[allow_multithreading] not enable parallel sort in i_key_sort

[alanlzhang]

in Cargo.toml, missing "i_key_sort/allow_multithreading"

allow_multithreading = ["dep:rayon"]

should be

allow_multithreading = ["dep:rayon", "i_key_sort/allow_multithreading"]

[NailxSharipov]

Hi @alanlzhang, thanks for this note

I ran a checkboard test for 3 cases:

without multithreading

2(5 0.7)     - 0.000002(-5.6)
4(25 1.4)     - 0.000015(-4.8)
8(113 2.1)     - 0.000079(-4.1)
16(481 2.7)     - 0.000404(-3.4)
32(1985 3.3)     - 0.002286(-2.6)
64(8065 3.9)     - 0.008519(-2.1)
128(32513 4.5)     - 0.040483(-1.4)
256(130561 5.1)     - 0.181508(-0.7)
512(523265 5.7)     - 0.901423(-0.0)
1024(2095105 6.3)     - 3.993353(0.6)
2048(8384513 6.9)     - 21.163190(1.3)

with multithreading (NO Fix)

2(5 0.7)     - 0.000002(-5.6)
4(25 1.4)     - 0.000015(-4.8)
8(113 2.1)     - 0.000079(-4.1)
16(481 2.7)     - 0.000404(-3.4)
32(1985 3.3)     - 0.002307(-2.6)
64(8065 3.9)     - 0.007591(-2.1)
128(32513 4.5)     - 0.036336(-1.4)
256(130561 5.1)     - 0.161347(-0.8)
512(523265 5.7)     - 0.751356(-0.1)
1024(2095105 6.3)     - 3.309321(0.5)
2048(8384513 6.9)     - 19.043071(1.3)

with multithreading (sort Fix)

2(5 0.7)     - 0.000002(-5.6)
4(25 1.4)     - 0.000014(-4.8)
8(113 2.1)     - 0.000078(-4.1)
16(481 2.7)     - 0.000391(-3.4)
32(1985 3.3)     - 0.002403(-2.6)
64(8065 3.9)     - 0.007267(-2.1)
128(32513 4.5)     - 0.029460(-1.5)
256(130561 5.1)     - 0.131319(-0.9)
512(523265 5.7)     - 0.624179(-0.2)
1024(2095105 6.3)     - 2.636597(0.4)
2048(8384513 6.9)     - 16.252444(1.2)

and yes with this fix it ran noticeably faster

[NailxSharipov]

fixed in #66