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Add blog post on CUDA.jl v5.4
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title = "CUDA.jl 5.4: Memory management mayhem"
author = "Tim Besard"
external = true
abstract = """
CUDA.jl 5.4 is an important release that comes with many memory-management related changes,
improving the performance and usability of several key features."""
+++

{{abstract}}

Before anything else, let's get the breaking changes out of the way. CUDA.jl v5.4 only
bumps the minor version, so it should be compatible with existing codebases. However, there
are a couple of API changes that, although covered by appropriate deprecation warnings,
applications should be updated to:

* The `CUDA.Mem` submodule has been removed. All identifiers have been moved to the parent
`CUDA` submodule, with a couple being renamed in the process:
- `Mem.Device` and `Mem.DeviceBuffer` have been renamed to `CUDA.DeviceMemory` (the same
applies to `Mem.Host` and `Mem.Unified`);
- enums from the `Mem` submodule have gained a `MEM` suffix, e.g., `Mem.ATTACH_GLOBAL` has
been renamed to `CUDA.MEM_ATTACH_GLOBAL`;
- `Mem.set!` has been renamed to `CUDA.memset`;
- `Mem.info()` has been renamed to `CUDA.memory_info()`;
* `CUDA.memory_status()` has been renamed to `CUDA.pool_status()`;
- `CUDA.available_memory()` has been renamed to `CUDA.free_memory()`.

The meat of this release is in the memory management improvements detailed below. These
changes can have a significant impact of the performance of your application, so it's
recommended to thoroughly test your application after upgrading!


## Eager garbage collection

Julia is a garbage collected language, which means that (GPU) allocations can fail because
garbage has piled up, necessitating a collection cycle. Previous versions of CUDA.jl handled
this at the allocation site, detecting out-of-memory errors and triggering the GC. This was
not ideal, as it could lead to significant pauses and a bloated memory usage.

To improve this, **CUDA.jl v5.4 more accurately keeps track of memory usage, and uses that
information to trigger the GC early at appropriate times**, e.g., when waiting for a kernel
to finish. This should lead to more predictable performance, both by distributing the cost
of garbage collection over time and by potentially masking it behind other operations.

For example, the following toy model implemented with Flux.jl allocates a ton of memory:

```julia
using CUDA, Flux
using MLUtils: DataLoader

n_obs = 300_000
n_feature = 1000
X = rand(n_feature, n_obs)
y = rand(1, n_obs)
train_data = DataLoader((X, y) |< gpu; batchsize = 2048, shuffle=false)

model = Dense(n_feature, >) |< gpu
loss(m, _x, _y) = Flux.Losses.mse(m(_x), _>)
opt_state = Flux.setup(Flux.Adam(), model)
Flux.train!(loss, model, train_data, opt_state)
for epoch in 1:100
Flux.train!(loss, model, train_data, opt_state)
end
```

Without eager garbage collection, this leads to expensive pauses while freeing a large
amount of memory at every epoch. We can simulate this by artificially limiting the memory
available to the GPU, while also disabling the new eager garbage collection feature by
setting the `JULIA_CUDA_GC_EARLY` environment variable to `false` (this is a temporary knob
that will be removed in the future, but may be useful now for evaluating the new feature):

```text
❯ JULIA_CUDA_GC_EARLY=false JULIA_CUDA_HARD_MEMORY_LIMIT=4GiB \
julia --project train.jl
...
[ Info: Epoch 90 train time 0.031s
retry_reclaim: freed 2.865 GiB
[ Info: Epoch 91 train time 0.031s
[ Info: Epoch 92 train time 0.027s
retry_reclaim: freed 2.865 GiB
[ Info: Epoch 93 train time 0.03s
retry_reclaim: freed 2.873 GiB
[ Info: Epoch 94 train time 0.031s
retry_reclaim: freed 2.873 GiB
[ Info: Epoch 95 train time 0.03s
retry_reclaim: freed 2.873 GiB
[ Info: Epoch 96 train time 0.031s
[ Info: Epoch 97 train time 0.027s
retry_reclaim: freed 2.873 GiB
[ Info: Epoch 98 train time 0.031s
retry_reclaim: freed 2.865 GiB
[ Info: Epoch 99 train time 0.031s
retry_reclaim: freed 2.865 GiB
[ Info: Epoch 100 train time 0.031s
[ Info: Total time 4.307s
```

