Re: [RFC v2 1/6] scsi: xarray hctl

[James Bottomley <jejb@linux.vnet.ibm.com>]

On Tue, 2020-05-26 at 11:39 -0700, Matthew Wilcox wrote:
> On Tue, May 26, 2020 at 07:53:35PM +0200, Hannes Reinecke wrote:
> > On 5/26/20 4:27 PM, Matthew Wilcox wrote:
> > > Ah, OK.  I think for these arrays you'd be better off accepting
> > > the cost of an extra 4 bytes in the struct scsi_device rather
> > > than the cost of storing the scsi_device at the LUN.
> > > 
> > > Let's just work an example where you have a 64-bit LUN with 4
> > > ranges, each of 64 entries (this is almost a best-case scenario
> > > for the XArray). [0,63], 2^62+[0,63], 2^63+[0,63],
> > > 2^63+2^62+[0,63].
> > > 
> > > If we store them sequentially in an allocating XArray, we take up
> > > 256 * 4 bytes = 1kB extra space in the scsi_device.  The XArray
> > > will allocate four nodes plus one node to hold the four nodes,
> > > which is 5 * 576 bytes (2780 bytes) for a total of 3804 bytes.
> > > 
> > > Storing them in at their LUN will allocate a top level node which
> > > covers bits 60-66, then four nodes, each covering bits of 54-59,
> > > another four nodes covering bits 48-53, four nodes for 42-47,
> > > ...  I make it 41 nodes, coming to 23616 bytes.  And the pointer
> > > chase to get to each LUN is ten deep.  It'll mostly be cached,
> > > but still ...
> > > 
> > 
> > Which is my worry, too.
> > In the end we're having a massively large array space (128bit if we
> > take the numbers as they stand today), of which only a _tiny_
> > fraction is actually allocated.
> 
> In your scheme, yes.  Do you ever need to look up scsi_device from
> a scsi_host with only the C:T:L as a key (and it's a situation where
> speed matters)?  Everything I've seen so far is about iterating every
> sdev in an shost, but maybe this is a "when you have a hammer"
> situation.

I don't believe we ever do.  We've arranged pretty much everything so
the SCSI device is immediately obtainable from the enclosing structure
(sysfs, rw, ioctl, interrupt from device etc).  The only time we do a
direct lookup by H:C:T:L is the very old scsi proc API, which we're
trying to deprecate.

James