| Message ID | 20200527141400.58087-1-hare@suse.de (mailing list archive) |
|---|---|
| Headers | show |
| Series | scsi: use xarray for devices and targets | expand |
On 2020-05-27 07:13, Hannes Reinecke wrote: > Hi all, > > based on the ideas from Doug Gilbert here's now my take on using > xarrays for devices and targets. > It revolves around two ideas: > - 'channel' and 'id' are never ever used to the full 32 bit range; > 'channels' are well below 10, and no driver is using more than > 16 bits for the id. So we can reduce the type of 'channel' and > 'id' to 16 bits, and use the 32 bit value 'channel << 16 | id' > as the index into the target xarray. > - Most SCSI LUNs are below 256 (to ensure compability with older > systems). So there we can use the LUN number as the index into > the xarray; for larger LUN numbers we'll allocate a separate > index. > > With these change we can implement an efficient lookup mechanism, > devolving into direct lookup for most cases. > And iteration should be as efficient as the current, list-based, > approach. > > This is compile-tested only, to give you an impression of the > overall idea and to get the discussion rolling. Hi Hannes, My understanding of the xarray concept is that it provides two advantages over using linked lists: - Faster lookups. - Requires less memory. Will we benefit from any of these advantages in the SCSI code? Hadn't James Bottomley already brought up that lookup by (channel, target, lun) only happens from some LLDs and from the procfs code? Are there any use cases where the number of SCSI devices is large enough to benefit from the memory reduction? Thanks, Bart.
On 5/27/20 6:36 PM, Bart Van Assche wrote: > On 2020-05-27 07:13, Hannes Reinecke wrote: >> Hi all, >> >> based on the ideas from Doug Gilbert here's now my take on using >> xarrays for devices and targets. >> It revolves around two ideas: >> - 'channel' and 'id' are never ever used to the full 32 bit range; >> 'channels' are well below 10, and no driver is using more than >> 16 bits for the id. So we can reduce the type of 'channel' and >> 'id' to 16 bits, and use the 32 bit value 'channel << 16 | id' >> as the index into the target xarray. >> - Most SCSI LUNs are below 256 (to ensure compability with older >> systems). So there we can use the LUN number as the index into >> the xarray; for larger LUN numbers we'll allocate a separate >> index. >> >> With these change we can implement an efficient lookup mechanism, >> devolving into direct lookup for most cases. >> And iteration should be as efficient as the current, list-based, >> approach. >> >> This is compile-tested only, to give you an impression of the >> overall idea and to get the discussion rolling. > > Hi Hannes, > > My understanding of the xarray concept is that it provides two > advantages over using linked lists: > - Faster lookups. > - Requires less memory. > > Will we benefit from any of these advantages in the SCSI code? Hadn't > James Bottomley already brought up that lookup by (channel, target, lun) > only happens from some LLDs and from the procfs code? > It's not only lookup, it's iteration in general. Which affects scanning and device removal; especially the latter is _very_ error prone (just look at scsi_target_reap etc), so any reduction in complexity is a good thing in general methinks. > Are there any use cases where the number of SCSI devices is large enough > to benefit from the memory reduction? > I would assume that we're seeing benefits as soon as we're in the range of tens to hundreds of devices; then list lookup will be eating up more time and space as xarrays. And the big benefit of using xarrays is that we will be alerted if an element with the same indices is being added; we've already had issues in the past here which are notoriously difficult to track down. Cheers, Hannes
On 2020-05-27 12:36 p.m., Bart Van Assche wrote: > On 2020-05-27 07:13, Hannes Reinecke wrote: >> Hi all, >> >> based on the ideas from Doug Gilbert here's now my take on using >> xarrays for devices and targets. >> It revolves around two ideas: >> - 'channel' and 'id' are never ever used to the full 32 bit range; >> 'channels' are well below 10, and no driver is using more than >> 16 bits for the id. So we can reduce the type of 'channel' and >> 'id' to 16 bits, and use the 32 bit value 'channel << 16 | id' >> as the index into the target xarray. >> - Most SCSI LUNs are below 256 (to ensure compability with older >> systems). So there we can use the LUN number as the index into >> the xarray; for larger LUN numbers we'll allocate a separate >> index. >> >> With these change we can implement an efficient lookup mechanism, >> devolving into direct lookup for most cases. >> And iteration should be as efficient as the current, list-based, >> approach. >> >> This is compile-tested only, to give you an impression of the >> overall idea and to get the discussion rolling. > > Hi Hannes, > > My understanding of the xarray concept is that it provides two > advantages over using linked lists: > - Faster lookups. > - Requires less memory. Bart, You might add these: - sane deletion semantics - inbuilt locking - inherently safer iterations, especially if marks are used Matthew can probably add more. > Will we benefit from any of these advantages in the SCSI code? Hadn't > James Bottomley already brought up that lookup by (channel, target, lun) > only happens from some LLDs and from the procfs code? The way that the SCSI object tree hangs together with doubly linked lists may have been a coherent design 20 years ago when JB wrote it, but it has been white-anted big time since then. The current state of that code is hard to defend. I have between 10 and 20 more examples of patently stupid things the current code does. See my exchange with JB concerning the starget iterator over its sdevs that decided to check on _every_ sdev in that host. That is done in several places. The redundant sdevs in a shost collection is probably the most glaring current design flaw. > Are there any use cases where the number of SCSI devices is large enough > to benefit from the memory reduction? I don't believe that overall memory usage is a problem. Fitting the sdev_s of hot devices in a smaller number of cache lines would help. That is where Ming Lei was looking that kicked off this exercise that has morphed into using xarray. Doug Gilbert