Replace khash with maps in ddoc_cache_lru

[nickva]

This is a companion PR to the previous one for couch_event_server [1].

Implementation notes:

• Some functionality was moved to helper functions (do_refresh, get_entries)

• The main concern was to make sure to return the updated map object, when before we just returned ok.

• ddoc_cache_lru already had an almost 100% test coverage so opted to rely those tests.

• Performance with maps seems to at least as good or better than with the khash nif [2].

[1] #4977
[2] https://gist.github.com/nickva/06f1511b7f9d0bbc2d9a0dfc2d36779e

Performance benchmark
(MacOS, Intel, Erlang/OTP 24)

With maps:

> ddoc_cache_bench:go().

Dbs    DocIds Revs    Open(Dt/Key) Close(Dt/Key)
100    100    5       202.5        11.1
1000   5      5       192.1        21.9
10000  1      1       176.8        65.6
1      10000  1       180.3        30.7
1      1      10000   185.9        32.1

GC Runs: 4733375

With the khash NIF:

> ddoc_cache_bench:go().

Dbs    DocIds Revs    Open(Dt/Key) Close(Dt/Key)
100    100    5       202.2        12.6
1000   5      5       193.2        26.0
10000  1      1       183.1        75.0
1      10000  1       182.8        40.0
1      1      10000   182.8        38.5

GC Runs: 4799318

[nickva]

Note: remove_entry/2 args were changed so that it can be directly used in a lists:foldl callback.

[davisp]

Goodbye, khash! You were such a good nif all these years!

[nickva]

@davisp I sorry. It was a good hashing data structure long before maps existed!

[nickva]

I did a run on Linux (Ubuntu) with Erlang 24:

With maps

ddoc_cache_bench:go().

Dbs    DocIds Revs    Open(Dt/Key) Close(Dt/Key)
100    100    5       182.0        8.8    
1000   5      5       178.6        17.6   
10000  1      1       159.3        54.9   
1      10000  1       160.6        28.6   
1      1      10000   166.1        28.4   

GC Runs: 4777049

With khash

> ddoc_cache_bench:go().

Dbs    DocIds Revs    Open(Dt/Key) Close(Dt/Key)
100    100    5       185.5        11.7   
1000   5      5       180.3        22.7   
10000  1      1       167.5        67.9   
1      10000  1       164.8        33.7   
1      1      10000   167.8        31.3   

GC Runs: 4836877

[jaydoane]

I ran some benchmarks with OTP 26 on an M1 Max.
With khash:

([email protected])1> ddoc_cache_bench:go().

Dbs    DocIds Revs    Open(Dt/Key) Close(Dt/Key)
100    100    5       111.0        5.7
1000   5      5       105.0        12.7
10000  1      1       99.0         43.4
1      10000  1       98.8         20.5
1      1      10000   98.7         17.9

GC Runs: 6386402

With maps:

Dbs    DocIds Revs    Open(Dt/Key) Close(Dt/Key)
100    100    5       86.7         4.4
1000   5      5       81.8         10.7
10000  1      1       77.3         43.1
1      10000  1       76.4         14.4
1      1      10000   78.0         15.1

GC Runs: 6332692

Looks like nice across the board improvement!

[jaydoane]

Have you considered adding ddoc_cache_bench.erl to the repo?

[nickva]

Hmm, that could work, but since ddoc_cache application restarts as part of test to get a clean slate, it would blow away the ddoc cache, so it might be a bit too aggressive.

[jaydoane]

Really nice improvement!