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Summary:
See Note [Supervisor deleter] for how SupervisedPtr works.
This design is not the obvious one, but there were a lot of
constraints feeding into it:
- It must support the reallocation usage-pattern, where, given
an existing Storage, we allocate a new region of memory,
copy the existing data to it, and then deallocate the old
region of memory.
- Creation of a deleter for memory MUST avoid dynamic allocations
in the common case. We've done some benchmarking in Caffe2
where dynamic allocation for deleters is ruinously expensive,
and it's really hard to avoid these performance tarpits in
very general function wrappers like std::function or
folly::Function (while benchmarking this, we discovered that
folly::Function's move constructor was way more expensive
than it should be).
- We need to be able to deallocate data that comes from external
sources, e.g., dlpack and numpy tensors. Most notably,
you often cannot deallocate these with merely the void*
data pointer; you need some extra, out-of-band information
(e.g., the managing struct) to deallocate it. Sometimes,
you may even want to resize data living in an external source!
- The "core" allocators need to support being wrapped in a Thrust
allocator, so you need to be implement the following two functions:
char* allocate(size_t);
void deallocate(char*, size_t);
- We need to support tensors which contain non-POD, non-trivially
copyable data; specifically tensors of std::string. This is
an upcoming requirement from Caffe2. It's dirty AF, but
it's really useful.
- It should use C++ standard library types like std::unique_ptr
(which is hugely problematic because std::unique_ptr doesn't
call the deleter when the pointer is null.)
Here is the billing of changes:
- Built-in support for realloc() has been DROPPED ENTIRELY.
Instead, you're expected to allocate and then copy from
the old memory to the new memory if you want to do a
reallocation. This is what you'd generally have expected
to occur; and axing realloc() from the design lets us avoid
some tricky correctness issues with std::realloc(), namely
the fact that we must refuse the realloc if the type of the
elements are not trivially copyeable. If it really matters,
we can add this back, but there really needs to be a good
explanation WHY you need fast resizing reallocations (by in
large, people don't resize their storages, and it should
be acceptable to have a performance degradation when they
do).
- TH_STORAGE_FREEMEM is no more; instead, if you want a
storage which doesn't free its result, you just give it
an empty deleter.
- What we used to call an "allocator" (really, a combined
object for allocating/deleting) has been split into two
concepts, an allocator, and a smart pointer (SupervisedPtr)
which knows how to delete data.
- Unlike previously, where THAllocator/THCDeviceAllocator
could have a per-tensor context storing extra information
(e.g., a pointer to the metadata you need to actually
free the tensor), there is no context in the allocator or
the deleter of the smart pointer; instead, the smart
pointer directly holds an owning reference to the
metadata necessary to free the data. This metadata
is *freshly manufactured* upon every allocation, which
permits us to resize tensors even in the absence of
built-in support for realloc().
- By default, allocators don't support "raw" allocations
and deallocations with raw pointers. This is because
some allocations may return a different context every
time, in which case you need to reconstruct the context
at delete time (because all you got was a void*, not
a unique_ptr that carries the deleter).
- The diff between at::Allocator and THCDeviceAllocator is a
bit larger:
- It used to return a cudaError_t. Now, allocators
are expected to check the error status immediately and throw
an exception if there was an error. It turns out that this
is what was immediately done after all occurrences of
allocate/release, so it wasn't a big deal (although some
subsidiary interfaces had to themselves be converted to
not return cudaError_t).
There is one notable exception to this, and it is how
we handle CUDA OOM: if this occurs, we attempt to return
unused memory to the system and try again. This is now
handled by a catch-all try-catch block. The cost of
catching the exception is probably the least of your worries
if you're about to OOM.
- It used to take the CUDA stream to perform the allocation
on as an argument. However, it turned out that all call
sites, this stream was the stream for the current device.
So we can push this into the allocator (and the choice,
in the future, could be made explicitly by twiddling
thread local state.)
- It held two extra methods, emptyCache and cacheInfo, specifically
for interacting with some state in THCCachingAllocator.
But this "generality" was a lie, since THCCachingAllocator
was the only allocator that actually implemented these
methods, and there is actually a bunch of code in THC
which assumes that it is the caching allocator that is
the underlying allocator for CUDA allocations. So I
folded these two methods into this interface as
THCCachingAllocator_emptyCache and THCCachingAllocator_cacheInfo.
- It held its context directly inside the THCDeviceAllocator
struct. This context has been moved out into whatever
is holding the at::Allocator*.
- The APIs for getting at allocators/deleters is now a little different.
