Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

More regular block sizes with targetCBlockSize #3917

Merged
merged 8 commits into from Feb 26, 2024
Merged

More regular block sizes with targetCBlockSize #3917

Changes from all commits
Commits
Show all changes
8 commits
Select commit Hold shift + click to select a range
038a8a9
targetCBlockSize: modified splitting strategy to generate blocks of m…
Cyan4973 Feb 26, 2024
1fafd0c
fix minor visual static analyzer warning
Cyan4973 Feb 26, 2024
e0412c2
fix extraneous semicolon ';'
Cyan4973 Feb 26, 2024
aa8592c
minor: reformulate nbSubBlocks assignment
Cyan4973 Feb 26, 2024
ef82b21
nit: comment indentation
Cyan4973 Feb 26, 2024
86db607
optimization: bail out faster in presence of incompressible data
Cyan4973 Feb 26, 2024
d23b95d
minor refactor for clarity
Cyan4973 Feb 26, 2024
8d31e8e
sizeBlockSequences() also tracks uncompressed size
Cyan4973 Feb 26, 2024
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Jump to
Jump to file
Failed to load files.
Diff view
Diff view
244 changes: 161 additions & 83 deletions lib/compress/zstd_compress_superblock.c
View file
Open in desktop
Expand Up @@ -390,23 +390,29 @@ static size_t ZSTD_estimateSubBlockSize_sequences(const BYTE* ofCodeTable,
return cSeqSizeEstimate + sequencesSectionHeaderSize;
}

static size_t ZSTD_estimateSubBlockSize(const BYTE* literals, size_t litSize,
typedef struct {
size_t estLitSize;
size_t estBlockSize;
} EstimatedBlockSize;
static EstimatedBlockSize ZSTD_estimateSubBlockSize(const BYTE* literals, size_t litSize,
const BYTE* ofCodeTable,
const BYTE* llCodeTable,
const BYTE* mlCodeTable,
size_t nbSeq,
const ZSTD_entropyCTables_t* entropy,
const ZSTD_entropyCTablesMetadata_t* entropyMetadata,
void* workspace, size_t wkspSize,
int writeLitEntropy, int writeSeqEntropy) {
size_t cSizeEstimate = 0;
cSizeEstimate += ZSTD_estimateSubBlockSize_literal(literals, litSize,
&entropy->huf, &entropyMetadata->hufMetadata,
workspace, wkspSize, writeLitEntropy);
cSizeEstimate += ZSTD_estimateSubBlockSize_sequences(ofCodeTable, llCodeTable, mlCodeTable,
int writeLitEntropy, int writeSeqEntropy)
{
EstimatedBlockSize ebs;
ebs.estLitSize = ZSTD_estimateSubBlockSize_literal(literals, litSize,
&entropy->huf, &entropyMetadata->hufMetadata,
workspace, wkspSize, writeLitEntropy);
ebs.estBlockSize = ZSTD_estimateSubBlockSize_sequences(ofCodeTable, llCodeTable, mlCodeTable,
nbSeq, &entropy->fse, &entropyMetadata->fseMetadata,
workspace, wkspSize, writeSeqEntropy);
return cSizeEstimate + ZSTD_blockHeaderSize;
ebs.estBlockSize += ebs.estLitSize + ZSTD_blockHeaderSize;
return ebs;
}

static int ZSTD_needSequenceEntropyTables(ZSTD_fseCTablesMetadata_t const* fseMetadata)
Expand All @@ -427,17 +433,51 @@ static size_t countLiterals(seqStore_t const* seqStore, const seqDef* sp, size_t
for (n=0; n<seqCount; n++) {
total += ZSTD_getSequenceLength(seqStore, sp+n).litLength;
}
DEBUGLOG(5, "countLiterals for %zu sequences from %p => %zu bytes", seqCount, (const void*)sp, total);
DEBUGLOG(6, "countLiterals for %zu sequences from %p => %zu bytes", seqCount, (const void*)sp, total);
return total;
}

#define BYTESCALE 256

static size_t sizeBlockSequences(const seqDef* sp, size_t nbSeqs,
size_t targetBudget, size_t avgLitCost, size_t avgSeqCost,
int firstSubBlock)
terrelln marked this conversation as resolved.
Show resolved Hide resolved
{
size_t n, budget = 0, inSize=0;
/* entropy headers */
size_t const headerSize = (size_t)firstSubBlock * 120 * BYTESCALE; /* generous estimate */
assert(firstSubBlock==0 || firstSubBlock==1);
budget += headerSize;

