Tango polling system
Tango workshop
Archamps (08/10/2005)
5-Oct-05
1
Tango workshop
Introduction to Tango polling


Every Tango device server process has a polling
thread
Its rule is to
 Poll
attribute and/or command (without input
parameters)
 Store result in a data cache (called polling buffer)
managed as a circular buffer
 Detect events and throw them to the CORBA
notification service
5-Oct-05
2
Tango workshop
Introduction to Tango polling


Each device has its own polling buffer
Polling buffer depth is tunable
 By
device (default is 10)
 By command/attribute for finer tuning

Client get their data from
 The
real device
 The last record in the polling buffer
 The polling buffer and in case of error from the real
device
 The choice is done by a Tango API client call
5-Oct-05
3
Tango workshop
Introduction to Tango polling

Configuring polling means
 Defining
which command/attribute must be polled
 Defining polling periods


Every polled object (command or attribute) has its own
polling period
Two way to configure polling
 From
the Tango database (using Jive)
 Directly in code (the way used by Pogo)
5-Oct-05
4
Tango workshop
Introduction to Tango polling

According to the way polling is configured (db or
code), polling starts
 At
device server process start-up time if configured
from database
 At the first request to read data from polling buffer
when configured by code
5-Oct-05
5
Tango workshop
The Polling has to be tuned

Nothing is magic!
 Don’t
believe that the polling thread is able to poll
everything at high frequency simply by decreasing
polling period parameter
 The hardware access time has to be taken into
account
 If you poll 5 objects (command or attribute) with 100
ms response time each, don’t try to poll them faster
than 500 ms…(That’s life…)
5-Oct-05
6
Tango workshop
Polling tuning rule

Reserve some time for clients other than the
polling thread to access your device
 The
polling thread should not access the device for
more than 50/60 % of time
 In the previous example, set polling period at 1
second leaving device free for external client during
50 % of time
5-Oct-05
7
Tango workshop
Checking polling status

The Tango device server process administration
device has one command to check polling
 Its
name is DevPollStatus
 This command returns




Polling buffer depth
Polling period
Time needed to execute last command or to read attribute
Delta time between the last recorded data (to check if it
follows polling period)
 Jive
5-Oct-05
displays the result of this command
8
Tango workshop
5-Oct-05
9
Tango workshop
De-tuned polling

It may happen that the hardware response time
dramatically increase (disconnected serial line..)
 The
polling thread is out of synchronization
 It tries to return on time by DISCARDING some work.
Att 1 reading time
Att 1
Att 2
Att 3
Att 4
Time
Att 2 and Att 3 WILL NOT be polled !!
5-Oct-05
10
Tango workshop
De-tuned polling

Without discarding some works, we could
saturate the device preventing any device
access for the external world
Att 1 reading time
Att 1
Att 2
Att 2 reading time
Att 3
Att 3 reading time
Att 4
Time
The device is not accessible any more for external world
5-Oct-05
11
Tango workshop
De-tuned polling

It could even be worse:
 The
polling thread is out of sync.
 Several polled objects are not polled any more
 The data stored within the polling buffer get older and
older
 They even become too old (4 times the polling period)
to be returned by the API. The API generates device
access to return data to client instead of getting data
from polling buffer
 We generate additional hardware access !!
5-Oct-05
12
Tango workshop
Tango kernel polling tuning

Tango kernel tries to automatically tunes the
polling thread
 At
device server process startup time for polling
configured from the database
 Every 500 poll times
 After a new object has been added to the polling
thread list of polled object
5-Oct-05
13
Tango workshop
Tango kernel polling tuning

Tuning at device server startup time
 We
don’t know attribute reading time nor command
execution time.
 The polling tops are equally spread in the smallest
polling period

Ex : 3 attributes polled at 600 mS, 1000 mS and 3000 mS.
The startup tuning will start polling at




T0 for attribute 1
T0 + 200 mS (600/3) for attribute 2
T0 + 400 mS ((600/3) * 2) for attribute 3
This is done by a temporary thread dedicated to
this issue
5-Oct-05
14
Tango workshop
Tango kernel polling tuning

Regular kernel polling tuning
 We
now know command execution time or read
attribute time
 The polling tops are spread in the smallest polling
period according to their previous response time
5-Oct-05
15
Tango workshop
Tango kernel polling tuning

Ex : One device with 3 attributes polled








Att 1 with polling period = 600 mS needing 400 mS to be read
Att 2 with polling period = 1000 mS needing 40 mS to be read
Att 3 with polling period = 3000 mS needing 10 mS to be read
Dead time is 150 mS (600 – (400 + 40 + 10))
Att 1 polled at T0
Att 2 polled at T0 + 450 mS (400 + (150 / 3))
Att 3 polled at T0 + 540 mS (400 + 40 + 2 * (150 / 3))
Note that in this case, device is free for external access
only 29% of time (10 + 3(40) + 5(400) = 2130 mS
compared to 3000 mS)
5-Oct-05
16
Tango workshop
Event system and polling


Event system needs polling
Don’t forget to take this into account when
defining polling period for attribute/command
 As
soon as one application subscribes to one event
on attribute, the event system ask polling thread to
poll this attribute with a default polling period set to 1
sec if not already polled
5-Oct-05
17
Tango workshop
Event system and de-tuned polling

Tango release 5.3
 If
the polling thread is out of sync and discards some
attribute reading
 If one event has been subscribed for this attribute
 An event with error “Out of sync” is forced
informing client that polling needs tuning
5-Oct-05
18
Tango workshop
Possible improvement (V 5.4 ?)


Add an attribute to device server process admin
device with read value set to polling thread
sleeping time during the last 5 seconds (or
something like)
In case of de-tuned polling, modify automatic
tuning that it does not discard always the same
polled object (with permutation of the polled list
object)
5-Oct-05
19
Tango workshop
Possible improvement (V 5.4 ?)

Stop attribute polling for attribute polled only for
event detection when no more clients are
interested in event (With the help of the event
heartbeat)
5-Oct-05
20