Previously, parallel_compression would only handle each job's results
after ALL jobs were successfully queued. This caused all src/dst
buffers to remain in memory until then!
It also polled to check whether a job completed, which is racy without
any memory barrier.
Now, we flush results as a side effect of completing a job. Completed
frames are placed in an ordered linked-list, and any eligible frames
are flushed. This may be zero or multiple frames, depending on the
order in which jobs finish.
This design also makes it simple to support streaming input, so that
is now available. Just pass `-` as the filename, and stdin/stdout will
be used for I/O.
As reported by @P-E-Meunier in https://github.com/facebook/zstd/issues/2662#issuecomment-1443836186,
seekable format ingestion speed can be particularly slow
when selected `FRAME_SIZE` is very small,
especially in combination with the recent row_hash compression mode.
The specific scenario mentioned was `pijul`,
using frame sizes of 256 bytes and level 10.
This is improved in this PR,
by providing approximate parameter adaptation to the compression process.
Tested locally on a M1 laptop,
ingestion of `enwik8` using `pijul` parameters
went from 35sec. (before this PR) to 2.5sec (with this PR).
For the specific corner case of a file full of zeroes,
this is even more pronounced, going from 45sec. to 0.5sec.
These benefits are unrelated to (and come on top of) other improvement efforts currently being made by @yoniko for the row_hash compression method specifically.
The `seekable_compress` test program has been updated to allows setting compression level,
in order to produce these performance results.
```
for f in $(find . \( -path ./.git -o -path ./tests/fuzz/corpora \) -prune -o -type f);
do
sed -i 's/Facebook, Inc\./Meta Platforms, Inc. and affiliates./' $f;
done
```
