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Re: FYI GDB on-disk .debug cache (mmapcache) [Re: Tasks]


On Thu, 07 Aug 2008 01:43:56 +0200, Tom Tromey wrote:
> * Maybe we could play some trick to speed up strcmp_iw_ordered.

But it affects only the CPU usage, not the disk access times.

> Jan> My idea was some on-disk cache, created on-demand in ~/.gdb_cache . There
> Jan> could also be some system-wide rpm debuginfo packages cache created in
> Jan> /usr/lib/debug during rpm %post by `gdb --readnow'.
> This is an interesting idea, though I find it a bit hard to reconcile
> with the I/O wait theory.  Perhaps we would be reading strictly less
> data.

Yes, I see now that the idea was naive.  I expected that with the prepared
memory data image just flushed to disk its reading will be cheap.
Unfortunately lookup_minimal_symbol_by_pc_section_1() does a binary search in
this data etc. so reading this mmap()ed image is still expensive.

> Anyway, I plan to do some more investigation.  I'm interested in your
> thoughts.

I would try dwarf2_build_psymtabs_easy() myself now, so far just with the
public symbols (regressing GDB).  If it would be fast GCC can provide even
indexes for the static symbols.


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