Make task dependency graph construction atomic #191
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…d deadlock
Submitted by Anton Schreiner from Intel Poland with a deep analysis of
the problem:
The important points are (should not happen as per my understanding of
the spec):
• Producers don’t wait for consumers
• The order of definitions of accessors in CG does matter – if we
submit two equal tasks (with different accessor order of two
buffers) from two different threads deadlock happens because one
buffer has the first task as producer and the second buffer has the
second task as producer – cyclic dependency.
triSYCL dependency handling:
a. Consumer – a task with read only access, Producer – a task with
any other type of accessor.
b. CG - command group
How it should be by spec:
The elementary node in dependency graph in SYCL is
CG. Dependencies for the CG are specified with accessors. The
order of definitions of accessors should not matter.
How it is implemented:
In triSYCL buffer keeps a use counter and a reference for the
latest producer and all the modifications are protected with
mutexes. A handler instance is made per CG submission which is
called task. At the moment when accessor is created it increments
use counter of the buffer and if the accessor wasn’t read only
then it replaces the reference to last producer of the buffer with
itself and saves the previous producer into a list inside task
class. Kernel invocations are specified with parallel_for kind of
methods of the task instance. Before the task runs its kernels it
first waits for all the producers in the tasks list of producers.
Problems:
• Producers don’t wait for consumers
• The order of definitions of accessors in CG does matters –
that’s why deadlock happens
Introduce the concept of finalization of a command group.
…ssors This remove one of the dead-lock cases submitted by Intel.
Simplify also the code by removing ad-hoc test or intermediate container.
keryell
changed the title
Do not add the third test yet because the anti-dependency is not managed yet.
Resolve conflict on include/CL/sycl/handler.hpp
keryell
changed the title
agozillon
reviewed
Jan 8, 2019
| associated to buffers */ | ||
| std::vector<std::unique_lock<std::mutex>> locks; | ||
| for (auto &b : buffers_in_use) | ||
| // Create a lock for the mutex, but doe not lock it yet |
include/CL/sycl/handler.hpp
Outdated
| @@ -31,6 +31,8 @@ | |||
| namespace cl { | |||
| namespace sycl { | |||
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Not relevant to the commit but if it's possible might as well change it to the newer nested namespaces definition e.g. namespace cl::sycl as was done for task.hpp, could also do it for queue.hpp if it requires it so we gradually move towards the new style.
tests/queue/deadlock_symetrical.cpp
Outdated
| /* RUN: %{execute}%s | ||
|
|
||
| Test that there is no deadlock in with massive parallel launch of | ||
| kernels using accessors in an asymmetrical creation, A B || B A. |
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should the name of this test be deadlock_asymmetrical.cpp rather than deadlock_symmetrical.cpp? Or are the comments for deadlock_symetrical.cpp and deadlock.cpp in reverse? I believe it's the former if something is incorrect
This should answer the review from Andrew Gozillon in triSYCL#191
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This should address some parts of #190 and other problems.