Strategy for compression levels 8 and 9

[fintelia]

What compression speed would we be OK for levels 8 and 9?

If we targeted around 15-20 MB/s and 5-10 MB/s respectively, then we could use (the slightly inaccurately named) optimal parsing strategy to get significantly better compression ratios. In my very preliminary experiments I've been able to hit:

fdeflate-opt:  19.0 MiB/s    22.62%
fdeflate-opt:   9.2 MiB/s    22.33%

I think I may be able to improve compression ratio further by adopting libdeflate's block splitting strategy, but their code is extremely well optimized, so replicating it isn't necessarily going to get as good speed

---

For reference, these are the current compression speeds on my system for qoibench:

fdeflate0:    17052.1 MiB/s   100.02%
fdeflate1:      390.2 MiB/s    24.41%
fdeflate2:      275.2 MiB/s    23.79%
fdeflate3:      223.2 MiB/s    23.77%
fdeflate4:      150.5 MiB/s    23.67%
fdeflate5:      105.6 MiB/s    23.66%
fdeflate6:       79.3 MiB/s    23.37%
fdeflate7:       60.6 MiB/s    23.23%

miniz_oxide:

miniz_oxide7:    32.6 MiB/s    23.44%
miniz_oxide8:    25.2 MiB/s    23.36%
miniz_oxide9:    21.6 MiB/s    23.31%

zlib-rs:

zlib-rs7:        64.0 MiB/s    23.28%
zlib-rs8:        37.5 MiB/s    23.04%
zlib-rs9:        24.6 MiB/s    22.86%

libdeflate:

libdeflate7:     95.5 MiB/s    23.36%
libdeflate8:     58.7 MiB/s    23.10%
libdeflate9:     46.6 MiB/s    23.04%
libdeflate10:    19.0 MiB/s    22.16%
libdeflate11:    11.4 MiB/s    21.88%
libdeflate12:     7.2 MiB/s    21.70%

zopfli:

zopfli[iter=1]:   0.6 MiB/s    21.78%
zopfli[default]: 0.27 MiB/s    21.47% 

[kornelski]

These speeds seem ok to me.

When I set max compression level, I care about size above all else.

level 8 being 2x faster than level 9 seems sensible. When the level 9 turns out to be too slow, that factor is significant enough.