This repository has been archived by the owner on Mar 21, 2024. It is now read-only.
-
Notifications
You must be signed in to change notification settings - Fork 187
Move and update "memory model" docs #301
Merged
Merged
Changes from all commits
Commits
Show all changes
8 commits
Select commit
Hold shift + click to select a range
6b015ef
Memory model documentation
gonzalobg c31c15d
Make site wider to support tables
gonzalobg 485a923
Fix site build scripts
gonzalobg 7ed814f
Fix typo
gonzalobg 0dab4ee
Update docs/extended_api/memory_model.md
gonzalobg 1d10a7f
Leaving sentence out
gonzalobg 383837e
Remove Threads section
gonzalobg 2d465e4
Fix broken links in extended_api
wmaxey File filter
Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
There are no files selected for viewing
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -1,3 +1,4 @@ | ||
FROM jekyll/jekyll:4.0 | ||
COPY Gemfile /Gemfile | ||
RUN /bin/bash -l -c "bundle install" | ||
RUN /bin/bash -l -c "bundle install" | ||
RUN /bin/bash -l -c "bundle add webrick" |
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -0,0 +1,176 @@ | ||
--- | ||
parent: Extended API | ||
nav_order: 0 | ||
--- | ||
|
||
# Memory model | ||
|
||
Standard C++ presents a view that the cost to synchronize threads is uniform and low. | ||
|
||
CUDA C++ is different: the cost to synchronize threads grows as threads are further apart. | ||
It is low across threads within a block, but high across arbitrary threads in the system running on multiple GPUs and CPUs. | ||
|
||
To account for non-uniform thread synchronization costs that are not always low, CUDA C++ extends the standard C++ memory model and concurrency facilities in the `cuda::` namespace with **thread scopes**, retaining the syntax and semantics of standard C++ by default. | ||
|
||
## Thread Scopes | ||
|
||
A _thread scope_ specifies the kind of threads that can synchronize with each other using synchronization primitive such as [`atomic`] or [`barrier`]. | ||
|
||
```cuda | ||
namespace cuda { | ||
|
||
enum thread_scope { | ||
thread_scope_system, | ||
thread_scope_device, | ||
thread_scope_block, | ||
thread_scope_thread | ||
}; | ||
|
||
} // namespace cuda | ||
``` | ||
|
||
[`atomic`]: synchronization_primitives/atomic.md | ||
[`barrier`]: synchronization_primitives/barrier.md | ||
|
||
### Scope Relationships | ||
|
||
Each program thread is related to each other program thread by one or more thread scope relations: | ||
- Each thread in the system is related to each other thread in the system by the *system* thread scope: `thread_scope_system`. | ||
- Each GPU thread is related to each other GPU thread in the same CUDA device by the *device* thread scope: `thread_scope_device`. | ||
- Each GPU thread is related to each other GPU thread in the same CUDA thread block by the *block* thread scope: `thread_scope_block`. | ||
- Each thread is related to itself by the `thread` thread scope: `thread_scope_thread`. | ||
|
||
## Synchronization primitives | ||
|
||
Types in namespaces `std::` and `cuda::std::` have the same behavior as corresponding types in namespace `cuda::` when instantiated with a scope of `cuda::thread_scope_system`. | ||
|
||
## Atomicity | ||
|
||
An atomic operation is atomic at the scope it specifies if: | ||
- it specifies a scope other than `thread_scope_system`, **or** | ||
|
||
the scope is `thread_scope_system` and: | ||
|
||
- it affects an object in [unified memory] and [`concurrentManagedAccess`] is `1`, **or** | ||
- it affects an object in CPU memory and [`hostNativeAtomicSupported`] is `1`, **or** | ||
- it is a load or store that affects a naturally-aligned object of sizes `1`, `2`, `4`, or `8` bytes on [mapped memory], **or** | ||
- it affects an object in GPU memory and only GPU threads access it. | ||
|
||
[mapped memory]: https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#mapped-memory | ||
|
||
Refer to the [CUDA programming guide] for more information on [unified memory], [mapped memory], CPU memory, and GPU peer memory. | ||
