Re: RFC: sorted value_string + bsearch

[Jakub Zawadzki <darkjames () darkjames ath cx>]

On Thu, Apr 22, 2010 at 10:47:14AM -0400, Ed Beroset wrote:
> I may have missed it, but have we measured to see if this is worth optimizing in the first place? 

I have some old callgrind log, where match_strval (for 58736 frames) is called a lot:

 from dissect_ieee80211 58,7k
 from ieee_80211_add_tagged_parameters 228k

It's problem with ieee80211 dissector, but when you grep dissector sources:

#v+
$ grep -Ir 'val_to_str' ./ | wc -l
3621

$ grep -Ir 'match_strval' ./ | wc -l
305
#v-

It's a common thing to use these functions.

> If not, I think I'd favor choosing code

Come on, this code require only adding:
   static const VALUE_STRING_FAST(some_value_string_array);

I don't see much difference between using:
   value_to_str_fast(..., &foo_fast) and value_to_str(..., foo)

If you don't like automatic variable naming I can replace VALUE_STRING_FAST with:

  #define VALUE_STRING_FAST_INIT(x) = { array_length(x)-1, x }
  static const value_string_fast foo = VALUE_STRING_FAST_INIT(bar);

Dissector maintainer is free to use old/new (or write his own) function he want,
but putting 57 new lines to core won't hurt.

> clarity over an inconsequential speedup.

ATM I don't have working profiler to benchmark wireshark, but I made some
_raw_ benchmark [1], for 5k entries and searching 0 to 6k three times

Average time for 16 tests: 
 val_to_str_fast: 0.02s
      val_to_str: 1.56s

Sorry for not providing real-life benchmark.

Cheers.

[1] http://szara.chmurka.net/value_string-bench.c

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