crash(1M) as follows. As root, issue the commands:

     # crash
     dumpfile = /dev/mem, namelist = /stand/unix, outfile = stdout
     Engine: 0 Procslot: 56 Lwpslot: 0
     > od segkmem_size
     c096c830:  02600000
     > q
     #
  

  In this example, the size for segkmem is 0x2600000 bytes in hexdecimal.

• DRV_SEGKMEM_BYTES specifies the quantity of kernel virtual memory which will be put aside for large driver mappings, especially those generated by physmap(D3). Typically, this parameter is incremented by driver package installation scripts rather than by the administrator. DRV_SEGKMEM_BYTES is the preferred tunable when setting aside kernel virtual memory for large driver mappings, but if you need to control the percentage of kernel virtual segments reserved for large mappings other than large device memory areas, use ZBM_LGMAP_PERCENT.