|
||
[mapped memory]: https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#mapped-memory | ||
[unified memory]: https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#um-unified-memory-programming-hd | ||
[CUDA programming guide]: https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html | ||
[`concurrentManagedAccess`]: https://docs.nvidia.com/cuda/cuda-runtime-api/structcudaDeviceProp.html#structcudaDeviceProp_116f9619ccc85e93bc456b8c69c80e78b | ||
[`hostNativeAtomicSupported`]: https://docs.nvidia.com/cuda/cuda-runtime-api/structcudaDeviceProp.html#structcudaDeviceProp_1ef82fd7d1d0413c7d6f33287e5b6306f | ||
|
||
## Data Races | ||
|
||
Modify [intro.races paragraph 21] of ISO/IEC IS 14882 (the C++ Standard) as follows: | ||
> The execution of a program contains a data race if it contains two potentially concurrent conflicting actions, at least one of which is not atomic ***at a scope that includes the thread that performed the other operation***, and neither happens before the other, except for the special case for signal handlers described below. Any such data race results in undefined behavior. [...] | ||
|
||
Modify [thread.barrier.class paragraph 4] of ISO/IEC IS 14882 (the C++ Standard) as follows: | ||
> 4. Concurrent invocations of the member functions of `barrier`, other than its destructor, do not introduce data races ***as if they were atomic operations***. [...] | ||
|
||
Modify [thread.latch.class paragraph 2] of ISO/IEC IS 14882 (the C++ Standard) as follows: | ||
> 2. Concurrent invocations of the member functions of `latch`, other than its destructor, do not introduce data races ***as if they were atomic operations***. | ||
|
||
Modify [thread.sema.cnt paragraph 3] of ISO/IEC IS 14882 (the C++ Standard) as follows: | ||
> 3. Concurrent invocations of the member functions of `counting_semaphore`, other than its destructor, do not introduce data races ***as if they were atomic operations***. | ||
|
||
Modify [thread.stoptoken.intro paragraph 5] of ISO/IEC IS 14882 (the C++ Standard) as follows: | ||
> Calls to the functions request_stop, stop_requested, and stop_possible do not introduce data races ***as if they were atomic operations***. [...] | ||
|
||
[thread.stoptoken.intro paragraph 5]: https://eel.is/c++draft/thread#stoptoken.intro-5 | ||
|
||
Modify [atomics.fences paragraph 2 through 4] of ISO/IEC IS 14882 (the C++ Standard) as follows: | ||
> A release fence A synchronizes with an acquire fence B if there exist atomic | ||
> operations X and Y, both operating on some atomic object M, such that A is | ||
> sequenced before X, X modifies M, Y is sequenced before B, and Y reads the | ||
> value written by X or a value written by any side effect in the hypothetical | ||
> release sequence X would head if it were a release operation, | ||
> ***and each operation (A, B, X, and Y) specifies a scope that includes the thread that performed each other operation***. | ||
|
||
> A release fence A synchronizes with an atomic operation B that performs an | ||
> acquire operation on an atomic object M if there exists an atomic operation X | ||
> such that A is sequenced before X, X modifies M, and B reads the value | ||
> written by X or a value written by any side effect in the hypothetical | ||
> release sequence X would head if it were a release operation, | ||
> ***and each operation (A, B, and X) specifies a scope that includes the thread that performed each other operation***. | ||
|
||
> An atomic operation A that is a release operation on an atomic object M | ||
> synchronizes with an acquire fence B if there exists some atomic operation X | ||
> on M such that X is sequenced before B and reads the value written by A or a | ||
> value written by any side effect in the release sequence headed by A, | ||
> ***and each operation (A, B, and X) specifies a scope that includes the thread that performed each other operation***. | ||
|
||
## Example: Message Passing | ||
|
||