/* first sequence => at least one sequence*/
budget += sp[0].litLength * avgLitCost + avgSeqCost;
if (budget > targetBudget) return 1;
inSize = sp[0].litLength + (sp[0].mlBase+MINMATCH);

/* loop over sequences */
for (n=1; n<nbSeqs; n++) {
size_t currentCost = sp[n].litLength * avgLitCost + avgSeqCost;
budget += currentCost;
inSize += sp[n].litLength + (sp[n].mlBase+MINMATCH);
/* stop when sub-block budget is reached */
if ( (budget > targetBudget)
/* though continue to expand until the sub-block is deemed compressible */
&& (budget < inSize * BYTESCALE) )
break;
}

return n;
}

#define CBLOCK_TARGET_SIZE_MIN 1340 /* suitable to fit into an ethernet / wifi / 4G transport frame */

/** ZSTD_compressSubBlock_multi() :
* Breaks super-block into multiple sub-blocks and compresses them.
* Entropy will be written to the first block.
* The following blocks will use repeat mode to compress.
* All sub-blocks are compressed blocks (no raw or rle blocks).
* @return : compressed size of the super block (which is multiple ZSTD blocks)
* Or 0 if it failed to compress. */
* Entropy will be written into the first block.
* The following blocks use repeat_mode to compress.
* Sub-blocks are all compressed, except the last one when beneficial.
* @return : compressed size of the super block (which features multiple ZSTD blocks)
* or 0 if it failed to compress. */
static size_t ZSTD_compressSubBlock_multi(const seqStore_t* seqStorePtr,
const ZSTD_compressedBlockState_t* prevCBlock,
ZSTD_compressedBlockState_t* nextCBlock,
Expand All @@ -452,7 +492,6 @@ static size_t ZSTD_compressSubBlock_multi(const seqStore_t* seqStorePtr,
const seqDef* const send = seqStorePtr->sequences;
const seqDef* sp = sstart; /* tracks progresses within seqStorePtr->sequences */
size_t const nbSeqs = (size_t)(send - sstart);
size_t nbSeqsPerBlock = nbSeqs;
const BYTE* const lstart = seqStorePtr->litStart;
const BYTE* const lend = seqStorePtr->lit;
const BYTE* lp = lstart;
Expand All @@ -465,90 +504,129 @@ static size_t ZSTD_compressSubBlock_multi(const seqStore_t* seqStorePtr,
const BYTE* llCodePtr = seqStorePtr->llCode;
const BYTE* mlCodePtr = seqStorePtr->mlCode;
const BYTE* ofCodePtr = seqStorePtr->ofCode;
size_t const minTarget = 1300; /* enforce minimum size, to reduce undesirable side effects */
size_t const minTarget = CBLOCK_TARGET_SIZE_MIN; /* enforce minimum size, to reduce undesirable side effects */
size_t const targetCBlockSize = MAX(minTarget, cctxParams->targetCBlockSize);
int writeLitEntropy = (entropyMetadata->hufMetadata.hType == set_compressed);
int writeSeqEntropy = 1;
size_t nbSubBlocks = 1;

DEBUGLOG(5, "ZSTD_compressSubBlock_multi (srcSize=%u, litSize=%u, nbSeq=%u)",
(unsigned)srcSize, (unsigned)(lend-lstart), (unsigned)(send-sstart));

if (nbSeqs == 0) {
/* special case : no sequence */
nbSeqsPerBlock = 0;
nbSubBlocks = 1;
} else {
/* let's start by a general estimation for the full block */
size_t const cBlockSizeEstimate =
if (nbSeqs > 0) {
EstimatedBlockSize const ebs =
ZSTD_estimateSubBlockSize(lp, nbLiterals,
ofCodePtr, llCodePtr, mlCodePtr, nbSeqs,
&nextCBlock->entropy, entropyMetadata,
workspace, wkspSize,
writeLitEntropy, writeSeqEntropy);
/* quick estimation */
nbSubBlocks = (cBlockSizeEstimate + (targetCBlockSize-1)) / targetCBlockSize;
assert(nbSubBlocks > 0);
if (nbSeqs > nbSubBlocks) {
nbSeqsPerBlock = nbSeqs / nbSubBlocks;
} else {
nbSeqsPerBlock = 1;
nbSubBlocks = nbSeqs;
size_t const avgLitCost = nbLiterals ? (ebs.estLitSize * BYTESCALE) / nbLiterals : BYTESCALE;
size_t const avgSeqCost = ((ebs.estBlockSize - ebs.estLitSize) * BYTESCALE) / nbSeqs;
const size_t nbSubBlocks = MAX((ebs.estBlockSize + (targetCBlockSize/2)) / targetCBlockSize, 1);
size_t n, avgBlockBudget, blockBudgetSupp=0;
avgBlockBudget = (ebs.estBlockSize * BYTESCALE) / nbSubBlocks;
DEBUGLOG(5, "estimated fullblock size=%u bytes ; avgLitCost=%.2f ; avgSeqCost=%.2f ; targetCBlockSize=%u, nbSubBlocks=%u ; avgBlockBudget=%.0f bytes",
(unsigned)ebs.estBlockSize, (double)avgLitCost/BYTESCALE, (double)avgSeqCost/BYTESCALE,
(unsigned)targetCBlockSize, (unsigned)nbSubBlocks, (double)avgBlockBudget/BYTESCALE);
/* simplification: if estimates states that the full superblock doesn't compress, just bail out immediately
* this will result in the production of a single uncompressed block covering @srcSize.*/
if (ebs.estBlockSize > srcSize) return 0;

