Introduce library-independent default compression level

Summary:
Previously we were using -1 as the default for every library, which was legacy from our zlib options. That worked for a while, but after zstd introduced https://github.com/facebook/zstd/commit/a146ee04ae5866b948be0c1911418e0436d80cb4, it started giving poor compression ratios by default in zstd.

This PR adds a constant to RocksDB public API, `CompressionOptions::kDefaultCompressionLevel`, which will get translated to the default value specific to the compression library being used in "util/compression.h". The constant uses a number that appears to be larger than any library's maximum compression level.
Closes https://github.com/facebook/rocksdb/pull/3895

Differential Revision: D8125780

Pulled By: ajkr

fbshipit-source-id: 2db157a89118cd4f94577c2f4a0a5ff31c8391c6

HISTORY.md

@@ -13,6 +13,7 @@
 * Now, `DBOptions::use_direct_io_for_flush_and_compaction` only applies to background writes, and `DBOptions::use_direct_reads` applies to both user reads and background reads. This conforms with Linux's `open(2)` manpage, which advises against simultaneously reading a file in buffered and direct modes, due to possibly undefined behavior and degraded performance.
 * Iterator::Valid() always returns false if !status().ok(). So, now when doing a Seek() followed by some Next()s, there's no need to check status() after every operation.
 * Iterator::Seek()/SeekForPrev()/SeekToFirst()/SeekToLast() always resets status().
+* Introduced `CompressionOptions::kDefaultCompressionLevel`, which is a generic way to tell RocksDB to use the compression library's default level. It is now the default value for `CompressionOptions::level`. Previously the level defaulted to -1, which gave poor compression ratios in ZSTD.
 
 ### New Features
 * Introduce TTL for level compaction so that all files older than ttl go through the compaction process to get rid of old data.

include/rocksdb/advanced_options.h

@@ -87,6 +87,14 @@ struct CompactionOptionsFIFO {
 
 // Compression options for different compression algorithms like Zlib
 struct CompressionOptions {
+  // RocksDB's generic default compression level. Internally it'll be translated
+  // to the default compression level specific to the library being used (see
+  // comment above `ColumnFamilyOptions::compression`).
+  //
+  // The default value is the max 16-bit int as it'll be written out in OPTIONS
+  // file, which should be portable.
+  const static int kDefaultCompressionLevel = 32767;
+
   int window_bits;
   int level;
   int strategy;
@@ -120,7 +128,7 @@ struct CompressionOptions {
 
   CompressionOptions()
       : window_bits(-14),
-        level(-1),
+        level(kDefaultCompressionLevel),
         strategy(0),
         max_dict_bytes(0),
         zstd_max_train_bytes(0) {}

include/rocksdb/options.h

@@ -198,11 +198,21 @@ struct ColumnFamilyOptions : public AdvancedColumnFamilyOptions {
   // Typical speeds of kSnappyCompression on an Intel(R) Core(TM)2 2.4GHz:
   //    ~200-500MB/s compression
   //    ~400-800MB/s decompression
+  //
   // Note that these speeds are significantly faster than most
   // persistent storage speeds, and therefore it is typically never
   // worth switching to kNoCompression.  Even if the input data is
   // incompressible, the kSnappyCompression implementation will
   // efficiently detect that and will switch to uncompressed mode.
+  //
+  // If you do not set `compression_opts.level`, or set it to
+  // `CompressionOptions::kDefaultCompressionLevel`, we will attempt to pick the
+  // default corresponding to `compression` as follows:
+  //
+  // - kZSTD: 3
+  // - kZlibCompression: Z_DEFAULT_COMPRESSION (currently -1)
+  // - kLZ4HCCompression: 0
+  // - For all others, we do not specify a compression level
   CompressionType compression;
 
   // Compression algorithm that will be used for the bottommost level that

util/compression.h

@@ -254,9 +254,15 @@ inline bool Zlib_Compress(const CompressionOptions& opts,
   // memLevel=9 uses maximum memory for optimal speed.
   // The default value is 8. See zconf.h for more details.
   static const int memLevel = 8;
+  int level;
+  if (opts.level == CompressionOptions::kDefaultCompressionLevel) {
+    level = Z_DEFAULT_COMPRESSION;
+  } else {
+    level = opts.level;
+  }
   z_stream _stream;
   memset(&_stream, 0, sizeof(z_stream));
-  int st = deflateInit2(&_stream, opts.level, Z_DEFLATED, opts.window_bits,
+  int st = deflateInit2(&_stream, level, Z_DEFLATED, opts.window_bits,
                         memLevel, opts.strategy);
   if (st != Z_OK) {
     return false;
@@ -719,9 +725,15 @@ inline bool LZ4HC_Compress(const CompressionOptions& opts,
   output->resize(static_cast<size_t>(output_header_len + compress_bound));
 
   int outlen;
+  int level;
+  if (opts.level == CompressionOptions::kDefaultCompressionLevel) {
+    level = 0;  // lz4hc.h says any value < 1 will be sanitized to default
+  } else {
+    level = opts.level;
+  }
 #if LZ4_VERSION_NUMBER >= 10400  // r124+
   LZ4_streamHC_t* stream = LZ4_createStreamHC();
-  LZ4_resetStreamHC(stream, opts.level);
+  LZ4_resetStreamHC(stream, level);
   const char* compression_dict_data =
       compression_dict.size() > 0 ? compression_dict.data() : nullptr;
   size_t compression_dict_size = compression_dict.size();
@@ -742,7 +754,7 @@ inline bool LZ4HC_Compress(const CompressionOptions& opts,
 #elif LZ4_VERSION_MAJOR  // r113-r123
   outlen = LZ4_compressHC2_limitedOutput(input, &(*output)[output_header_len],
                                          static_cast<int>(length),
-                                         compress_bound, opts.level);
+                                         compress_bound, level);
 #else                    // up to r112
   outlen =
       LZ4_compressHC_limitedOutput(input, &(*output)[output_header_len],
@@ -807,15 +819,23 @@ inline bool ZSTD_Compress(const CompressionOptions& opts, const char* input,
   size_t compressBound = ZSTD_compressBound(length);
   output->resize(static_cast<size_t>(output_header_len + compressBound));
   size_t outlen;
+  int level;
+  if (opts.level == CompressionOptions::kDefaultCompressionLevel) {
+    // 3 is the value of ZSTD_CLEVEL_DEFAULT (not exposed publicly), see
+    // https://github.com/facebook/zstd/issues/1148
+    level = 3;
+  } else {
+    level = opts.level;
+  }
 #if ZSTD_VERSION_NUMBER >= 500  // v0.5.0+
   ZSTD_CCtx* context = ZSTD_createCCtx();
   outlen = ZSTD_compress_usingDict(
       context, &(*output)[output_header_len], compressBound, input, length,
-      compression_dict.data(), compression_dict.size(), opts.level);
+      compression_dict.data(), compression_dict.size(), level);
   ZSTD_freeCCtx(context);
 #else   // up to v0.4.x
   outlen = ZSTD_compress(&(*output)[output_header_len], compressBound, input,
-                         length, opts.level);
+                         length, level);
 #endif  // ZSTD_VERSION_NUMBER >= 500
   if (outlen == 0) {
     return false;