Experimental Physics and
Industrial Control System

Benjamin Franksen <[email protected]> · Mon, 2 Feb 2009 23:01:25 +0100

On Montag, 2. Februar 2009, Till Straumann wrote:
> Benjamin Franksen wrote:
> > Our idea for an efficient implementation is based on the observation
> > that in a typical EPICS database there are only very few fields per
> > record that actually receive dbPuts (or get updated directly from
> > within base). Only for these fields is it necessary to provide storage
> > for the time of the last update. This storage will be allocated on
> > demand; each record must maintain a (typically very short) list of
> > (field index, timestamp) pairs; let us call it the 'field update list'
> > or short 'update list'. When reading the timestamp for a field, first
> > search the update list and use the contained stamp if one is found,
> > otherwise use the TIME field of the record.
>
> How does that implement (b) ? If a field is never updated,
> shouldn't that field then report the time when iocInit() was
> executed?

Ah, a good objection. It looks as if we need to distinguish between fields 
that have

 i)   never been changed (timestamp = iocInit time)
 ii)  last been updated by record support (timestamp = record's TIME field)
 iii) last been updated via dbPut or from base (timestamp = see update list)

On could use a bit field to distinguish between (i) and (ii or iii). On 
dbGet (with timestamp requested) we would have to

 - search the update list, if found -> iii
 - if not, check the bitfield for i or ii

This increases the memory overhead by an array of noOfRecordFields/8 bytes, 
i.e. typically no more than 16 bytes per record (exact number depending on 
the record type, of course). Dynamically allocating the array (at init 
time) would be simpler (no change to dbToRecordtypeH) at the cost of yet 
another pointer sized field. Note: existing record supports work w/o 
changes.

Yea, right, implementing the whole proposal is probably not done in a 
weekend, as I originally conceived. I still think it is something we should 
strive for. What about Ralph's suggestion to offer it as a codeathon 
assignment?

Cheers
Ben

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System