From my understanding, OpenCL context is an abstraction layer. But where does it physically lie (host RAM, GPU memory... or somewhere in the air)?
I just want to understand how it would be possible for the multiple GPUs using the same OpenCL platform can access the memory buffer objects without having to explicitly transferring files in between the host.
The OpenCL cl::Context object, like all Objects of the OpenCL headers, is located in CPU RAM.
The cl::Buffer object itself also resides in CPU RAM, but allocates a region of memory in GPU VRAM and stores a pointer to it to allow host<->device data transfer over PCIe.
Multiple GPUs within a platform can - if supported by extensions - access each other's memory through remote direct memory access (RDMA). If supported, a GPU then knows the pointer to the array in the other GPU's memory.
Related
I am fairly new to OpenCL and though I have understood everything up until now, but I am having trouble understanding how buffer objects work.
I haven't understood where a buffer object is stored. In this StackOverflow question it is stated that:
If you have one device only, probably (99.99%) is going to be in the device. (In rare cases it may be in the host if the device does not have enough memory for the time being)
To me, this means that buffer objects are stored in device memory. However, as is stated in this StackOverflow question, if the flag CL_MEM_ALLOC_HOST_PTR is used in clCreateBuffer, the memory used will most likely be pinned memory. My understanding is that, when memory is pinned it will not be swapped out. This means that pinned memory MUST be located in RAM, not in device memory.
So what is actually happening?
What I would like to know what do the flags:
CL_MEM_USE_HOST_PTR
CL_MEM_COPY_HOST_PTR
CL_MEM_ALLOC_HOST_PTR
imply about the location of buffer.
Thank you
Let's first have a look at the signature of clCreateBuffer:
cl_mem clCreateBuffer(
cl_context context,
cl_mem_flags flags,
size_t size,
void *host_ptr,
cl_int *errcode_ret)
There is no argument here that would provide the OpenCL runtime with an exact device to whose memory the buffer shall be put, as a context can have multiple devices. The runtime only knows as soon as we use a buffer object, e.g. read/write from/to it, as those operations need a command queue that is connected to a specific device.
Every memory object an reside in either the host memory or one of the context's device's memories, and the runtime might migrate it as needed. So in general, every memory object, might have a piece of internal host memory within the OpenCL runtime. What the runtime actually does is implementation dependent, so we cannot not make too many assumptions and get no portable guarantees. That means everything about pinning etc. is implementation-dependent, and you can only hope for the best, but avoid patterns that will definitely prevent the use of pinned memory.
Why do we want pinned memory?
Pinned memory means, that the virtual address of our memory page in our process' address space has a fixed translation into a physical memory address of the RAM. This enables DMA (Direct Memory Access) transfers (which operate on physical addresses) between the device memory of a GPU and the CPU memory using PCIe. DMA lowers the CPU load and possibly increases copy speed. So we want the internal host storage of our OpenCL memory objects to be pinned, to increase the performance of data transfers between the internal host storage and the device memory of an OpenCL memory object.
As a basic rule of thumb: if your runtime allocates the host memory, it might be pinned. If you allocate it in your application code, the runtime will pessimistically assume it is not pinned - which usually is a correct assumption.
CL_MEM_USE_HOST_PTR
Allows us to provide memory to the OpenCL implementation for internal host-storage of the object. It does not mean that the memory object will not be migrated into device memory if we call a kernel. As that memory is user-provided, the runtime cannot assume it to be pinned. This might lead to an additional copy between the un-pinned internal host storage and a pinned buffer prior to device transfer, to enable DMA for host-device-transfers.
CL_MEM_ALLOC_HOST_PTR
We tell the runtime to allocate host memory for the object. It could be pinned.
CL_MEM_COPY_HOST_PTR
We provide host memory to copy-initialise our buffer from, not to use it internally. We can also combine it with CL_MEM_ALLOC_HOST_PTR. The runtime will allocate memory for internal host storage. It could be pinned.
Hope that helps.
The specification is (deliberately?) vague on the topic, leaving a lot of freedom to implementors. So unless an OpenCL implementation you are targeting makes explicit guarantees for the flags, you should treat them as advisory.
First off, CL_MEM_COPY_HOST_PTR actually has nothing to do with allocation, it just means that you would like clCreateBuffer to pre-fill the allocated memory with the contents of the memory at the host_ptr you passed to the call. This is as if you called clCreateBuffer with host_ptr = NULL and without this flag, and then made a blocking clEnqueueWriteBuffer call to write the entire buffer.
