Last modified: 11 November 2011
Name: H5Pset_chunk_cache
Signature:
herr_t H5Pset_chunk_cache( hid_t dapl_id, size_t rdcc_nslots, size_t rdcc_nbytes, double rdcc_w0 )

Purpose:
Sets the raw data chunk cache parameters.

Motivation:
H5Pset_chunk_cache is used to adjust the chunk cache parameters on a per-dataset basis, as opposed to a global setting for the file. The optimum chunk cache parameters vary wildly with different data layout and access patterns, so for optimal performance they must be set individually for each dataset. It may also be beneficial to reduce the size of the chunk cache for datasets whose performance is not important in order to save memory space.

Description:
H5Pset_chunk_cache sets the number of elements, the total number of bytes, and the preemption policy value in the raw data chunk cache on a dataset access property list. After calling this function, the values set in the property list will override the values in the file's file access property list.

The raw data chunk cache inserts chunks into the cache by first computing a hash value using the address of a chunk, then using that hash value as the chunk's index into the table of cached chunks. The size of this hash table, i.e., and the number of possible hash values, is determined by the rdcc_nslots parameter. If a different chunk in the cache has the same hash value, this causes a collision, which reduces efficiency. If inserting the chunk into cache would cause the cache to be too big, then the cache is pruned according to the rdcc_w0 parameter.

Parameters:
hid_t dapl_id      IN: Dataset access property list identifier.
size_t rdcc_nslots   IN:The number of chunk slots in the raw data chunk cache for this dataset. Increasing this value reduces the number of cache collisions, but slightly increases the memory used. Due to the hashing strategy, this value should ideally be a prime number. As a rule of thumb, this value should be at least 10 times the number of chunks that can fit in rdcc_nbytes bytes. For maximum performance, this value should be set approximately 100 times that number of chunks.

The default value is 521. If the value passed is H5D_CHUNK_CACHE_NSLOTS_DEFAULT, then the property will not be set on dapl_id and the parameter will come from the file access property list used to open the file.

size_t rdcc_nbytes   IN: The total size of the raw data chunk cache for this dataset. In most cases increasing this number will improve performance, as long as you have enough free memory.

The default size is 1 MB. If the value passed is H5D_CHUNK_CACHE_NBYTES_DEFAULT, then the property will not be set on dapl_id and the parameter will come from the file access property list.

double rdcc_w0   IN: The chunk preemption policy for this dataset. This must be between 0 and 1 inclusive and indicates the weighting according to which chunks which have been fully read or written are penalized when determining which chunks to flush from cache. A value of 0 means fully read or written chunks are treated no differently than other chunks (the preemption is strictly LRU) while a value of 1 means fully read or written chunks are always preempted before other chunks. If your application only reads or writes data once, this can be safely set to 1. Otherwise, this should be set lower, depending on how often you re-read or re-write the same data.

The default value is 0.75. If the value passed is H5D_CHUNK_CACHE_W0_DEFAULT, then the property will not be set on dapl_id and the parameter will come from the file access property list.

Returns:
Returns a non-negative value if successful; otherwise returns a negative value.

Example Usage:
The following code sets the chunk cache to use a hash table with 12421 elements and a maximum size of 16 MB, while using the preemption policy specified for the entire file:
H5Pset_chunk_cache(dapl_id, 12421, 16*1024*1024, H5D_CHUNK_CACHE_W0_DEFAULT);
Fortran90 Interface: h5pset_chunk_cache_f
SUBROUTINE h5pset_chunk_cache_f(dapl_id, rdcc_nslots, rdcc_nbytes, rdcc_w0, hdferr)
  IMPLICIT NONE
  INTEGER(HID_T), INTENT(IN) :: dapl_id      ! Dataset access property list 
                                             ! identifier.
  INTEGER(SIZE_T), INTENT(IN) :: rdcc_nslots ! The number of chunk slots in the 
                                             ! raw data chunk cache for this 
                                             ! dataset.
  INTEGER(SIZE_T), INTENT(IN) :: rdcc_nbytes ! The total size of the raw data 
                                             ! chunk cache for this dataset.
  REAL, INTENT(IN) :: rdcc_w0                ! The chunk preemption policy for 
                                             ! this dataset.
  INTEGER, INTENT(OUT) :: hdferr             ! Error code
                                             ! 0 on success and -1 on failure
END SUBROUTINE h5pset_chunk_cache_f
    

See Also:
H5Pget_chunk_cache,   H5Pset_cache

History:
Release     Change
1.8.3 C function introduced in this release.