The `avgJobSize` must not be lower than 256 KB for single-pass mode.
In `zstd.h` we say the minimum value for `ZSTD_p_jobSize` is 1 MB,
so ensure that we always pick a size >= 1 MB.
Found by libFuzzer fuzzer tests with large input limits.
Integrate ldm into zstdmt by running it in serial and in order in the first
step of each job, in the same place as the hash gets updated. The input
buffer is sized to fit the whole LDM window and 2 full buffers of slack.
Input buffers cannot be reused until the LDM step is done with them.
After the LDM step is finished, the jobs don't actually have access to the
full window, only the overlap.
Tested on a few different multi-GB files with and without sanitizers,
and with different numbers of threads.
* Computes the XXH hash in the worker threads.
* Workers get a sequence number and wait until ther number shows up. On
error, ensures that its sequence is finished, so future threads don't
get blocked.
* Sets up for ldm integration, which will go in the same spot.
Setting `loadedDictEnd` was accidently removed from `ZSTD_loadDictionaryContent()`,
which means that dictionary compression will only be able to reference the parts of
the dictionary within the window. The spec allows us to reference the entire
dictionary so long as even one byte is in the window.
`ZSTD_enforceMaxDist()` incorrectly always allowed offsets up to `loadedDictEnd`
beyond the window, even once the dictionary was out of range.
When overflow protection kicked in, the check `current > loadedDictEnd + maxDist`
is incorrect if `loadedDictEnd` isn't reset back to zero. `current` could be reset
below the value, which would incorrectly allow references beyond the window. This
bug is present in `master`, but is very hard to trigger, since it requires both
dictionaries and data which triggers overflow correction.
Summary:
Allocate a single input buffer large enough to house each job, as well as
enough space for the IO thread to write 2 extra buffers. One goes in the
`POOL` queue, and one to fill, and then block on a full `POOL` queue.
Since we can't overlap with the prefix, we allocate space for 3 extra
input buffers.
Test Plan:
* CI
* With and without ASAN/UBSAN run zstdmt with different number of threads
on two large binaries, and verify that their checksums match.
* Test on the tip of the zstdmt ldm integration.
Reviewers: cyan
Differential Revision: https://phabricator.intern.facebook.com/D7284007
Tasks: T25664120
Summary:
* Expose the reference external sequences API for zstdmt.
Allows external sequences of any length, which get split when necessary.
* Reset the LDM window when the context is reset.
* Store the maximum number of LDM sequences.
* Sequence generation now returns the number of last literals.
* Fix sequence generation to not throw out the last literals when blocks of
more than 1 MB are encountered.
Expose reference external sequence API
* Expose the reference external sequences API for zstdmt.
* Allows external sequences of any length, which get split when necessary.
* Reset the LDM window when the context is reset.
* Store the maximum number of LDM sequences.
* Sequence generation now returns the number of last literals.
* Fix sequence generation to not throw out the last literals when blocks of
more than 1 MB are encountered.
Test Plan:
* CI
* Test the zstdmt ldm integration stacked on top of this diff
Reviewers: cyan
Differential Revision: https://phabricator.intern.facebook.com/D7283968
Tasks: T25664120
* Expose the reference external sequences API for zstdmt.
Allows external sequences of any length, which get split when necessary.
* Reset the LDM window when the context is reset.
* Store the maximum number of LDM sequences.
* Sequence generation now returns the number of last literals.
* Fix sequence generation to not throw out the last literals when blocks of
more than 1 MB are encountered.
The overflow protection is broken when the window log is `> (3U << 29)`, so 31.
It doesn't work when `current` isn't around `1U << windowLog` ahead of `lowLimit`,
and the the assertion `current > newCurrent` fails. This happens when the same
context is used many times over, but with a large window log, like in zstdmt.
Fix it by triggering correction based on `nextSrc - base` instead of `lowLimit`.
The added test fails before the patch, and passes after.
* `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()`.
When the dictionary is <= 8 bytes, no data is loaded from the dictionary.
In this case the repcodes weren't set, because they were inserted after the
size check. Fix this problem in general by first setting the cdict state to
a clean state of an empty dictionary, then filling the state from there.
Shaves 492,076 B off of the `ZSTD_CDict`.
The size of a `ZSTD_CDict` created from a 112,640 B dictionary is:
| Level | Before (B) | After (B) |
|-------|------------|-----------|
| 1 | 648,448 | 156,412 |
| 3 | 1,140,008 | 647,932 |
ZSTD_compress() and friends would treat an empty input as an unknown size
when selecting parameters. Thus, they would drastically overallocate the
context. Tell ZSTD_getParams() that the source size is 1 when it is empty.
It isn't useful in any case to repeat default tables.
Saves a few bytes on Silesia, since we don't trigger the dictionary
heuristic.
Before: 211988480 => 73651998 bytes
After: 211988480 => 73651721 bytes
In `ZSTD_compressBegin_advanced()`, `ZSTD_parameters` are used to set the
compression parameters, but the level didn't get set to `CLEVEL_CUSTOM`, so
`ZSTD_compressBlock()` used the wrong parameters when checking the source
size.
* Enforce smaller maximum values for parameters
* Adjust parameters to the source size
The memory usage is reduced by about 5x, which makes the fuzzers run at
least twice as fast, even more so with ASAN/MSAN enabled.
* Maximum window size in 32-bit mode is 1GB, since allocations for 2GB fail
on my Mac.
* Maximum window size in 64-bit mode is 2GB, since that is the largest
power of 2 that works with the overflow prevention.
* Allow `--long=windowLog` to set the window log, along with
`--zstd=wlog=#`. These options also set the window size during
decompression, but don't override `--memory=#` if it is set.
* Present a helpful error message when the window size is too large during
decompression.
* The long range matcher defaults to a hash log 7 less than the window log,
which keeps it at 20 for window log 27.
* Keep the default long range matcher window size and the default maximum
window size at 27 for the API and CLI.
* Add tests that use the maximum window size and hash size for compression
and decompression.
The timer used was only accurate up to 0.01 seconds. This timer is accurate up to 1 ns.
It is a monotonic timer that measures the real time difference, not on CPU time.
Nick Terrell