程序代写代做代考 flex Microsoft PowerPoint – MPI-4 [Compatibility Mode]
Microsoft PowerPoint – MPI-4 [Compatibility Mode]
1Computer Science, University of Warwick
MPI functions
MPI is a complex system comprising of numerous
functions with various parameters and variants
Six of them are indispensable, but can write a large
number of useful programs already
Other functions add flexibility (datatype),
robustness (non-blocking send/receive), efficiency
(communication mode), modularity
(communicators, groups) or convenience
(collective operations, topology).
In the lectures, we are going to cover most
commonly encountered functions
2Computer Science, University of Warwick
Derived datatype
– In MPI_Send, the data to be send, expressed as a
triple(addr, count, datatype), have to occupy
contiguous memory address
– Now, there are two problems:
1. What if the data do not occupy contiguous address
2. What if the data to be sent do not have the same data type.
3Computer Science, University of Warwick
Derived datatype
– Two possible solutions to problem 1:
1. Call a MPI_Send for each data block occupying contiguous
space.
2. Copy the non-contiguous data to a contiguous buffer space
and then call a MPI_Send to send the data in the buffer
– Solution to problem 2:
Call a MPI_Send for each data block which has the same datatype
and occupies the contiguous space.
These solutions are tedious and don’t have good performance
4Computer Science, University of Warwick
Derived datatype
– Derived datatype allows the users to construct (derive) their
own data types
– Derived datatypes can be used in
– Grouping non-contiguous data.
– Grouping data of different datatypes
– Provides the opportunity to send this kind of data
conveniently and potentially improving performance
5Computer Science, University of Warwick
Memory Layout of a Datatype
– The memory layout (mapping) of a datatype in
MPI is expressed as
{(type_0, offset_0), (type_1, offset_1), …, (type_n,
offset_n)}
6Computer Science, University of Warwick
Same type in
each block
Same number of
elements in
each block
Same distance
between
consecutive
block
MPI_Type_vector Yes Yes Yes
MPI_Type_create_in
dexed_block
Yes Yes No
MPI_Type_indexed Yes No No
MPI_Type_struct No No No
A block is a contiguous data items
Three Attributes to characterize non-contiguous data
7Computer Science, University of Warwick
Same type in
each block
Same number of
elements in
each block
Same distance
between
consecutive
block
MPI_Type_vector Yes Yes Yes
MPI_Type_create_in
dexed_block
Yes Yes No
MPI_Type_indexed Yes No No
MPI_Type_struct No No No
8Computer Science, University of Warwick
MPI_Type_vector
MPI_Type_Vector(count, blocklen, stride, oldtype, newtype)
Defines a derived type newtype comprising count consecutive blocks
of data elements with datatype oldtype, with each block containing
blocklen data elements, and the start of successive blocks separated
by stride data elements. E.g.
float data [1024];
MPI_Datatype floattype;
MPI_TYPE_vector (3, 2, 4, MPI_FLOAT, &floattype);
MPI_Type_commit (&floattype);
MPI_Send (data, 1, floattype, dest, tag, MPI_COMM_WORLD);
MPI_Type_free (&floattype)
9Computer Science, University of Warwick
MPI_Type_vector
The code in last slide is equivalent to the following code
MPI_FLOAT *buff, *data;
j=0;
for (i=0; i<3; i++) { buff[j] = data [i*4]; buff[j+1]=data[i*4+1]; j+=2;} MPI_Send (buff, 6, MPI_FLOAT, dest, tag, MPI_COMM_WORLD); The memory layout in the example (assume MPI_FLOAT is 16 bits) Old datatype = {(MPI_FLOAT, 0)} New datatype = { (MPI_FLOAT, 0), (MPI_FLOAT, 16), (MPI_FLOAT, 64), (MPI_FLOAT, 80), (MPI_FLOAT, 128), (MPI_FLOAT, 144) } 10Computer Science, University of Warwick Same type in each block Same number of elements in each block Same distance between consecutive block MPI_Type_vector Yes Yes Yes MPI_Type_create_in dexed_block Yes Yes No MPI_Type_indexed Yes No No MPI_Type_struct No No No 11Computer Science, University of Warwick MPI_Type_create_indexed_block int MPI_Type_create_indexed_block( int count, int blocklength, int array_of_offsets[], MPI_Datatype oldtype, MPI_Datatype *newtype ); Parameters: count [in] length of array of displacements (integer) blocklength [in] size of block (integer) array_of_offsets [in] array of displacements (array of integer) oldtype [in] old datatype (handle) newtype [out] new datatype (handle) 12Computer Science, University of Warwick MPI_Type_create_indexed_block int MPI_Type_create_indexed_block( int count, int blocklength, int array_of_displacements[], MPI_Datatype oldtype, MPI_Datatype *newtype ); Let oldtype = {(double, 0), (char, 32)} with extent 40 bits. Let D = (4, 1). After calling MPI_Type_indexed(2, 2, D, oldtype, newtype) newtype= { (double, 