Re: -var-update @

[Vladimir Prus <vladimir at codesourcery dot com>]

On Friday 28 March 2008 17:31:33 Marc Khouzam wrote:
> 
> > > 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.

You can freeze variable objects that are not visible to the user,
and -var-update * won't fetch those. Please see the -var-set-frozen
command.

> 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?

There's no hard limit. The practical limit depends on target and can only
be found empirically.

- Volodya