Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Mon, 23 Nov 2015 17:09:27 -0600

On 11/23/2015 04:41 PM, Michael Davidsaver wrote:
> On 11/23/2015 05:17 PM, Andrew Johnson wrote:
>> Hi Ralph,
>> 
>> On 11/23/2015 03:54 PM, Ralph Lange wrote:
>>> ... I guess we never thought about this memlocking thing being
>>> done when you start a client on a real-time system. Doesn't
>>> make much sense to memlock, but running a client on a real-time
>>> system is certainly legal.
>> I was never completely happy with this mlockall() solution but
>> didn't really know why until now.
> 
> I'm not a huge fan of doing this by default, even for IOCs.  I
> can't

It is only tried if once() is able to use SCHED_FIFO, so it's not
quite by default, and the process also has to have CAP_IPC_LOCK for
the mlockall() to succeed.

> cite any specific bad behavior, but I think the potential for
> surprises is there, especially w/ MCL_FUTURE.  The closest I can
> come to specifics is wondering how this mixes with the OOM killer
> on Linux (which is enough fun as is).

Agreed...

>> ... I think it probably makes more sense for the application
>> itself (i.e. the IOC's startup script) to explicitly ask for
>> mlockall() to be called, but I don't know whether we could
>> implement that level of change in 3.15.
> 
> How about moving the call to mlockall() from once() in osdThread
> to iocInit()?  Perhaps with a global variable as a conditional?
> For 3.15 the default would be to call it.

For portability the mlockall() call itself must be implemented in a
new libCom/osi routine, called say epicsThreadMLockAll(), with dummy
implementations where there's no OS-specific equivalent.

The global variable would be registered in dbCore.dbd.

> This could prevent non-IOC processes from being effected.

<pedantic>s/effected/affected/</pedantic>

- Andrew

-- 
Light thinks it travels faster than anything but it is wrong.
No matter how fast light travels, it finds the darkness has
always got there first, and is waiting for it.
    -- Terry Pratchett, Reaper Man

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System