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Grok Tile Cache Architecture

Grok provides a multi-level tile caching system designed for efficient random-access decompression of large JPEG 2000 images. The cache minimizes redundant parsing, reduces memory consumption, and avoids unnecessary network fetches when reading from cloud storage.

Cache Strategies

Tile caching is controlled by a strategy bitmask set on the codec before decompression. Strategies can be combined with bitwise OR.

Constant Value Description
GRK_TILE_CACHE_NONE 0 No caching — each decompress is independent
GRK_TILE_CACHE_IMAGE 1 Cache decompressed tile images for reuse
GRK_TILE_CACHE_ALL 2 Cache everything (processors + images)
GRK_TILE_CACHE_LRU 4 LRU eviction with compressed chunk cache and re-decompression

Tile Lifecycle

1. First Encounter (SOT Parsed)

When a tile's SOT (Start of Tile-part) marker is parsed, a TileProcessor is created and its tilePartSeq_ is populated with byte offsets and lengths of every tile part in the codestream. This metadata constitutes a partial TLM-like database — even when the codestream lacks TLM markers.

2. Decompression

The T2 (packet parsing) and T1 (wavelet + entropy decoding) stages produce a GrkImage containing the decompressed pixel data. The cache entry is marked clean (dirty_ = false).

3. Cache Hit

Subsequent decompress calls for the same tile find a clean entry with a valid image and return immediately — no reprocessing.

4. SOT-Cached Fast Path (Non-TLM)

When decompressing a different tile from the same codestream, the SOTs of previously encountered tiles are already cached. On a later decode targeting one of those tiles, decompressFromCachedTileParts() seeks directly to each tile part using the cached byte offsets — no sequential codestream re-walk required. This is the mechanism described in issue #396.

5. LRU Eviction

When GRK_TILE_CACHE_LRU is active and maxActiveTiles is set, exceeding the limit evicts the least-recently-used tile's decompressed data via release(GRK_TILE_CACHE_LRU). The tile processor and its SOT metadata survive so the tile can be re-decompressed without re-parsing the codestream (issue #398).

6. Re-decompression

An evicted tile is re-decompressed by calling reinitForReDecompress(), which recreates the internal Tile structure, then runs the T2+T1 pipeline from cached compressed data. The CompressedChunkCache (see below) provides the compressed tile parts so no network re-fetch is needed (issue #399).

Compressed Chunk Cache

The CompressedChunkCache is a per-file LRU cache that holds compressed tile part data in memory after it has been fetched from the codestream (local or network). It serves two purposes:

  1. Avoids network re-fetches — When a tile is LRU-evicted and later needed again, its compressed data is retrieved from this cache rather than re-fetching from S3 / cloud storage.

  2. Manages memory with a high water mark — The cache enforces a configurable memory budget (issue #397). When the budget is exceeded, the least-recently-used tile's compressed buffers are spilled to disk and freed from memory.

Memory Budget Configuration

The budget is controlled by environment variables, checked in this order:

Variable Default Description
GRK_CACHEMAX 256 MB Grok-specific cache limit
GDAL_CACHEMAX 256 MB GDAL-compatible cache limit (fallback)

Values can be specified as:

  • Raw bytes: 268435456
  • With suffix: 256M, 256MB, 1G, 1GB

Disk Spillover

When the in-memory budget is exceeded, evicted tiles are serialized to a temporary disk cache (DiskCache). On reload, the compressed buffers and memory streams are recreated transparently. This allows the system to handle codestreams much larger than available RAM.

Configuration Summary

Setting How to Set Description
Cache strategy grk_decompress_set_params() → tile_cache_strategy Bitmask of GRK_TILE_CACHE_* constants
Max active tiles grk_decompress_set_params() → max_active_tiles LRU limit (0 = unlimited)
Memory budget GRK_CACHEMAX or GDAL_CACHEMAX env var Compressed chunk cache size

Example: Multi-Region Decompress with LRU

grk_decompress_parameters params;
grk_decompress_default_parameters(&params);
params.tile_cache_strategy = GRK_TILE_CACHE_IMAGE | GRK_TILE_CACHE_LRU;
params.max_active_tiles = 4;  // Keep at most 4 tiles decompressed

grk_codec* codec = grk_decompress_init(&stream, &params);

// Decompress tiles in any order — previously parsed SOT metadata is reused,
// evicted tiles are re-decompressed from cached compressed data.
grk_decompress_tile(codec, 10, &image);
grk_decompress_tile(codec, 3, &image);   // SOT fast path — no re-walk
grk_decompress_tile(codec, 10, &image);  // Cache hit — instant return
grk_decompress_tile(codec, 50, &image);  // May evict LRU tile