160), (char, 192), (double, 200), (char, 232) (double, 40), (char, 72), (double, 80), (char, 112) } 13Computer Science, University of Warwick Same type in each block Same number of elements in each block Same distance between consecutive block MPI_Type_vector Yes Yes Yes MPI_Type_create_in dexed_block Yes Yes No MPI_Type_indexed Yes No No MPI_Type_struct No No No 14Computer Science, University of Warwick MPI_Type_Indexed MPI_Type_Indexed (count, lengths[], offsets[], oldtype, newtype) Used in the case where the elements in the datatype to be constructed have the same type, but each block has different number of elements and the distance between two consecutive blocks are different Used to define a type comprising one or more blocks of a primitive or previously defined datatype, where block lengths and the displacement between blocks are specified in arrays The above call defines a type newtype comprising count consecutive blocks of data elements with type oldtype, with block i having a displacement of offsets data elements and containing lengths data elements 15Computer Science, University of Warwick MPI_Type_Indexed MPI_Type_Indexed(count, lengths[], offsets[], oldtype, newtype) Let oldtype = {(double, 0), (char, 32)} with extent 40 bits. Let B = (1, 3) and Let D = (1, 4). After calling MPI_Type_indexed(2, B, D, oldtype, newtype) newtype= { (double, 40), (char, 72) (double, 160), (char, 192), (double, 200), (char, 232), , (double, 240), (char, 272) } 16Computer Science, University of Warwick Same type in each block Same number of elements in each block Same distance between consecutive block MPI_Type_vector Yes Yes Yes MPI_Type_create_in dexed_block Yes Yes No MPI_Type_indexed Yes No No MPI_Type_struct No No No 17Computer Science, University of Warwick MPI_Type_struct int MPI_Type_struct(int count, int *array_of_blocklengths, MPI_int *array_of_displacements, MPI_Datatype *array_of_types, MPI_Datatype *newtype) The derived datatype includes different datatypes, each block with different displacements and different data elements 18Computer Science, University of Warwick MPI_Type_struct Let oldtype1 = {(double, 0), (char, 32bits)} with extent 5 bytes, and oldtype2 = {(float, 0), (char, 16bits)} with extend 3 bytes. Let B = (3, 1) and D = (20, 2) and C=(oldtype1, oldtype2). After calling MPI_Type_struct (2, B, D, C, newtype) newtype= { (double, 20), (char, 24), (double, 25), (char, 29) (double, 30), (char, 34), (float, 2), (char, 4) } 19Computer Science, University of Warwick Same type in each block Same number of elements in each block Same distance between consecutive block MPI_Type_vector Yes Yes Yes MPI_Type_create_in dexed_block Yes Yes No MPI_Type_indexed Yes No No MPI_Type_struct No No No Derived datatype summary oldtype3 20Computer Science, University of Warwick How are the data with derived data type are sent? - Pack the data elements specified by the memory layout into the contiguous space in system buffer - MPI library sends the data in system buffer 21Computer Science, University of Warwick Why using derived data type can improve performance? -Two possible solutions to non-contiguous data: 1. Call a MPI_Send for each data block that occupies contiguous space. Calling MPI_Send multiple times, which occurs higher overhead (e.g., handshaking between sender and receiver) 2. Copy the non-contiguous data to a contiguous buffer space and then call a MPI_Send to send the data in the buffer Needs to copy the data twice: 1) copy the non-contiguous data to a contiguous buffer space; 2) copy the data in the buffer to system buffer 22Computer Science, University of Warwick MPI_Type_contiguous MPI_Type_contiguous(3, MPI_REAL, newtype) // returns a new datatype that represents the concatenation of 3 instances of // MPI_REAL. MPI_Type_commit(newtype) // commits the datatype, must be done before communication MPI_Send(buff, 1, newtype, dest, tag, MPI_COMM_WORLD) // sends the data at location data to dest MPI_Type_free(newtype) // frees the datatype This is equivalent to the following single call MPI_SEND(buff, 3, MPI_REAL, dest, tag, MPI_COMM_WORLD) Where the elements to be sent are already contiguous int MPI_Type_contiguous( int count, MPI_Datatype old_type, MPI_Datatype *new_type_p ); 23Computer Science, University of Warwick MPI_Type_contiguous MPI_Type_contiguous(3, MPI_REAL, newtype) Old data type = {(MPI_REAL, 0)} Memory layout of the newtype (assume MPI_REAL occupies 32 bits) New datatype = {(MPI_REAL, 0), (MPI_REAL,32), (MPI_REAL, 64)} 24Computer Science, University of Warwick Summary of MPI Point-to-point communication Collective communication Communication modes Virtual topology Derived datatype
