* `ZSTD_ldm_generateSequences()` generates the LDM sequences and
stores them in a table. It should work with any chunk size, but
is currently only called one block at a time.
* `ZSTD_ldm_blockCompress()` emits the pre-defined sequences, and
instead of encoding the literals directly, it passes them to a
secondary block compressor. The code to handle chunk sizes greater
than the block size is currently commented out, since it is unused.
The next PR will uncomment exercise this code.
* During optimal parsing, ensure LDM `minMatchLength` is at least
`targetLength`. Also don't emit repcode matches in the LDM block
compressor. Enabling the LDM with the optimal parser now actually improves
the compression ratio.
* The compression ratio is very similar to before. It is very slightly
different, because the repcode handling is slightly different. If I remove
immediate repcode checking in both branches the compressed size is exactly
the same.
* The speed looks to be the same or better than before.
Up Next (in a separate PR)
--------------------------
Allow sequence generation to happen prior to compression, and produce more
than a block worth of sequences. Expose some API for zstdmt to consume.
This will test out some currently untested code in
`ZSTD_ldm_blockCompress()`.
by invoking time() once per batch, instead of once per compression / decompression.
Batch is dynamically resized so that each round lasts approximately 1 second.
Also : increases time accuracy to nanosecond
as it's faster, due to one memory scan instead of two
(confirmed by microbenchmark).
Note : as ZSTD_reduceIndex() is rarely invoked,
it does not translate into a visible gain.
Consider it an exercise in auto-vectorization and micro-benchmarking.
On my laptop:
Before:
./zstd32 -b --zstd=wlog=27 silesia.tar enwik8 -S
3#silesia.tar : 211984896 -> 66683478 (3.179), 97.6 MB/s , 400.7 MB/s
3#enwik8 : 100000000 -> 35643153 (2.806), 76.5 MB/s , 303.2 MB/s
After:
./zstd32 -b --zstd=wlog=27 silesia.tar enwik8 -S
3#silesia.tar : 211984896 -> 66683478 (3.179), 97.4 MB/s , 435.0 MB/s
3#enwik8 : 100000000 -> 35643153 (2.806), 76.2 MB/s , 338.1 MB/s
Mileage vary, depending on file, and cpu type.
But a generic rule is : x86 benefits less from "long-offset mode" than x64,
maybe due to register pressure.
On "entropy", long-mode is _never_ a win for x86.
On my laptop though, it may, depending on file and compression level
(enwik8 benefits more from "long-mode" than silesia).
This makes it easier to explain that nbWorkers=0 --> single-threaded mode,
while nbWorkers=1 --> asynchronous mode (one mode thread on top of the "main" caller thread).
No need for an additional asynchronous mode flag.
nbWorkers>=2 works the same as nbThreads>=2 previously.
Nick Terrell