Re: [RFC PATCH 0/4] scsi: use xarray for devices and targets

[Bart Van Assche <bvanassche@acm.org>]

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.