# RFC 26: GDAL Block Cache Improvements

Authors: Tamas Szekeres, Even Rouault

Contact: szekerest@gmail.com, even.rouault at spatialys.com

Status: Adopted, implemented

Implementation version: GDAL 2.1

## Summary and rationale

GDAL maintains an in-memory cache for the raster blocks fetched from the drivers and ensures that the second attempt to access the same block will be served from the cache instead of the driver. This cache is maintained in a per-band fashion and an array is allocated for the pointers for each blocks (or sub-blocks). This approach is not sufficient with large raster dimensions (or large virtual rasters ie. with the WMS/TMS driver), which may cause out of memory errors in GDALRasterBand::InitBlockInfo, as raised in #3224

For example, a band of a dataset at level 21 with a GoogleMaps tiling requires 2097152x2097152 tiles of 256x256 pixels. This means that GDAL will try to allocate an array of 32768x32768 = 1 billion elements (32768 = 2097152 / 64). The size of this array is 4 GB on a 32-bit build, so it cannot be allocated at all. And it is 8 GB on a 64-bit build (even if this is generally only virtual memory reservation but not actually allocation of physical pages of memory, due to over-commit mechanism of the operating system). At dataset closing, this means that those 1 billion cells will have to be explored to discover remaining cached blocks. In reality, all above figures must be multiplied by 3 for a RGB (or 4 for a RGBA) dataset.

In the hash set implementation, memory allocation depends directly on the number of cached blocks. Typically with the default GDAL_CACHEMAX size of 40 MB, only 640 blocks of 256x256 pixels can be simultaneously cached (for all datasets).

## Main concepts

Awareness of thread-safety issues is crucial in the design of block caching. In gdalrasterblock.cpp, a static linked list is maintained so as to track the access order of the blocks and keep the size of the cache within a desired limit by dropping the oldest blocks out of the list. This linked list is shared among all the datasets and bands in GDAL (protected by a hRBMutex) and a thread on each band, when reading a new block, may also trigger a GDALRasterBand::UnreferenceBlock call on another band within the scope of this mutex. GDALRasterBand::FlushBlock will also access the data structure of the band level cache by removing the corresponding tile from the array or the hashtable.

In GDAL 2.0, some issues related to thread-safety (#3225, #3226) have been fixed and this RFC still preserves those scenarios as safe.

The changes of this RFC consist in moving away from the GDALRasterBand class the logic to access to a cached block, to add or remove it. This is done with the new GDALAbstractBandBlockCache class. The current array based logic is moved into the new GDALArrayBandBlockCache class, and the new hashset based logic in GDALHashsetBandBlockCache.

For the array based implementation, due to the "static" nature of the hosting structure (an array), no special care is needed when reading or writing a cell from concurrent threads. The only special care that must be taken is to prevent a given cell (block) to be accessed concurrently. For example we want to avoid TryGetLockedBlockRef() to return a block that is being freed by another thread from GDALRasterBlock::FlushCacheBlock() or Internalize(). For that, the nRefCount member of GDALRasterBlock is now accessed and modified only through atomic functions to increase, decrease or compare-and-swap its value.

For the hash set based implementation, the base implementation of hash set data structure done in in cpl_hash_set.h / cpl_hash_set.cpp is not thread safe by default. So GDALHashsetBandBlockCache has a dedicated mutex to protect all reads, additions and removals from the hash set. No dead-lock with the hRBMutex can occurs since no operations done under the hashset mutex involves calling any method from GDALRasterBlock.

We could potentially have reused the hRBMutex to protect the hash set, but this would have increased the contention of the hRBMutex unnecessarily.

By default, the selection between the array based and the hashtable based approaches is based on the following rule: if the dataset has more than 1 million blocks, the hashset based implementation is used, otherwise the array based implementation is used. The new GDAL_OF_ARRAY_BLOCK_ACCESS and GDAL_OF_HASHSET_BLOCK_ACCESS open flags can also be passed to GDALOpenEx() to override this choice. The `GDAL_BAND_BLOCK_CACHE` configuration option can also be set to ARRAY or HASHSET.

The hashset based implementation could potentially be the default implementation in all cases (performance comparisons done with the autotest/cpp/testblockcache utility with 4 or 8 cores show non measurable differences), but in theory the array based implementation offers less contention of the hRBMutex, so should be more scalable when using lots of cores. And as work has been done during GDAL 2.0 to improve the scalability, it might be prudent for now to remain on the array based implementation on rasters of modest size.

