question concerning proper usage of kernel variables net.bpf.bufsize and vm_kmem_size_max

Raymond Owens owensr at
Sat Jul 29 00:02:41 UTC 2006


By hardware cache size, you are referring to the processor cache? If the box has two processors, should the value used for cache size in this calculation be doubled? In very general terms, what is the link between the net.bpf.bufsize and the cache?  Thanks for info..

  R. B. Riddick wrote:

  >--- Raymond Owens <owensr at> wrote:
  >>Can VM_KMEM_SIZE_MAX be set manually with sysctl?
  >No, but you could set it with this procedure:
  >1. Insert the lines
  >  vm.kmem_size=123456789
  >  vm.kmem_size_max=1234567890
  >  /boot/loader.conf
  >2. reboot
  >That should change those values...
  >(see src/sys/kern/kern_malloc.c)
  >I wonder, why your box needs such a big buffer? Do u have network traffic
  >bursts or so?
  Regardless what purpose is for, the net.bpf.bufsize should never
  set above hardware cache size. The best (optimal size) is 50% - 80%
  of the hardware cache size, unless original BPF is modified in some
  way I do not know.
  Such high bufsize will degrade performance.

  ------------ Jin Guojun ----------- v --- jin at ---
  Distributed Systems Department
  Lawrence Berkeley National Laboratory, Berkeley, CA 94720

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