/* compress and write sub-blocks */
assert(nbSubBlocks>0);
for (n=0; n < nbSubBlocks-1; n++) {
/* determine nb of sequences for current sub-block + nbLiterals from next sequence */
size_t const seqCount = sizeBlockSequences(sp, (size_t)(send-sp),
avgBlockBudget + blockBudgetSupp, avgLitCost, avgSeqCost, n==0);
/* if reached last sequence : break to last sub-block (simplification) */
assert(seqCount <= (size_t)(send-sp));
if (sp + seqCount == send) break;
assert(seqCount > 0);
/* compress sub-block */
{ int litEntropyWritten = 0;
int seqEntropyWritten = 0;
size_t litSize = countLiterals(seqStorePtr, sp, seqCount);
const size_t decompressedSize =
ZSTD_seqDecompressedSize(seqStorePtr, sp, seqCount, litSize, 0);
size_t const cSize = ZSTD_compressSubBlock(&nextCBlock->entropy, entropyMetadata,
sp, seqCount,
lp, litSize,
llCodePtr, mlCodePtr, ofCodePtr,
cctxParams,
op, (size_t)(oend-op),
bmi2, writeLitEntropy, writeSeqEntropy,
&litEntropyWritten, &seqEntropyWritten,
0);
FORWARD_IF_ERROR(cSize, "ZSTD_compressSubBlock failed");

/* check compressibility, update state components */
if (cSize > 0 && cSize < decompressedSize) {
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Now that we've removed the restriction that blocks must not be expanded, I think we could remove this cSize < decompressedSize check, and just check that it isn't beyond the maximum block size.

I don't think we should do it in this PR, because we want to cherry pick it into a release, but we should consider it.

DEBUGLOG(5, "Committed sub-block compressing %u bytes => %u bytes",
(unsigned)decompressedSize, (unsigned)cSize);
assert(ip + decompressedSize <= iend);
ip += decompressedSize;
lp += litSize;
op += cSize;
llCodePtr += seqCount;
mlCodePtr += seqCount;
ofCodePtr += seqCount;
/* Entropy only needs to be written once */
if (litEntropyWritten) {
writeLitEntropy = 0;
}
if (seqEntropyWritten) {
writeSeqEntropy = 0;
}
sp += seqCount;
blockBudgetSupp = 0;
} }
/* otherwise : do not compress yet, coalesce current sub-block with following one */
}
/* Note: this is very approximative. Obviously, some sub-blocks will be larger and others smaller.
* But the contract of this feature has always been approximative, so for now we'll leverage it for speed.
* It can be refined later, for closer-to-target compressed block size, if it ever matters. */
}

/* write sub-blocks */
{ size_t n;
size_t nbSeqsToProcess = 0;
for (n=0; n < nbSubBlocks; n++) {
int const lastSubBlock = (n==nbSubBlocks-1);
size_t const nbSeqsLastSubBlock = nbSeqs - (nbSubBlocks-1) * nbSeqsPerBlock;
size_t nbSeqsSubBlock = lastSubBlock ? nbSeqsLastSubBlock : nbSeqsPerBlock;
size_t seqCount = nbSeqsToProcess+nbSeqsSubBlock;
size_t litSize = lastSubBlock ? (size_t)(lend-lp) : countLiterals(seqStorePtr, sp, seqCount);
int litEntropyWritten = 0;
int seqEntropyWritten = 0;
const size_t decompressedSize =
ZSTD_seqDecompressedSize(seqStorePtr, sp, seqCount, litSize, lastSubBlock);
size_t const cSize = ZSTD_compressSubBlock(&nextCBlock->entropy, entropyMetadata,
sp, seqCount,
lp, litSize,
llCodePtr, mlCodePtr, ofCodePtr,
cctxParams,
op, (size_t)(oend-op),
bmi2, writeLitEntropy, writeSeqEntropy,
&litEntropyWritten, &seqEntropyWritten,
lastBlock && lastSubBlock);
nbSeqsToProcess = seqCount;
FORWARD_IF_ERROR(cSize, "ZSTD_compressSubBlock failed");

