RE: -var-update @

["Marc Khouzam" <marc dot khouzam at ericsson dot com>]

> > DSF only updates varObj that are visible on screen.  So currently, it always
> > uses -var-update with a single varObj name (never use *).
> 
> Which must mean that there is a round trip to the target for each variable
> object that needs to be updated.
> 
> This is sounds similar to the previous discussion about using
> "-var-list-children --all-values".  There Daniel stated that "for a lot of
> embedded targets [...] reading memory becomes the dominant time delay".
> 
> Can someone give some typical numbers for "round trip time" vs "reading memory"
> time.  In my naive understanding of embedded targets, I would have thought the
> "round trip time" might be large due to a slow serial link, while "reading
> memory" wouldn't change much as all RAM is pretty much the same.  Or is the
> latter slow because of the time taken to transfer any unneeded extra data back
> to the host?

I'm not familiar with such numbers myself, although I would be interested in
finding out.

However, I wanted to point out that there are currently two possible options
for var-update

-var-update <singleVarObj>
-var-update *
(we'll ignore the new -var-update @, which does not affect the discussion)

For DSF, which tries to minimize the amount of work done, we can:

1- use multiple var-update <singleVarObj>, which typically results in 
about 5 or 6 var-updates being sent (only 5 or 6 variables are visible on-screen).
Then GDB on the target reads the memory for those 5 or 6 varObjs.

2- use var-update *, which results in a single -var-update, but which
makes GDB on the target read the memory of all varObjects, which can be
anywhere from, say, 5 to 5000, or even more.

As you can see, option 2 does not scale, irrespective of which is 
the true bottleneck, the round-trip time, or the target memory access.
That is why DSF does not use var-update *.

But you are right that less round-trips would be even better.
So, to improve option 1, Vladimir's suggestion of a batch -var-update
  -var-update <var1> <var2> ...
would be better (although DSF is not currently setup for it.)

BTW, is there a limit (enforced or recommended) on the number of varObj that
can be created?

BR,
Marc