Regarding allocation modes:
CL_MEM_USE_HOST_PTR - this means you've pre-allocated some memory, correctly aligned, and would like to use this as backing memory for the buffer. The implementation can still allocate device memory and copy back and forth between your buffer and the allocated memory, if the device does not support directly accessing host memory, or if the driver decides that a shadow copy to VRAM will be more efficient than directly accessing system memory. On implementations that can read directly from system memory though, this is one option for zero-copy buffers.
CL_MEM_ALLOC_HOST_PTR - This is a hint to tell the OpenCL implementation that you're planning to access the buffer from the host side by mapping it into host address space, but unlike CL_MEM_USE_HOST_PTR, you are leaving the allocation itself to the OpenCL implementation. For implementations that support it, this is another option for zero copy buffers: create the buffer, map it to the host, get a host algorithm or I/O to write to the mapped memory, then unmap it and use it in a GPU kernel. Unlike CL_MEM_USE_HOST_PTR, this leaves the door open for using VRAM that can be mapped directly to the CPU's address space (e.g. PCIe BARs).
Default (neither of the above 2): Allocate wherever most convenient for the device. Typically VRAM, and if memory-mapping into host memory is not supported by the device, this typically means that if you map it into host address space, you end up with 2 copies of the buffer, one in VRAM and one in system memory, while the OpenCL implementation internally copies back and forth between the 2.
Note that the implementation may also use any access flags provided ( CL_MEM_HOST_WRITE_ONLY, CL_MEM_HOST_READ_ONLY, CL_MEM_HOST_NO_ACCESS, CL_MEM_WRITE_ONLY, CL_MEM_READ_ONLY, and CL_MEM_READ_WRITE) to influence the decision where to allocate memory.
Finally, regarding "pinned" memory: many modern systems have an IOMMU, and when this is active, system memory access from devices can cause IOMMU page faults, so the host memory technically doesn't even need to be resident. In any case, the OpenCL implementation is typically deeply integrated with a kernel-level device driver, which can typically pin system memory ranges (exclude them from paging) on demand. So if using CL_MEM_USE_HOST_PTR you just need to make sure you provide appropriately aligned memory, and the implementation will take care of pinning for you.
I have a piece of code in which I use clCreateBuffer with the CL_MEM_ALLOC_HOST_PTR flag and I realised that this allocates memory from the device. Is that correct and I'm missing something from the standard?
CL_MEM_ALLOC_HOST_PTR: This flag specifies that the application wants the OpenCL implementation to allocate memory from host accessible memory.
Personally I understood that that buffer should be a host-side buffer that, later on, can be mapped using clEnqueueMapBuffer.
Follows some info about the device I'm using:
Device: Tesla K40c
Hardware version: OpenCL 1.2 CUDA
Software version: 352.63
OpenCL C version: OpenCL C 1.2
It is described as
OpenCL implementations are allowed to cache the buffer contents
pointed to by host_ptr in device memory. This cached copy can be used
when kernels are executed on a device.
in
https://www.khronos.org/registry/OpenCL/sdk/1.0/docs/man/xhtml/clCreateBuffer.html
The description is for CL_MEM_USE_HOST_PTR but it is only different by its allocator from CL_MEM_ALLOC_HOST_PTR. USE uses host-given pointer, ALLOC uses opencl implementation's own allocators return value.
The caching is not doable for some integrated-gpu types so its not always true.
The key phrase from the spec is host accessible:
This flag specifies that the application wants the OpenCL implementation to allocate memory from host accessible memory.
It doesn't say it'll be allocated in host memory: it says it'll be accessible by the host.
This includes any memory that can be mapped into CPU-visible memory addresses. Typically some, if not all VRAM in a discrete graphics device will be available through a PCI memory range exposed in one of the BARs - these get mapped into the CPU's physical memory address space by firmware or the OS. They can be used similarly to system memory in page tables and thus made available to user processes by mapping them to virtual memory addresses.
The spec even goes on to mention this possibility, at least in combination with another flag:
CL_MEM_COPY_HOST_PTR can be used with CL_MEM_ALLOC_HOST_PTR to initialize the contents of the cl_mem object allocated using host-accessible (e.g. PCIe) memory.
If you definitely want to use system memory for a buffer (may be a good choice if GPU access to it is sparse or less frequent than CPU acccess), allocate it yourself and wrap it in a buffer with CL_MEM_USE_HOST_PTR. (Which may still end up being cached in VRAM, depending on the implementation.)
