MT compression generates a single frame.
Multi-threading operates by breaking the frames into independent sections.
But from a decoder perspective, there is no difference :
it's just a suite of blocks.
Problem is, decoder preserves repCodes from previous block to start decoding next block.
This is also valid between sections, since they are no different than changing block.
Previous version would incorrectly initialize repcodes to their default value at the beginning of each section.
When using them, there was a mismatch between encoder (default values) and decoder (values from previous block).
This change ensures that repcodes won't be used at the beginning of a new section.
It works by setting them to 0.
This only works with regular (single segment) variants : extDict variants will fail !
Fortunately, sections beyond the 1st one belong to this category.
To be checked : btopt strategy.
This change was only validated from fast to btlazy2 strategies.
In some complex scenarios (free() without finishing compression),
it is possible that some resources are still into jobs
and not collected back into pools.
In which case, previous version of free() would miss them.
This would be equivalent to a leak.
New version ensures that it even foes after such resource.
It requires job consumers to properly mark resources as released,
by replacing entries by NULL after releasing back to the pool.
Obviously, it's not recommended to free() zstdmt context mid-term,
still that's now a supported scenario.
The same methodology is also used to ensure proper resource collection
after an error is detected.
Still to do :
- detect compression errors (not just allocation ones)
- properly manage resource when init() is called without finishing previous compression.
The main issue was to avoid a caller to continually loop on {flush,end}Stream()
when there was nothing ready to be flushed but still some compression work ongoing in a worker thread.
The continuous loop would have resulted in wasted energy.
The new version makes call to {flush,end}Stream blocking when there is nothing ready to be flushed.
Of course, if all worker threads have exhausted job, it will return zero (all flush completed).
Note : There are still some remaining issues to report error codes
and properly collect back resources into pools when an error is triggered.
In previous version, main function would return early when detecting a job error.
Late threads resources were therefore not collected back into pools.
New version just register the error, but continue the collecting process.
All buffers and context should be released back to pool before leaving main function.
Result from getBuffer and getCCtx could be NULL when allocation fails.
Now correctly checks : job creation stop and last job reports an allocation error.
releaseBuffer and releaseCCtx are now also compatible with NULL input.
Identified a new potential issue :
when early job fails, later jobs are not collected for resource retrieval.
Since the result of mt compression is a single frame,
changed naming, which implied the concatenation of multiple frames.
minor : ensures that content size is written in header
The new strategy involves cutting frame at block level.
The result is a single frame, preserving ZSTD_getDecompressedSize()
As a consequence, bench can now make a full round-trip,
since the result is compatible with ZSTD_decompress().
This strategy will not make it possible to decode the frame with multiple threads
since the exact cut between independent blocks is not known.
MT decoding needs further discussions.
use ZSTD_freeCCtxPool() to release the partially created pool.
avoids to duplicate logic.
Also : identified a new difficult corner case :
when freeing the Pool, all CCtx should be previously released back to the pool.
Otherwise, it means some CCtx are still in use.
There is currently no clear policy on what to do in such a case.
Note : it's supposed to never happen.
Since pool creation/usage is static, it has no external user,
which limits risks.
Yann Collet
Yann Collet
Nick Terrell
Przemyslaw Skibinski
Chocobo1