Not completely linked with this RFC, a few changes have been done to limit the number of allocation/deallocation of objects (GDALRasterBlock instances, as well as an internal element of CPLHashSet), which has an effect on scalability since memory allocation routines involve synchronization between threads.

## Implementation

To implement the addition the following changes is made in the GDAL codebase:

- port/cpl_hash_set.cpp / port/cpl_hash_set.h: CPLHashSetClear() function added to remove all the elements in one operation.

- port/cpl_hash_set.cpp / port/cpl_hash_set.h: CPLHashSetRemoveDeferRehash() function added to remove one element quickly. That is to say the potential resizing of the array used internally is deferred to a later operation

- port/cpl_hash_set.cpp / port/cpl_hash_set.h: improvements to "recycle" links from the linked lists and avoid useless malloc()/free().

- port/cpl_atomic_ops.cpp: addition of CPLAtomicCompareAndExchange()

- gcore/gdal.h: additions of GDAL_OF_DEFAULT_BLOCK_ACCESS, GDAL_OF_ARRAY_BLOCK_ACCESS and GDAL_OF_HASHSET_BLOCK_ACCESS values.

- gcore/gdal_priv.h: definition of GDALAbstractBandBlockCache class, and GDALArrayBandBlockCacheCreate() and GDALHashSetBandBlockCacheCreate() functions. Modifications of GDALRasterBand, GDALDataset and GDALRasterBlock definitions.

- gcore/gdalrasterband.cpp: InitBlockInfo() instantiates the appropriate band block cache implementation.

- gcore/gdalrasterband.cpp: the AdoptBlock(), UnreferenceBlock(), FlushBlock() and TryGetLockedBlockRef() methods delegate to the actual band block cache implementation.

- gcore/gdalrasterband.cpp: AddBlockToFreeList() is added and delegate to GDALAbstractBandBlockCache

- gcore/gdalrasterblock.cpp: SafeLockBlock() is replaced by TakeLock()

- gcore/gdalrasterblock.cpp: RecycleFor() method added to recycle an existing block object to save a few new/delete calls (used by GDALAbstractBandBlockCache::CreateBlock())

- gcore/gdalrasterblock.cpp: Internalize() or FlushCacheBlock() no longer directly free a block (they still free or recycle its pData member), but provide it to GDALRasterBand::AddBlockToFreeList() for layer reuse.

- gcore/gdalrasterblock.cpp: DropLockForRemovalFromStorage() is added to avoid racing destruction of blocks between GDALRasterBand::FlushCache() or FlushBlock() with GDALRasterBlock::Internalize() or FlushCacheBlock().

- gcore/gdalabstractbandblockcache.cpp: added. Contains logic to keep instantiated GDALRasterBlock that were discarded by the global block manager for their later reuse. Saves a few new/delete calls.

- gcore/gdalarraybandblockcache.cpp: the GDALArrayBandBlockCache class implementation with mostly the existing code

- gcore/gdalhashsetbandblockcache.cpp: the new GDALHashsetBandBlockCache class implementation

## Backward Compatibility

This implementation retains the backward compatibility with the existing API. The C++ ABI of GDALRasterBand, GDALDataset and GDALRasterBlock is modified.

## Performance impacts

The array based implementation after this RFC should still show the same performance than the current implementation (potentially very slightly improved with the recycling of blocks). Confirmed by tests with autotest/cpp/testblockcache.

## Documentation

This change doesn't affect the existing user documentation.

## Testing

The autotest/cpp/testblockcache utility is now run by the "quick_test" target of autotest/cpp/Makefile with `GDAL_BAND_BLOCK_CACHE=HASHSET` in additions to the array based implementation.

A new autotest/cpp/testblockcachelimits utility has been developed to test a few racing situations. As races are hard to trigger, the code of GDALRasterBlock has been instrumented to allow sleeping in particular places, enabling races to be reliably simulated.

## Implementation

Tamas Szekeres had provided an initial version of this RFC. It has been restructured and ported on GDAL 2.0 by Even Rouault (sponsored by LINZ (Land Information New Zealand))

## References

The proposed implementation lies in the "rfc26_bandblockcache" branch of the https://github.com/rouault/gdal2/tree/rfc26_bandblockcache repository.

The list of changes: https://github.com/rouault/gdal2/compare/rfc26_bandblockcache

Related bugs: #3264, #3224.

## Voting History

+1 from EvenR, DanielM, TamasS. +0 from JukkaR