I'd like to know what exactly happens when we assign a memory object to a context in OpenCL.
Does the runtime copies the data to all of the devices which are associated with the context?
I'd be thankful if you help me understand this issue :-)
Generally and typically the copy happens when the runtime handles the clEnqueueWriteBuffer / clEnqueueReadBuffer commands.
However, if you created the memory object using certain combinations of flags, the runtime can choose to copy the memory sooner than that (like right after creation) or later (like on-demand before running a kernel or even on-demand as it needs it). Vendor documentation often indicates if they take special advantage of any of these flags.
A couple of the "interesting" variations:
Shared memory (Intel Ingrated Graphics GPUs, AMD APUs, and CPU drivers): You can allocate a buffer and never copy it to the device because the device can access host memory.
On-demand paging: Some discrete GPUs can copy buffer memory over PCIe as it is read or written by a kernel.
Those are both "advanced" usage of OpenCL buffers. You should probably start with "regular" buffers and work your way up if they don't do what you need.
This post describes the extra flags fairly well.
Consider the following code which creates a buffer memory object from an array of double's of size size:
coef_mem = clCreateBuffer(context, CL_MEM_READ_WRITE | CL_MEM_COPY_HOST_PTR, (sizeof(double) * size), arr, &err);
Consider it is passed as an arg for a kernel. There are 2 possibilities depending on the device on which the kernel is running:
The device is same as host device
The device is other than host device
Here are my questions for both the possibilities:
At what step is the memory transferred to the device from the host?
How do I measure the time required for transferring the memory from host to device?
How do I measure the time required for transferring the memory from
device's global memory to private memory?
Is the memory still transferred if the device is same as host device?
Will the time required to transfer from host to device be greater
than the time required for transferring from device's global memory
to private memory?
At what step is the memory transferred to the device from the host?
The only guarantee you have is that the data will be on the device by the time the kernel begins execution. The OpenCL specification deliberately doesn't mandate when these data transfers should happen, in order to allow different OpenCL implementations to make decisions that are suitable for their own hardware. If you only have a single device in the context, the transfer could be performed as soon as you create the buffer. In my experience, these transfers usually happen when the kernel is enqueued (or soon after), because that is when the implementation knows that it really needs the buffer on a particular device. But it really is completely up to the implementation.
How do I measure the time required for transferring the memory from host to device?
Use a profiler, which usually shows when these transfers happen and how long they take. If you transfer the data with clEnqueueWriteBuffer instead, you could use the OpenCL event profiling system.
How do I measure the time required for transferring the memory from device's global memory to private memory?
Again, use a profiler. Most profilers will have a metric for the achieved bandwidth when reading from global memory, or something similar. It's not really an explicit transfer from global to private memory though.
Is the memory still transferred if the device is same as host device?
With CL_MEM_COPY_HOST_PTR, yes. If you don't want a transfer to happen, use CL_MEM_USE_HOST_PTR instead. With unified memory architectures (e.g. integrated GPU), the typical recommendation is to use CL_MEM_ALLOC_HOST_PTR to allocate a device buffer in host-accessible memory (usually pinned), and access it with clEnqueueMapBuffer.
Will the time required to transfer from host to device be greater than the time required for transferring from device's global memory to private memory?
Probably, but this will depend on the architecture, whether you have a unified memory system, and how you actually access the data in kernel (memory access patterns and caches will have a big effect).
Ex: To perform an algorithm on an array, we must use a buffer created with an array.
But with a Intel/AMD CPU, it use the DDR of the system like Global Memory.
Finally, the table is created twice. Is there a way to use the table already in memory without allocating buffer.
You can ask OpenCL to use the original memory area by setting the CL_MEM_USE_HOST_PTR flag when creating the buffer.
If the kernel is run on a CPU no memory copy will occur.
If run on a GPU a copy might occur if the OpenCL runtime thinks it's more suitable.
The CPU has access to the machine's memory, but doesn't have access to the GPU's memory. Likewise, the GPU has access to its own memory, but not to the host machine's. This is the reason that you must transfer the information between those - they are two completely separate memory spaces.
As opposed to gpgpu, with OpenCL the kernel might run on the CPU itself, so no need to copy the buffer; but OpenCL still always requires you to explicitly transfer the memory, it's just that its implementation will ignore it if it's running on the host computer.