Parallel Programming Guide for HP-UX Systems

Troubleshooting
False cache line sharing
Chapter 9180
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Working with unaligned arrays
The most common cache-thrashing complication using arrays and loops
occurs when arrays assigned within a loop are unaligned with each other.
There are several possible causes for this:
• Arrays that are local to a routine are allocated on the stack.
• Array dummy arguments might be passed an element other than the
first in the actual argument.
• Array elements might be assigned with different offset indexes.
Consider the following Fortran code:
COMMON /OKAY/ X(112,100)
...
CALL UNALIGNED (X(I,J))
...
SUBROUTINE UNALIGNED (Y)
REAL*4 Y(*)
! Y(1) PROBABLY NOT ON A CACHE LINE BOUNDARY
The address of Y(1) is unknown. However, if elements of Y are heavily
assigned in this routine, it may be worthwhile to compute an alignment,
given by the following formula:
LREM = LSIZE - ( (
( LOC(Y(1))-4, LSIZE*x) + 4) /x)
where
LSIZE is the appropriate cache line size in words
x is the data size for elements of Y
For this case, LSIZE on V2250 servers is 32 bytes in single precision
words (8 words). Note that:
( ( MOD ( LOC(Y(1))-4, LSIZE*4) + 4) /4)
returns a value in the set 1, 2, 3, ..., LSIZE,soLREM is in the range 0 to 7.
Then a loop such as: