copying pkgsrc tree around jails on beefy machine - fast on 2nd iteration, a bit slower than first on 3rd iteration

[Matthew Dillon]

:
:If the machine has 3-4GB of memory why shouldn't the vnode and buffer
:cache do some better auto tunning.  If the kernel has a VM system
:shouldn't the kernel be aware of how much memory is in use for the
:entire system and make all of the system caches larger ?

    The kernel is fairly conservative with regards to the number of vnodes
    it allows to be cached because the memory required to hold the vnodes
    and related VM structures can exceed the kernel's own fairly limited
    address space.  When doing copy operations on large numbers of small
    files, the vnode limit will often be hit before the memory limit.
    (When copying larger files the memory limit is hit before the vnode
    limit).

    You can adjust the number of vnodes the kernel caches with the
    'kern.maxvnodes' sysctl.  So, e.g. /usr/pkgsrc has around 105000
    files so if you want to support caching of a cpdup you would need
    to be able to cache ~210000 vnodes.  If you make the number too
    big, however, you run a serious risk of running the kernel out of 
    KVM.

    Unless you actually *NEED* to cache whole file structures for
    performance reasons in a production environment, I would *NOT*
    recommend that the system default be changed.

					-Matt
					Matthew Dillon 
					<dillon at xxxxxxxxxxxxx>