Re: [FFmpeg-devel] AVDictionary vs. AVSet (AVDictionary2 approximation)

[Michael Niedermayer <michael@niedermayer.cc>]

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On Mon, Apr 14, 2025 at 01:02:00PM +0000, softworkz . wrote:
> 
> 
> > -----Original Message-----
> > From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of
> > softworkz .
> > Sent: Montag, 14. April 2025 14:40
> > To: FFmpeg development discussions and patches <ffmpeg-
> > devel@ffmpeg.org>
> > Subject: Re: [FFmpeg-devel] AVDictionary vs. AVSet (AVDictionary2
> > approximation)
> > 
> > 
> > 
> > > -----Original Message-----
> > > From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of
> > > Michael Niedermayer
> > > Sent: Montag, 14. April 2025 13:33
> > > To: FFmpeg development discussions and patches <ffmpeg-
> > > devel@ffmpeg.org>
> > > Subject: [FFmpeg-devel] AVDictionary vs. AVSet (AVDictionary2
> > > approximation)
> > >
> > > Hi
> > >
> > > I just posted a AVSet implementation i wrote in the last 2 days (yes
> > > thats
> > > why i did dissapear for the last 2 days)
> > >
> > > My plan was to use that AVSet as basis for AVDictionary2 in case
> > > benchmarks indicate that its worth it, so is it ?
> > >
> > > with 3 entries (100000 runs)
> > > AVDictionary    0.040sec
> > > AVSet           0.027sec
> > >
> > > with 5 entries (100000 runs)
> > > AVDictionary    0.065sec
> > > AVSet           0.042sec
> > >
> > > with 10 entries (100000 runs)
> > > AVDictionary    0.193sec
> > > AVSet           0.087sec
> > >
> > > with 100 entries (100000 runs)
> > > AVDictionary    8.7  sec
> > > AVSet           1.4  sec
> > >
> > > with 1000 entries (1000 runs)
> > > AVDictionary    8.0   sec
> > > AVSet           0.240 sec
> > >
> > > with 10000 entries (10 runs)
> > > AVDictionary    7.2   sec
> > > AVSet           0.042 sec
> > >
> > >
> > > I was a bit surprised for the 3 and 5 entry case, maybe my benchmark
> > > is buggy or
> > > AVSet is, but then AVDictionary is pretty bad with memory
> > allocations
> > >
> > > AVDictionary needs to strdup every key and value, needs to allocate
> > > the AVDictionary itself and reallocs the entry array each time
> > > thats 10 memory allocation related calls for adding 3 entries
> > >
> > > while AVSet allocates the AVSet and then uses av_fast_realloc() for
> > > the array
> > > and theres nothing else, the key/value goes in that array too
> > >
> > >
> > > bechmark code used is below:
> > >
> > >
> > > #if 0
> > >     for (int runs = 0; runs < 100000; runs++) {
> > >         AVSet *set = av_set_new(strcmp, NULL, NULL);
> > >         for(int pass = 0; pass < 2; pass++) {
> > >             unsigned r = 5;
> > >             for(int i=0; i<100; i++) {
> > >                 r = r*123 + 7;
> > >                 char str[2*7] = "TESTXXTESTXX";
> > >                 str[4] = r;
> > >                 str[5] = r>>8;
> > >                 if(pass == 0) {
> > >                     av_set_add(set, str, 2*7, 0);
> > >                 } else {
> > >                     av_set_get(set, NULL, str, NULL);
> > >                 }
> > >             }
> > >         }
> > >         av_set_free(&set);
> > >     }
> > > #else
> > >     for (int runs = 0; runs < 100000; runs++) {
> > >         AVDictionary *dict = NULL;
> > >         for(int pass = 0; pass < 2; pass++) {
> > >             unsigned r = 5;
> > >             for(int i=0; i<100; i++) {
> > >                 r = r*123 + 7;
> > >                 char str[7] = "TEST";
> > >                 str[4] = r;
> > >                 str[5] = r>>8;
> > >                 if(pass == 0) {
> > >                     av_dict_set(&dict, str, str, 0);
> > >                 } else {
> > >                     av_dict_get(dict, str, NULL, 0);
> > >                 }
> > >             }
> > >         }
> > >         av_dict_free(&dict);
> > >     }
> > > #endif
> > >
> > >
> > > --
> > 
> > Hi Michael,
> > 
> > 
> > what's not quite realistic is that all keys are starting with the same
> > 4 characters. This affects the lookups of course - and probably
> > (maybe) not equally for both sides.
> > 
> > Doesn't the code create duplicate keys (at least when it gets > 65536
> > it will for sure) ?
> > 
> > So, I think, the keys should be completely random (all chars).
> > 
> > I would also check whether the lookups are successful (just to be
> > sure).
> 
> Sorry, I forgot the most important one: 
> 
> Timing for population and lookup should be measured separately..

