Re: [patch for playing] 2.5.65 patch to support > 256 disks

Andrew Morton
2003-03-22T12:05:50Z

Douglas Gilbert <dougg@torque.net> wrote:
>
> Andrew Morton wrote:
> > Douglas Gilbert <dougg@torque.net> wrote:
> >
> >>>Slab: 464364 kB
> >>
> >
> > It's all in slab.
> >
> >
> >>I did notice a rather large growth of nodes
> >>in sysfs. For 84 added scsi_debug pseudo disks the number
> >>of sysfs nodes went from 686 to 3347.
> >>
> >>Does anybody know what is the per node memory cost of sysfs?
> >
> >
> > Let's see all of /pro/slabinfo please.
>
> Andrew,
> Attachments are /proc/slabinfo pre and post:
> $ modprobe scsi_debug add_host=42 num_devs=2
> which adds 84 pseudo disks.
>

OK, thanks. So with 48 disks you've lost five megabytes to blkdev_requests
and deadline_drq objects. With 4000 disks, you're toast. That's enough
request structures to put 200 gigabytes of memory under I/O ;)

We need to make the request structures dymanically allocated for other
reasons (which I cannot immediately remember) but it didn't happen. I guess
we have some motivation now.