This message discusses IOC CPU Priorities.
How about the following priorities, from highest to lowest.
IOCHIGH
IOCMEDIUM
CAHIGH
IOCLOW
CAMEDIUM
CALOW
CAUDP and CATCP
BACKGROUND
All scan tasks can be IOCHIGH, IOCMEDIUM, IOCLOW, or BACKGROUND.The
actual choices an application developer is given are: background, low,
medium, and high.
Connected CA clients can be CAHIGH, CAMEDIUM, CALOW, or BACKGROUND. The
actual choices a CA user is given are: background, low, medium, high.
CAUDP is the task listening for ca search requests. CATCP is the task
listening for ca TCP connection requests.
Discussion:
This allows the IOC application developer the ability to guarantee that
important IOC based needs are satisfied first. At the same time, for
heavily loaded IOCs, the developer can assign periodic, I/O interrupt,
and event scanned records to a priority of IOCLOW or BACKGROUND. Thus
she/he can assure that non-time critical IOC functions do not lock out
CA client requests.
For BACKGROUND tasks it would be nice to turn on round-robin scheduling.
The problem is that vxWorks requires round-robin scheduling for all
tasks or for none (I am looking at a vxWorks 5.1 manual). Note that this
can be used now by just inserting a call to kernelTimeSlice in a st.cmd
file. Note that round-robin scheduling was not always a feature of
vxWorks.
Another feature of vxWorks that was not always available is priority
inheritance semaphores. Currently EPICS never uses this feature. I don't
know what the overhead is for using this feature. This is extremely
important because EPICS uses semaphores extensively. If priority
inheritance slows down semTakes considerably it would severly degrade
ioc performance.
If anyone has performance figures for either round-robin scheduling
and/or priority inversion, please send a message to tech-talk.
Marty
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