Hi Tim, explanation by example:
First the familiar; get the field of a quadrupole, using
the eget EPICS 4 command, telling it to use the Channel Access protocol
(-p ca). I could have added -m to say monitor it.
$ eget -p ca QUAD:LTU1:880:BACT
14.036
Now, what are the optics right now at that quad? That’s
computed by the online model. The Twiss parameters are available as a structure,
returned over pvAccess (V4):
[physics@lcls-srv01 ~/greg]$ eget -s QUAD:LTU1:880:TWISS
non-normative type
structure
double energy 14.525
double psix 75.1573
double alphax 85.5249
double betax 1.83101
double etax -0.0874823
double etaxp 0.00345222
double psiy 63.7057
double alphay 26.1492
double betay 3.69231
double etay -3.02964e-05
double etayp 3.23509e-06
double z 3555.94
double leff 0.316
double sleff 0.158
int ordinality 3364
Ok, if that’s the optics right now, let’s compare those to the *design* Twiss
parameters, from the model run we did for this machine configuration. Note
arguments.
[physics@lcls-srv01 ~/greg]$ eget -s QUAD:LTU1:880:TWISS -a type design -a run 46696
non-normative type
structure
double energy 14.583
double psix 75.2175
double alphax 131.746
double betax 3.47598
double etax -0.0815512
double etaxp 0.0042281
double psiy 64.2223
double alphay 38.2057
double betay 3.74104
double etay -5.1665e-05
double etayp 3.3515e-06
double z 3555.94
double leff 0.316
double sleff 0.158
int ordinality 3360
Is that a bit clearer? In shameless self promotion - you might see the talk I
gave for the ESS (Lund) EPICS meeting. It has a number of examples of V4 from
different labs, plus some timing measurements and basic architecture outline.
https://indico.esss.lu.se/event/507/session/2/contribution/51
Cheers
Greg
> On Dec 5, 2016, at 5:26 PM, Ralph Lange <[email protected]> wrote:
>
> Hi Tim,
>
> Answering a specific one of your questions:
>
> On Tue, Dec 6, 2016 at 7:45 AM, Madden, Timothy J. <[email protected]> wrote:
> General question:
> I know RPC stands for remote procedure call, but what does it mean in epics V4?
> What does it do that epics V3 does not do?
>
> Maybe it is easier to answer in terms of data structures:
>
> "Normal" connections and subscriptions are using a single data type, which may be a structure in V4, but it is fixed during the connection lifetime, and it is the same structure on read, write and subscription operations.
>
> RPC type connections may have different data types on write and read, and the returned structure may change with every call of the method.
>
> Thus, the RPC type connections are well suited to middle-layer type services, where the data written (think: small list of arguments) are very different from the returned values (think: big arrays of structures that depend on the sent arguments).
>
> Cheers,
> ~Ralph
>
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