The following example passes a message stored to the `x` variable by a thread in block `0` to a thread in block `1` via the flag `f`: | ||
|
||
<table class="display"> | ||
<tr class="header"><td colspan="2" markdown="span" align="center"> | ||
`int x = 0;`<br> | ||
`int f = 0;` | ||
</td></tr> | ||
<tr class="header"> | ||
<td markdown="span" align="center"> | ||
**Thread 0 Block 0** | ||
</td><td markdown="span" align="center"> | ||
**Thread 0 Block 1** | ||
</td> | ||
</tr> | ||
<tr> | ||
<td markdown="span"> | ||
`x = 42;`<br> | ||
`cuda::atomic_ref<int, cuda::thread_scope_device> flag(f);`<br> | ||
`flag.store(1, memory_order_release);` | ||
</td> | ||
<td markdown="span"> | ||
`cuda::atomic_ref<int, cuda::thread_scope_device> flag(f);`<br> | ||
`while(flag.load(memory_order_acquire) != 1);`<br> | ||
`assert(x == 42);` | ||
</td> | ||
</tr> | ||
</table> | ||
|
||
In the following variation of the previous example, two threads concurrently access the `f` object without synchronization, which leads to a **data race**, and exhibits **undefined behavior**: | ||
|
||
<table> | ||
<tr><td colspan="2" markdown="span" align="center"> | ||
`int x = 0;`<br> | ||
`int f = 0;` | ||
</td></tr> | ||
<tr> | ||
<td markdown="span" align="center"> | ||
**Thread 0 Block 0** | ||
</td><td markdown="span" align="center"> | ||
**Thread 0 Block 1** | ||
</td> | ||
</tr> | ||
<tr> | ||
<td markdown="span"> | ||
`x = 42;`<br> | ||
gonzalobg marked this conversation as resolved.
Show resolved
Hide resolved
|
||
`cuda::atomic_ref<int, cuda::thread_scope_block> flag(f);`<br> | ||
`flag.store(1, memory_order_release); // UB: data race` | ||
</td> | ||
<td markdown="span"> | ||
`cuda::atomic_ref<int, cuda::thread_scope_device> flag(f);`<br> | ||
`while(flag.load(memory_order_acquire) != 1); // UB: data race`<br> | ||
`assert(x == 42);` | ||
</td> | ||
</tr> | ||
</table> | ||
|
||
While the memory operations on `f` - the store and the loads - are atomic, the scope of the store operation is "block scope". Since the store is performed by Thread 0 of Block 0, it only includes all other threads of Block 0. However, the thread doing the loads is in Block 1, i.e., it is not in a scope included by the store operation performed in Block 0, causing the store and the load to not be "atomic", and introducing a data-race. | ||
|
||
For more examples see the [PTX memory consistency model litmus tests]. | ||
|
||
[PTX memory consistency model litmus tests]: https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#axioms | ||
[intro.races paragraph 21]: https://eel.is/c++draft/intro.races#21 | ||
[thread.barrier.class paragraph 4]: https://eel.is/c++draft/thread.barrier.class#4 | ||
[thread.latch.class paragraph 2]: https://eel.is/c++draft/thread.latch.class#2 | ||
[thread.sema.cnt paragraph 3]: https://eel.is/c++draft/thread.sema.cnt#3 | ||
[atomics.fences paragraph 2 through 4]: https://eel.is/c++draft/atomics.fences#2 |
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Oops, something went wrong.
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
There was a problem hiding this comment.
Choose a reason for hiding this comment
The reason will be displayed to describe this comment to others. Learn more.
Why separate this part out? Isn't this whole thing logically equivalent to just listing all 5 bullets in a single list?
There was a problem hiding this comment.
Choose a reason for hiding this comment
The reason will be displayed to describe this comment to others. Learn more.
Yes, you are right. The numbered list below says that this "boolean" expression is true: 1 || (2 && (3 || 4 || 5 || 6)).
I got feedback that this was hard to follow when it is all together, so I decided to split it when the parenthesis open.
In rendered markdown it looked ok to me, but if this can be improved I'd like to improve it.
An atomic operation is atomic at the scope it specifies if:
thread_scope_system
, orthread_scope_system
andconcurrentManagedAccess
] is1
, orhostNativeAtomicSupported
] is1
, or1
,2
,4
, or8
bytes on [mapped memory], or