With eager garbage collection enabled, more frequent but less costly pauses result in
significantly improved performance:

```text
❯ JULIA_CUDA_GC_EARLY=true JULIA_CUDA_HARD_MEMORY_LIMIT=4GiB \
julia --project wip.jl
...
[ Info: Epoch 90 train time 0.031s
maybe_collect: collected 1.8 GiB
maybe_collect: collected 1.8 GiB
[ Info: Epoch 91 train time 0.033s
maybe_collect: collected 1.8 GiB
[ Info: Epoch 92 train time 0.031s
maybe_collect: collected 1.8 GiB
[ Info: Epoch 93 train time 0.031s
maybe_collect: collected 1.8 GiB
[ Info: Epoch 94 train time 0.03s
maybe_collect: collected 1.8 GiB
[ Info: Epoch 95 train time 0.03s
maybe_collect: collected 1.8 GiB
maybe_collect: collected 1.8 GiB
[ Info: Epoch 96 train time 0.033s
maybe_collect: collected 1.8 GiB
[ Info: Epoch 97 train time 0.03s
maybe_collect: collected 1.8 GiB
[ Info: Epoch 98 train time 0.03s
maybe_collect: collected 1.8 GiB
[ Info: Epoch 99 train time 0.03s
maybe_collect: collected 1.8 GiB
[ Info: Epoch 100 train time 0.03s
[ Info: Total time 3.76s
```

Eager garbage collection is driven by a heuristic that considers the current memory
pressure, how much memory was freed during previous collections, and how much time that
took. It is possible that the current implementation is not optimal, so if you encounter
performance issues, please file an issue.


## Tracked memory allocations

When working with multiple GPUs, it is important to differentiate between the device that
memory was allocated on, and the device used to execute code. Practically, this meant that
users of CUDA.jl had to manually remember that allocating and using `CuArray` objects
(typically) needed to happen with the same device active. The same is true for streams,
which are used to order operations executing on a single GPU.

To improve this, **CUDA.jl now keeps track of the device that owns the memory, and the
stream last used to access it, enabling the package to "do the right thing" when using that
memory** in kernels or with library functionality. This does **not** mean that CUDA.jl will
automatically switch the active device: We want to keep the user in control of that, as it
often makes sense to access memory from another device, if your system supports it.

Let's break down what the implications are of this change.

**1. Using multiple GPUs**

If you have multiple GPUs, it may be possible that direct P2P access between devices is
possible (e.g., using NVLink, or just over PCIe). In this case, CUDA.jl will now
automatically configure the system to allow such access, making it possible to seamlessly
use memory allocated on one device in kernels executing on a different device:

```julia
julia> # Allocate memory on device 0
device!(0)
CuDevice(0): Tesla V100-PCIE-16GB
julia> a = CuArray([1]);

julia> # Use on device 1
device!(1)
CuDevice(1): Tesla V100S-PCIE-32GB
julia> a .+ 1;
```

If P2P access between devices is not possible, CUDA.jl will now raise an error instead of
throwing an illegal memory access error as it did before:

```julia
julia> # Use on incompatible device 2
device!(2)
CuDevice(5): NVIDIA GeForce GTX 1080 Ti
julia> a .+ 1
ERROR: cannot take the GPU address of inaccessible device memory.

You are trying to use memory from GPU 0 on GPU 2.
P2P access between these devices is not possible;
either switch to GPU 0 by calling `CUDA.device!(0)`,
or copy the data to an array allocated on device 2.
```

As the error message suggests, you can always copy memory between devices using the
`copyto!` function. In this case, CUDA.jl will fall back to staging the copy on the host
when P2P access is not possible.

**2. Using multiple streams**

Streams are used to order operations executing on a single GPU. In CUDA.jl, every Julia task
has its own stream, making it very easy to group independent operations together, and make
it possible for the GPU to potentially overlap execution of these operations.

