Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Fri, 8 Aug 2014 11:13:08 -0500

Hi Ralph,

On 08/08/2014 10:19 AM, Ralph Lange wrote:
> I was looking at db/dbCaTest.c today, and I found that
> 
> dbcar() is looping through all record instances of all types, and taking
> dbLockSetGblLock() when investigating a CA link.
> dbcaStats() is looping through all record instances of all types, and
> *NOT* taking dbLockSetGblLock() when investigating a CA link.
> 
> Why? Why not?
> 
> dbcar() is doing more CA investigation, and using the formatter in-between.
> But other than that, dbcaStats() could also encounter another thread
> changing the link type from CA to DB or CONSTANT.

As the guilty party my reasoning probably went something like this:
Links don't get changed very often; if a change does occur at the moment
when we are counting a specific link, the result is that the statistics
could be off by one that one time, but the consequence of that
miss-count is effectively nil — the number was changing at the time
anyway, so is really wrong? The user would only really notice if the
number of CA links gets counted as less than the number of connected
links, resulting in *pdiscon < 0.

> Can anyone explain why these similar cases are handled differently?

I didn't want to lock the global lock every time we redo the statistics
count (which happens frequently if you're running devIocStats) as that
could affect IOC performance.

A better implementation might keep separate (atomic) running counter
variables inside dbCa.c and have dbcaStats just return the values from
those counters. dbcar could compare its counts with those (it still has
to scan the whole database anyway). Using atomic counters wouldn't
prevent dbcaStats from returning *pdiscon < 0 though, we'd need a single
lock to cover both counters to prevent that.

HTH,

- Andrew
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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System