optimizing a DSO

optimizing a DSO

[Vivek Verma]

I am trying to speedup the load and startup time of a shared library. 
After reading Ulrich Drepper's paper on "How to write shared libraries", 
it seems that the easiest thing to try would be to reduce the number of 
symbols that are  globally visible. After carefully adding 
__attribute__((visibility ("default"))) to only the symbols that should 
be globally visible and using the gcc option -fvisibility=hidden to hide 
all symbols by default, I managed to reduce the number of globally 
visible symbols. But now, it seems that even though the number of 
symbols needing relocation has decreased, the cost of searching for a 
symbol in the "optimized" dso has gone up. Here is the output from 
"eu-readelf -I" before and after reducing the number of globally visible 
symbols. It seems that the cost of both successful and unsuccessful 
lookup has gone up. I haven't done any profiling yet but I am guessing 
that my runtime symbol lookup cost will go up. Is this normal and am I 
missing something?

BEFORE:

Histogram for bucket list length in section [ 1] '.gnu.hash' (total of 
4099 buckets):
Addr: 0x0000000000000158  Offset: 0x000158  Link to section: [ 2] '.dynsym'
Symbol Bias: 652
Bitmask Size: 4096 bytes  26% bits set  2nd hash shift: 15
Length  Number  % of total  Coverage
     0    1123       27.4%
     1    1470       35.9%     28.1%
     2     955       23.3%     64.7%
     3     391        9.5%     87.1%
     4     132        3.2%     97.2%
     5      23        0.6%     99.4%
     6       5        0.1%    100.0%
Average number of tests:   successful lookup: 1.617107
                         unsuccessful lookup: 1.274945


AFTER:

Histogram for bucket list length in section [ 1] '.gnu.hash' (total of 
2053 buckets):
Addr: 0x0000000000000158  Offset: 0x000158  Link to section: [ 2] '.dynsym'
Symbol Bias: 652
Bitmask Size: 4096 bytes  21% bits set  2nd hash shift: 15
Length  Number  % of total  Coverage
     0     288       14.0%
     1     576       28.1%     14.7%
     2     575       28.0%     44.1%
     3     367       17.9%     72.2%
     4     165        8.0%     89.0%
     5      64        3.1%     97.2%
     6      16        0.8%     99.6%
     7       2        0.1%    100.0%
Average number of tests:   successful lookup: 1.916007
                         unsuccessful lookup: 1.90794