Re: parallel loading of debug info in gdb

On Wed, Jan 8, 2025 at 7:45 AM Tom Tromey <[email protected]> wrote:

> > Yes dependency is a problem, but some dependent libs are naturally
> > independent to be read by parallel. If there could be some way to
> > specify this, it would be great.
>
> If you try git gdb, you can enable background reading with
> "maint set dwarf synchronous off".
>


Thanks, using this command in gdb15.2 indeed make the initial loading
faster.


>
> (There's been at least one request to re-enable this by default for GDB
> 16... I wasn't sure if we should, though, considering that the data race
> fixes are pretty new.)
>
> Anyway, background reading is lazier -- the reading is started but gdb
> will only pause for it if it is truly needed.
>
> Note that the library debug info isn't really read in parallel.  Or, it
> kind of is, depending on how the scheduling shakes out.  That is, the
> DWARF reader looks at the number of worker threads and splits its work
> up accordingly.  "Overlap" can only really happen if one such worker
> finishes and then the next library's job is scheduled.
>
> This might be a bad strategy overall.  Like, testing on a big machine
> showed that the reader didn't parallelize past 8 CPUs, so maybe it would
> be better to limit the DWARF reader to 8 tasks.  Nobody has worked on
> figuring out why it doesn't scale better, though, and I wouldn't want to
> change this without knowing more.
>
> Tom
>