The problem is the lack of dispatch buffers when thousands of loops with nowait,
about 10 iterations each, are executed by hundreds of threads. We only have
built-in 7 dispatch buffers, but there is a need in dozens or hundreds of
buffers.
The problem can be fixed by setting KMP_MAX_DISP_BUF to bigger value. In order
to give users same possibility I changed build-time control into run-time one,
adding API just in case.
This change adds an environment variable KMP_DISP_NUM_BUFFERS and a new API
function kmp_set_disp_num_buffers(int num_buffers).
The KMP_DISP_NUM_BUFFERS envirable works only before serial initialization,
because during the serial initialization we already allocate buffers for the hot
team, so it is too late to change the number of buffers later (or we need to
reallocate buffers for all teams which sounds too complicated). The
kmp_set_defaults() routine does not work for this envirable, because it calls
serial initialization before reading the parameter string. So a new routine,
kmp_set_disp_num_buffers(), is created so that it can set our internal global
variable before the library initialization. If both the envirable and API used
the envirable wins.
Differential Revision: http://reviews.llvm.org/D20697
llvm-svn: 271318
This change has OMP_WAIT_POLICY=active to mean that threads will busy-wait in
spin loops and virtually never go to sleep. OMP_WAIT_POLICY=passive now means
that threads will immediately go to sleep inside a spin loop. KMP_BLOCKTIME was
the previous mechanism to specify this behavior via KMP_BLOCKTIME=0 or
KMP_BLOCKTIME=infinite, but the standard OpenMP environment variable should
also be able to specify this behavior.
Differential Revision: http://reviews.llvm.org/D18577
llvm-svn: 265339
Jonathan Peyton