Sure, for the v2 (AVMap) i just posted

with TESTXX / TESTXX strings where XX is random

1000 entries
  5354505 decicycles in av_map_add,     512 runs,      0 skips
  4040575 decicycles in av_map_get,     512 runs,      0 skips
148082828 decicycles in av_dict_set,     512 runs,      0 skips
145828939 decicycles in av_dict_get,     512 runs,      0 skips

100 entries
 332015 decicycles in av_map_add,     512 runs,      0 skips
 193726 decicycles in av_map_get,     512 runs,      0 skips
1697242 decicycles in av_dict_set,     512 runs,      0 skips
1392837 decicycles in av_dict_get,     512 runs,      0 skips

10 entries
  21142 decicycles in av_map_add,     512 runs,      0 skips
  11395 decicycles in av_map_get,     512 runs,      0 skips
  45663 decicycles in av_dict_set,     512 runs,      0 skips
  19756 decicycles in av_dict_get,     512 runs,      0 skips

5 entries
   9210 decicycles in av_map_add,     512 runs,      0 skips
   4870 decicycles in av_map_get,     511 runs,      1 skips
  18823 decicycles in av_dict_set,     512 runs,      0 skips
   5483 decicycles in av_dict_get,     512 runs,      0 skips

3 entries
   5693 decicycles in av_map_add,     512 runs,      0 skips
   2645 decicycles in av_map_get,     512 runs,      0 skips
  11462 decicycles in av_dict_set,     511 runs,      1 skips
   2532 decicycles in av_dict_get,     512 runs,      0 skips



with XXST / XXST strings where XX is random

1000 entries
 5321153 decicycles in av_map_add,     512 runs,      0 skips
 4295153 decicycles in av_map_get,     512 runs,      0 skips
70417784 decicycles in av_dict_set,     512 runs,      0 skips
68188612 decicycles in av_dict_get,     512 runs,      0 skips

100 entries
 322872 decicycles in av_map_add,     512 runs,      0 skips
 216032 decicycles in av_map_get,     511 runs,      1 skips
1022088 decicycles in av_dict_set,     512 runs,      0 skips
 723612 decicycles in av_dict_get,     512 runs,      0 skips

10 entries
  20993 decicycles in av_map_add,     512 runs,      0 skips
  11744 decicycles in av_map_get,     512 runs,      0 skips
  38945 decicycles in av_dict_set,     512 runs,      0 skips
  11308 decicycles in av_dict_get,     512 runs,      0 skips

5 entries
  10007 decicycles in av_map_add,     511 runs,      1 skips
   5004 decicycles in av_map_get,     512 runs,      0 skips
  17374 decicycles in av_dict_set,     511 runs,      1 skips
   3848 decicycles in av_dict_get,     512 runs,      0 skips

3 entries
   5896 decicycles in av_map_add,     512 runs,      0 skips
   2765 decicycles in av_map_get,     512 runs,      0 skips
  11396 decicycles in av_dict_set,     511 runs,      1 skips
   2029 decicycles in av_dict_get,     512 runs,      0 skips


[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The worst form of inequality is to try to make unequal things equal.
-- Aristotle

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