- Previously there were a bunch of static variables you could get
the address of (e.g., &THDefaultAllocator); now there is a
function getTHDefaultAllocator().
- Some "allocators" didn't actually know how to allocate (e.g.,
the IPC "allocator"). These have been deleted; instead, you
can wrap the produced pointers into SupervisedPtr using
an appropriate makeSupervisedPtr() static method.
- Storage sharing was a lot of work to wrangle, but I think I've
tamed the beast.
- THMapAllocator and its "subclasses" have been refactored to
be proper, honest to goodness C++ classes. I used the enum
argument trick to get "named" constructors. We use inheritance
to add refcounting and management (in libshm). What we previously
called the "Context" class (Context has been dropped from the name)
is now the supervisor for the data.
- Sometimes, we need to pull out the file descriptor from a
tensor. Previously, it was pulled out of the allocator context.
Now, we pull it out of the supervisor of the SupervisorPtr,
using the static method fromSupervisedPtr(), which uses the
deleter as the typeid, and refines the type if it matches.
- I renamed the std::function deleter into
InefficientStdFunctionSupervisor, to emphasize the fact that it does
a dynamic allocation to save the std::function deleter.
TODO:
- Windows libshm is in shambles and needs to be fixed.
Perhaps for the future:
- newFromFd is now unconditionally calling cudaPointerGetAttributes
even though this is unnecessary, because we know what the device
is from higher up in the callstack. We can fix this by making
newWithDataAndAllocator also take an explicit device argument.
- Consider statically distinguishing between allocators that
support raw_allocate/raw_deallocate, and those which don't.
The Thrust constraint applies only to the CUDA device allocator;
you never need to allocate CPU memory this way
- Really want to get rid of storage views. Ugh.
Nontrivial bugs I noticed when preparing this patch:
- I forgot to placement-new unique pointers and attempted to
assign them directly on uninitialized memory; very bad! Sam
Gross has encouraged me to replace this with a proper constructor
but I keep putting it off, because once everything goes in
StorageImpl there really will be a proper constructor.
- I rewrote a number of APIs to use newWithDataAndAllocator
instead of newWithAllocator, calling the allocator at the
call site (because they required "allocation context" which
we no longer give to "allocators"). When I did this, I forgot
to insert the multiplication with sizeof(real) to scale from
numels to number of bytes.
- The implementation of swap on storages was missing it for
scalarType and backend. It was benign (because the only case
we call swap is when these are the same), but I fixed it anyway.
- I accidentally returned a nullptr unique_ptr with no deleter,
even though there was a legitimate one. This matters, because
some code still shoves its hands in the deleter context to
get extra metadata about the function.
- I used std::move() on a unique_ptr, and then did a boolean
test on the pointer aftewards (always false!)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/9358
Reviewed By: SsnL
Differential Revision: D8811822
Pulled By: ezyang
fbshipit-source-id: 4befe2d12c3e7fd62bad819ff52b054a9bf47c75
49 lines
1.1 KiB
C++
49 lines
1.1 KiB
C++
#define __STDC_FORMAT_MACROS
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#include "torch/csrc/python_headers.h"
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#ifdef _MSC_VER
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#include <Windows.h>
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#endif
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#include <structmember.h>
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#define THP_HOST_HALF
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#include <stdbool.h>
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#include <TH/TH.h>
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// See Note [TH abstraction violation]
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// - Used to get at the allocator associated with a storage
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#include <TH/THStorage.hpp>
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#include <torch/csrc/finalizer.h>
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#include <libshm.h>
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#include "THP.h"
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#include "copy_utils.h"
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#include "DynamicTypes.h"
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#ifdef USE_CUDA
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#include <THC/THCStorage.hpp>
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#endif
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#include "generic/Storage.cpp"
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#include <TH/THGenerateAllTypes.h>
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#include "generic/Storage.cpp"
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#include <TH/THGenerateHalfType.h>
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// NB: If you ever divest libtorch of USE_CUDA, you'll have to virtualize
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// the CUDA call.
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template<>
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void THPPointer<THStorage>::free() {
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if (ptr) {
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if (ptr->data_ptr.device().is_cpu()) {
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THStorage_free(ptr);
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} else {
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AT_ASSERT(ptr->data_ptr.device().is_cuda());
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#ifdef USE_CUDA
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THCStorage_free(at::globalContext().lazyInitCUDA(), ptr);
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#else
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AT_ERROR("Cannot free THCStorage when not built with CUDA");
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#endif
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}
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}
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}
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