system slowdown - vnode related

[Masachika ISHIZUKA]

>>>>   I have still vnodes problem in 4.8-stable with /sys/kern/vfs_subr.c
>>>> 1.249.2.30.
>>>> 
>>>>   % sysctl kern.maxvnodes
>>>>   kern.maxvnodes: 17979
>>>>   % sysctl vm.zone | grep VNODE
>>>>   VNODE:           192,        0,  18004,    122,    18004
>>> 
>>> This looks pretty normal to me for a quiescent system.
>>
>>   I think the used(18004) exceeds maxvnodes(17979), isn't it ?
> 
> Only by a little bit.  maxvnodes isn't a hard limit, since making
> it a hard limit would lead to deadlocks.  Instead, the system
> garbage collects vnodes to keep the number roughly in line with
> maxvnodes.  Judging by the numbers above, it's doing a pretty good
> job, but that's probably because, from the looks of it, you
> just booted the system.

  Hi, David-san.
  Thank you for mail.
  I understood.

> The reason it might make sense to increase maxvnodes is that
> having vnlru work overtime to keep your vnode count low may
> result in vnodes being freed that are still needed, e.g. by the
> buffer cache.  This would cause the slowdown you were mentioning.

  I will try to increase kern.maxvnodes when the machine is slowdown.
But I can not reproduce slowdown in experimental environment, yet.

>>> Did you get a backtrace from the panics?
>> 
>>   It's too hard for me. Is there any way to do it ?
> 
> The panics might be unrelated to the number of vnodes, so it's
> important that we have additional information.  See:
> 
> http://www.freebsd.org/doc/en/books/developers-handbook/kerneldebug.html

  I'll try.  Thank you very much.

-- 
ishizuka at ish.org