var create through mi mode on std::istream sometimes very slow

Björn Carlsson
Thu Mar 5 09:02:00 GMT 2020

I need some help to further investigate a problem.
When using VSCode to debug using gdb, it unfortunately load the
variables for all the frames in current thread.
VSCode uses gdb's mi mode to get a vizualisation of these variables.

Most var-create takes 1 or 2 ms. But a few takes hundreds or a couple
of thousand ms.
The worst is std::istream taking around 2 seconds to be created, from the log:
1: (51975) <-1343-var-create - * "is"
1: (54172) ->1343^done,name="var80",numchild="0",value="{...}",type="std::istream
1: (54172) ->(gdb)
1: (54172) 1343: elapsed time 2197

I can't say that I can reproduce it while running normal gdb, not
using mi mode. And I can't reproduce in a simple small test project,
with just a main and one function call with an std::istream, then the
var-create is down to around 1 ms.

When I test it now I run gdb without my gdbinit to avoid the influence
of our pretty-printers.
I did also test to register a pretty-printer for std::istream that
just returned a static text, it got the time down to around 1400 ms.

Running gdb 9.1 (same problem in 8.2)
Ubuntu 18.04
C++ (a rather big executable with around 100 threads)

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