svn commit: r274017 - head/sys/kern

[Mateusz Guzik]

On Mon, Nov 03, 2014 at 04:21:02PM +0800, Julian Elischer wrote:
> On 11/3/14, 3:46 PM, Mateusz Guzik wrote:
> >Author: mjg
> >Date: Mon Nov  3 07:46:51 2014
> >New Revision: 274017
> >URL: https://svnweb.freebsd.org/changeset/base/274017
> >
> >Log:
> >   Provide an on-stack temporary buffer for small ioctl requests.
> I'm not sure I like this. We don't know how many more levels
> of stack may be needed.
> I know that machines are getting faster with more memory,
> but the current move towards bloating out the
> worries me.  we started out with a single page of stack (SHARED
> with the U-area!). I think we are now at several pages.. I forget, is
> it 8?
> I'm open to being persuaded but I think we need to have a discussion
> about stack usage. We used to say that anything greater that, say
> 64 bytes should probably be allocated.
> 

Depends on the arch, 2 on arm and mips, 4 on the rest. Roughly speaking.

If 128 in ioctl is really such a big concern I'm happy to make it
smaller, but I do prefer to have a buffer covering some stuff.

Just booting the system gives:
size	occurences
4	  118
8	  60
16	   3
20	   8
24	   33
32	   127
36	   8
40	   16
44	   268
48	   278
68	   1
72	   31
80	   8
192	    1
236	    55
256	    1
288	    22
817	    8

Building the kernel gives me over 38000 occurences of size 44 (separate question
is what is that ioctl and is it really necessary to call it).

That said, maybe 128 was indeed too big. 64 sounds like a nice round number to me.

-- 
Mateusz Guzik <mjguzik gmail.com>