shdict_get flag comparison

[Miniwoffer]

Proposal

Add optional flags argument to shmdict_get(_stale)? that only returns value if flags do not match.
And a 3rd (4th for _stale) return value to indicate if it is a flag match.

Example usage

local object, stale_data, lru_flag = lru:get(key)
local serialized, shm_flag, flag_match = shm:get(key, object and lru_flag)

if flag_match then
    return object
end

if serialized then
    object, err = cjson.decode(serialized)
    if object then
        lru:set(key, object, 0, shm_flag)
    end

    return object, err
end

Why?

Allocating and collecting the memory nedded to return a value can be
very expensive when value is not needed.

Bencmarking

Link to benchmark repo
Read only benchmarking, with random access order.
Tested with 5 000 000 shm lookups on a single worker.

Value 2048 bytes string
  Results: ~75.8% faster

Value 1024 bytes string
  Results: ~46.8% faster

Value 256 bytes string
  Results: ~24.7% faster

Value 16 byte string
  Results: ~19.8% faster

Value number
  Results: ~5.4% faster

Pull requests

• stream-lua-nginx Add support for shdict_get flag comparison stream-lua-nginx-module#265
• lua-nginx-module Add support for shdict_get flag comparison #1922
• lua-resty-core Add support for shdict_get flag comparison lua-resty-core#362

[doujiang24]

Hi @Miniwoffer
I think we also need to think about if the new API is slower than before when there is no flag used, could you please provide some bench results?

[Miniwoffer]

Sure, i had som trouble getting consistent results betwene runs, but managed to get some more or less reliable data.
Benchmarked with numbers, no random order and ran it for 1 800 000 000 gets.

With commits:
728602.810000 std-dev +- 3111.248909

Without commits
727996.340000 std-dev +- 2020.834505

0.083307% +0.149138 -0.149968

[Miniwoffer]

Any progress?

[zhuizhuhaomeng]

@Miniwoffer, please rebase to the master and change the lua-resty-core in .travis.yml to your branch.

[zhuizhuhaomeng]

we need to update the api in doc/HttpLuaModule.wiki.
run ngx-releng in openresty-devel-utils repo, it will convert doc/HttpLuaModule.wiki to README.markdown

[Miniwoffer]

@zhuizhuhaomeng Done, travis is green on lua-nginx-module but not lua-resty-core. But master is also red, so i am unsure if its me.

[Miniwoffer]

Okay so there are some tests in lua-resty-core that uses get and get_stale with two arguments. Dont know why those tests specify a secound argument, so would need some guidance on how to alter those tests.

Tests in question are t/stdict.t(Test 24 and 25)

Also want me to write tests for lua-rest-core even though the tests in lua-nginx-module should cover everything?

[Miniwoffer]

Fixed so Test 24 and 25 in t/shdict.t pass, just changed so key == nil is check first.

[zhuizhuhaomeng]

lua-resty-core still failed.
https://app.travis-ci.com/github/openresty/lua-resty-core/builds/239052924

[Miniwoffer]

@zhuizhuhaomeng Yeah sorry about that, thought stream was generated from http so didn't implement anything in stream. I ported tho code and the tests over to the stream module.

[Miniwoffer]

@zhuizhuhaomeng Think you might have missed the openresty/stream-lua-nginx-module#265 pull request.

Also am i the one who should revert the .travis.yml(cc12437, openresty/stream-lua-nginx-module@e53cdc7, openresty/lua-resty-core@358f770) changes?

Il add a summary of pull requests here and in the original post

Pull requests

• stream-lua-nginx Add support for shdict_get flag comparison stream-lua-nginx-module#265
• lua-nginx-module Add support for shdict_get flag comparison #1922
• lua-resty-core Add support for shdict_get flag comparison lua-resty-core#362

[zhuizhuhaomeng]

@Miniwoffer No, you don't need a revert .travis.yml.