Re: [RFC 0/6] Backing Store for sysfs

[Greg KH <greg@kroah.com>]

On Mon, Oct 06, 2003 at 11:01:11PM +0530, Dipankar Sarma wrote:
> On Mon, Oct 06, 2003 at 09:08:46AM -0700, Greg KH wrote:
> > On Mon, Oct 06, 2003 at 02:29:15PM +0530, Maneesh Soni wrote:
> > > 
> > > 				2.6.0-test6		With patches.
> > > -----------------
> > > dentry_cache (active)		2520			2544
> > > inode_cache (active)		1058			1050
> > > LowFree			875032 KB		874748 KB
> > 
> > So with these patches we actually eat up more LowFree if all sysfs
> > entries are searched, and make the dentry_cache bigger?  That's not good :(
> 
> My guess is that those 24 dentries are just noise. What we should
> do is verify with a large number of devices if the numbers are all
> that different after a walk of the sysfs tree.

Ok, a better test would be with a _lot_ of devices.  Care to test with a
lot of scsi debug devices?

> > Remember, every kobject that's created will cause a call to
> > /sbin/hotplug which will cause udev to walk the sysfs tree to get the
> > information for that kobject.  So I don't see any savings in these
> > patches, do you?
> 
> Assuming that unused files/dirs are aged out of dentry and inode cache,
> it should benefit. The numbers you should look at are -
> 
> --------------------------------------------------------
> After mounting sysfs
> -------------------
> dentry_cache (active)           2350                    1321
> inode_cache (active)            1058                    31
> LowFree                         875096 KB               875836 KB
> --------------------------------------------------------
> 
> That saves ~800KB. If you just mount sysfs and use a few files, you
> aren't eating up dentries and inodes for every file in sysfs. How often 
> do you expect hotplug events to happen in a system ?

Every kobject that is created and is associated with a subsystem
generates a hotplug call.  So that's about every kobject that we care
about here :)

> Some time after a hotplug event, dentries/inodes will get aged out and
> then you should see savings. It should greatly benefit in a normal
> system.

Can you show this happening?

> Now if the additional kobjects cause problems with userland hotplug, then 
> that needs to be resolved. However that seems to be a different problem 
> altogether. Could you please elaborate on that ?

No, I don't think the additional ones you have added will cause
problems, but can you verify this?  Just log all hotplug events
happening in your system (point /proc/sys/kernel/hotplug to a simple
logging program).

But again, I don't think the added overhead you have added to a kobject
is acceptable for not much gain for the normal case (systems without a
zillion devices.)

thanks,

greg k-h