if (cSize > 0 && cSize < decompressedSize) {
DEBUGLOG(5, "Committed sub-block compressing %u bytes => %u bytes",
(unsigned)decompressedSize, (unsigned)cSize);
assert(ip + decompressedSize <= iend);
ip += decompressedSize;
lp += litSize;
op += cSize;
llCodePtr += seqCount;
mlCodePtr += seqCount;
ofCodePtr += seqCount;
/* Entropy only needs to be written once */
if (litEntropyWritten) {
writeLitEntropy = 0;
}
if (seqEntropyWritten) {
writeSeqEntropy = 0;
}
sp += seqCount;
nbSeqsToProcess = 0;
} /* if (nbSeqs > 0) */

/* write last block */
DEBUGLOG(2, "Generate last sub-block: %u sequences remaining", (unsigned)(send - sp));
{ int litEntropyWritten = 0;
int seqEntropyWritten = 0;
size_t litSize = (size_t)(lend - lp);
size_t seqCount = (size_t)(send - sp);
const size_t decompressedSize =
ZSTD_seqDecompressedSize(seqStorePtr, sp, seqCount, litSize, 1);
size_t const cSize = ZSTD_compressSubBlock(&nextCBlock->entropy, entropyMetadata,
sp, seqCount,
lp, litSize,
llCodePtr, mlCodePtr, ofCodePtr,
cctxParams,
op, (size_t)(oend-op),
bmi2, writeLitEntropy, writeSeqEntropy,
&litEntropyWritten, &seqEntropyWritten,
lastBlock);
FORWARD_IF_ERROR(cSize, "ZSTD_compressSubBlock failed");

/* update pointers, the nb of literals borrowed from next sequence must be preserved */
if (cSize > 0 && cSize < decompressedSize) {
DEBUGLOG(2, "Last sub-block compressed %u bytes => %u bytes",
(unsigned)decompressedSize, (unsigned)cSize);
assert(ip + decompressedSize <= iend);
ip += decompressedSize;
lp += litSize;
op += cSize;
llCodePtr += seqCount;
mlCodePtr += seqCount;
ofCodePtr += seqCount;
/* Entropy only needs to be written once */
if (litEntropyWritten) {
writeLitEntropy = 0;
}
if (seqEntropyWritten) {
writeSeqEntropy = 0;
}
/* otherwise : coalesce current block with next one */
sp += seqCount;
}
}


if (writeLitEntropy) {
DEBUGLOG(5, "Literal entropy tables were never written");
ZSTD_memcpy(&nextCBlock->entropy.huf, &prevCBlock->entropy.huf, sizeof(prevCBlock->entropy.huf));
Expand All @@ -565,7 +643,7 @@ static size_t ZSTD_compressSubBlock_multi(const seqStore_t* seqStorePtr,
/* some data left : last part of the block sent uncompressed */
size_t const rSize = (size_t)((iend - ip));
size_t const cSize = ZSTD_noCompressBlock(op, (size_t)(oend - op), ip, rSize, lastBlock);
DEBUGLOG(5, "Generate last uncompressed sub-block of %u bytes", (unsigned)(rSize));
DEBUGLOG(2, "Generate last uncompressed sub-block of %u bytes", (unsigned)(rSize));
FORWARD_IF_ERROR(cSize, "ZSTD_noCompressBlock failed");
assert(cSize != 0);
op += cSize;
Expand All @@ -581,8 +659,8 @@ static size_t ZSTD_compressSubBlock_multi(const seqStore_t* seqStorePtr,
}
}

DEBUGLOG(5, "ZSTD_compressSubBlock_multi compressed %u subBlocks: total compressed size = %u",
(unsigned)nbSubBlocks, (unsigned)(op-ostart));
DEBUGLOG(5, "ZSTD_compressSubBlock_multi compressed all subBlocks: total compressed size = %u",
(unsigned)(op-ostart));
return (size_t)(op-ostart);
}

Expand Down