On Mon, Jan 11, 2021 at 07:33:15PM +0100, Greg KH wrote:
> On Mon, Jan 11, 2021 at 06:20:29PM +0000, Matthew Wilcox wrote:
> > > efficient in what way? Space or faster lookup?
> >
> > Both, but primarily space.
> >
> > The radix tree underlying the xarray allows N consecutive entries with
> > the same value to be represented as a single entry; if there are at
> > least 64 entries then we get to skip an entire level of the tree (saving
> > 1/7 of a page). Of course, we'd need to go from the 'head' pointer to
> > the correct pointer, something like p += rdev - p->rdev.
>
> How much "space" are you talking about here?
576 bytes -- 1/7 of a page.
> A "normal" machine has about 100-200 char devices. Servers, maybe more,
> but probably not.
>
> The kobject being used previously wasn't really "small" at all, so odds
> are any conversion to not use it like this will be better overall.
Yes.
> > > THis shouldn't be on a "fast" lookup path, so I doubt that's worth
> > > optimizing for. Space, maybe, for systems with thousands of scsi
> > > devices, but usually they just stick to the block device, not a char
> > > device from what I remember.
> >
> > /dev/sgX is a chardev?
>
> I sure hope no one is using /dev/sgX for tens of thousands of block
> device accesses, if so, they have bigger problems than this :)
There is one sgX char dev for every /dev/sdN, so anyone with a thousand
SCSI devices also has a thousand char devices. On the other hand,
they're added one at a time, so there is no chance to optimise here:
cdev = cdev_alloc();
...
error = cdev_add(cdev, MKDEV(SCSI_GENERIC_MAJOR, sdp->index), 1);
