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Re: [RFC] Proposal for a new DWARF name index section
On Wed, Dec 2, 2009 at 11:38 AM, Daniel Jacobowitz <email@example.com> wrote:
> Well, inherent in the cache approach (IMO) is a system-provided cache;
> for installed libraries, the cache data could be added to a debuginfo
> file. ?Of course, that assumes GDB's format stays "relatively stable"
> across GDB updates.
FWIW, I've used the following approach on a previous product X:
- As new binary is detected, a copy of X is invoked to parse all
the needed debug info into internal form and written to a cache file.
- Once the copy exits, the cache file is directly mmap()ed by X.
- Cache files older than 1 week, and cache files prepared from
binaries which no longer exist in their original location are
pruned to keep cache size down.
The cache file contains version of X, so when a new version of X
is shipped, the cache is automatically rebuilt.
It also contains path/timestamp/inode/size for the target binary,
so if e.g. one of the shared libs has been rebuilt since last run,
only that one shared library must be re-processed.
This trades startup speed against disk space, and disk is usually
very cheap now.
One of our typical usage scenarios is a tiny executable linked with
1000+ C++ shared libraries. Simply re-running the test a second time
in a row in GDB takes 1+ minutes, as GDB discards and re-reads the
debug info for each solib (it used to take 6+ minutes before my dwarf
The major CPU consumers in my tests are now:
CPU: AMD64 processors, speed 2200 MHz (estimated)
Counted CPU_CLK_UNHALTED events (Cycles outside of halt state) with a
unit mask of 0x00 (No unit mask) count 100000
samples % symbol name
43092 8.2847 read_partial_die
38243 7.3525 strcmp_iw_ordered
36744 7.0643 read_attribute_value
28887 5.5537 cpname_parse
28849 5.5464 d_print_comp
27731 5.3315 htab_hash_string
21975 4.2248 cp_canonicalize_string
20736 3.9866 load_partial_dies
18098 3.4795 cpname_lex
15649 3.0086 lookup_minimal_symbol
15156 2.9138 msymbol_hash_iw
14185 2.7272 htab_find_slot_with_hash
I am guessing that a GDB cache of pre-canonicalized strings would
save a *lot* of CPU under this scenario, and there is no reason
you can't put any other indices into the cache, or to have a stable
format of the cache file -- newer version of GDB will simply rebuild
what it needs on demand.