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

[Dipankar Sarma <dipankar@in.ibm.com>]

On Mon, Oct 06, 2003 at 11:09:07AM -0700, Greg KH wrote:
> On Mon, Oct 06, 2003 at 11:31:19PM +0530, Dipankar Sarma wrote:
> No.  My main point is that for every hotplug event (which is caused by a
> kobject being created or destroyed), udev will run and look at the sysfs
> entry for the kobject (by using libsysfs which reads in all of the
> kobject information, including attributes).  This is a normal event, so
> we have to care about what happens after running 'find' on the sysfs
> tree as that is basically what will always happen.
> 
> Does that make more sense?  We can't just look at what happens with this
> patch without actually accessing all of the sysfs tree, as that will be
> the "normal" case.

That sounds odd. So, udev essentially results in a frequent and continuous
"find /sys" ? That doesn't sound good. You are unnecessarily adding
pressure on vfs (dcache specially). We will discuss this offline then
and see what needs to be done.

> > > Can you show this happening?
> > 
> > It should be easy to demonstrate. That is how dentries/inodes
> > work for on-disk filesystems. If Maneesh's patch didn't work that
> > way, then the whole point is lost. I hope that is not the case.
> 
> Me too.  It's just that the free memory numbers didn't show much gain
> with this patch on his system.  That worries me.

Well, Maneesh didn't post numbers after letting the system age out
sysfs dentries/inodes. Maneesh can you post some such numbers ?


> > > 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.)
> > 
> > IIRC, Maneesh test machine is a 2-way P4 xeon with six scsi disks and savings
> > are of about 800KB. That is as normal a case as it gets, I think.
> > It only gets better as you have more devices in your system.
> 
> 800Kb after running find?  I don't see that :)

No, those numbers were for just mounting sysfs. More numbers tomorrow.

Thanks
Dipankar