Before CUDA.jl v5.4, users had to be careful about synchronizing data used in multiple
tasks. It was recommended, for example, to end every data-producing task with an explicit
call to `synchronize()`, or alternatively make sure to `device_synchronize()` at the start
of a data-consuming task. Now that CUDA.jl keeps track of the stream used to last access
memory, it can automatically synchronize streams when needed:

```julia
# Allocate some data
a = CUDA.zeros(4096, 4096)
b = CUDA.zeros(4096, 4096)
#synchronize() # No longer needed

# Perform work on a task
t = @async begin
a * b
#synchronize() # No longer needed
end

# Fetch the results
c = fetch(t)
```

**3. Using capturing APIs**

All of the above is implemented by piggybacking on the function that converts memory objects
to pointers, in the assumption that this will be the final operation before the memory is
used. This is generally true, with one important exception: APIs that capture memory. For
example, when recording an operation using the CUDA graph APIs, a memory address may be
captured and used later without CUDA.jl being aware of it.

CUDA.jl accounts for this by detecting conversions during stream capture, however, some APIs
may not covered yet. If you encounter issues with capturing APIs, let us know, and keep
using additional synchronization calls to ensure correctness.


## Unified memory iteration

Unified memory is a feature of CUDA that allows memory to be accessed from both the CPU and
the GPU. We have now greatly **improved the performance of using unified memory with CPU
code that iterates over elements** of a `CuArray`. Although this is typically unwanted,
triggering the dreaded "scalar indexing" error when accessing device memory in such a way,
it can be useful when incrementaly porting code to the GPU.

Concretely, accessing elements of a unified `CuArray` on the CPU is much faster now:

```julia-repl
julia> # Reference
a = [1];
julia> @btime $a[];
1.959 ns (0 allocations: 0 bytes)
julia> b = cu(a; unified=true);
julia> # Before
@btime $b[]
2.617 μs (0 allocations: 0 bytes);
julia> # After
@btime $b[];
4.140 ns (0 allocations: 0 bytes)
```

Notice the different unit! This has a massive impact on real-life performance, for example,
as demonstrated by calling `foldl` which does not have a GPU-optimized implementation:

```julia-repl
julia> a = cu(rand(1024, 1024); unified=true);
julia> # Before
@b foldl(+, a)
4.210 s (9 allocs: 208 bytes, without a warmup)
julia> # After
@b foldl(+, a)
3.107 ms (9 allocs: 208 bytes)
```

For completeness, doing this with regular device memory triggers a scalar indexing error:

```julia-repl
julia> a = cu(rand(1024, 1024));
julia> foldl(+, a)
ERROR: Scalar indexing is disallowed.
```

These changes should make it easier to port applications to the GPU by incrementally moving
parts of the codebase to the GPU without having to worry about the performance of accessing
memory from the CPU. The only requirement is to use unified memory, e.g., by calling `cu`
with `unified=true`, or setting the CUDA.jl preference `default_memory` to use unified
memory by default. However, as unified memory comes with a slight cost, and results in
synchronous allocation behavior, it is still recommended to switch back to regular device
memory when your application has been fully ported to the GPU.


## Other changes

To keep this post from becoming even longer, a quick rundown of other changes:

* [@wsmoses](https://github.com/wsmoses) introduced initial support for automatic
differentiation of heterogeneous host/device code using Enzyme.jl. Before, you would have
to differentiate through host and device code separately, and manually set up rules for
crossing the host/device boundary. Now, you can differentiate through entire applications
with ease;
* `CUDA.@profile` now [automatically detects external
profilers](https://github.com/JuliaGPU/CUDA.jl/pull/2339), so it should not be required to
specify `external=true` anymore when running under NSight;
* [Exception output has been improved](https://github.com/JuliaGPU/CUDA.jl/pull/2342), only
reporting a single error message instead of generating output on each thread, and better
forwarding the exception type;
* Cached handles from libraries [will now be
freed](https://github.com/JuliaGPU/CUDA.jl/pull/2352) when under memory pressure;
* Tegra devices [are now supported](https://github.com/JuliaGPU/CUDA.jl/pull/2374) by our
artifacts, obviating the use of a local toolkit;
* Support for [CUDA 12.5](https://github.com/JuliaGPU/CUDA.jl/pull/2392) has been added, as
well as [initial support for Julia 1.12](https://github.com/JuliaGPU/CUDA.jl/pull/2390).

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