Consider the following. In a context there exist two buffers allocated in device memory, buffer A and buffer B. One buffer contains a pointer to something in another buffer. Assuming the host will propery keep the buffers alive between kernel invocations, is it safe to have this setup? Particularly is it guaranted that the implementation will not move buffers around thus invalidating the pointers?
It seems not, atleast if the context has more than one device.
Сlause 5.4.4 Migrating Memory Objects of the specification among other things states:
Typically, memory objects are implicitly migrated to a device for
which enqueued commands, using the memory object, are targeted.
And there seems to be no way to prohibit this migration, and no information on what happens if there is only one device in a context.
Alas it appears that the only way to keep addressing consistent is to allocate one huge buffer and do manual memory-management of it's contents storing all addresses as offsets from the beginning of the buffer.
OpenCL 1.2 does not support pointers to pointers in buffers, but it seems that OpenCL 2.0 will allow this. See the slide titled "SVM: Shared Virtual Memory" in this presentation.
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 am trying to understand when to use CL_MEM_USE_HOST_PTR on a CPU-GPU Soc by Intel.
Reading this guide I came across:
If your application uses a specific memory management algorithm, or if
you want to wrap existing native application memory allocations, you
can pass a pointer to clCreateBuffer along with the
CL_MEM_USE_HOST_PTR flag.
Can someone explain with an example what is the meaning of: specific memory management algorithm, and wrap existing native application memory allocations.
CL_MEM_USE_HOST_PTR flag means, that memory for OpenCL memory object will not be allocated on Device side, but will be used from memory, allocated on Host side. Though, memory content may be cached (this is opaque to user).
Imagine, that you have complicated library, which has it's own sophisticated memory allocation mechanisms (e. g. with reference counting), etc. It's not that easy (usually - impossible) to allocate OpenCL memory objects "by hand", as they must have same lifetime to objects, allocated by library, (possibly - same alignment), etc.
In that case much easier way it to use CL_MEM_USE_HOST_PTR flag, when creating OpenCL memory objects. All objects handling will be done under-the-hood. This way can save you a lot of pain especially when you're working with big projects, implemented on plain C, in which memory objects processing is always tricky.
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.
I am writing a multi-GPU parallel algorithm. One of the issues I am facing is to find out what would happen if I push one cl_mem to multiple devices, and let them run the same kernel at the same time. The kernel will make change to the memory passed to device.
It is very time consuming to code and debug OpenCL code. So before I start doing it I want to take some advices from fellow Stackoverflow users - I want to know the consequence of doing such thing, in both of below scenarios (e.g will there be any exception raised during execution? Are data synchronized? When CL_MEM_COPY_HOST_PTR is used is the same region of memory pointed by this cl_mem get properly copied to device? etc.):
The memory is created with CL_MEM_COPY_HOST_PTR
The memory is created with CL_MEM_USE_HOST_PTR
I don't see anything explicit in the OpenCL specifications that guarantees that data will be synchronised across devices. I don't see how the OpenCL implementation would know how to distribute a buffer across multiple devices and how to aggregate those buffers again later.
The approach I've adopted is to create a separate context, read, write and kernel exec queues for each device. I then create separate buffers on each device and enqueue writes/reads to move data to/from the devices. Hence I explicitly handle all of that myself.
I'd like a better solution, but at least the above method works and doesn't rely on anything that is implementation specific.
Appendix A of the OpenCL Specification explains the required synchronization for objects shared between different command queues.
Basically it says you should use OpenCL events and clFlush to synchronize execution between the command queues. The OpenCL implementation will synchronize the contents of the memory objects between the different devices of the OpenCL context. USE/COPY _HOST_PTR does not make any difference, but USE_HOST_PTR will avoid a couple of extra copies of the data in host memory. Use clEnqueueMapBuffer to synchronize bits with the host at the end.
I've a vector on the host and I want to halve it and send to the device. Doing a benchmark shows that CL_MEM_ALLOC_HOST_PTR is faster than CL_MEM_USE_HOST_PTR and much faster than CL_MEM_COPY_HOST_PTR. Also memory analysis on device doesn't show any difference in the buffer size created on device. This differs from the documentation of the mentioned flag on Khronos- clCreateBuffer. Does anyone know what's going on?
The answer by Pompei 2 is incorrect. The specification makes no guarantee as to where the memory is allocated but only how it is allocated. CL_MEM_ALLOC_HOST_PTR makes the clCreateBuffer allocate the host side memory for you. You can then map this into a host pointer using clEnqueueMapBuffer. CL_MEM_USE_HOST_PTR will cause the runtime to scoop up the data you give it into a OpenCL buffer.
Pinned memory is achieved through the use of CL_MEM_ALLOC_HOST_PTR: the runtime is able to allocate the memory as it can.
All this performance is implementation dependant. Reading section 3.1.1 more carefully will show that in one of the calls (with no CL_MEM flag) NVIDIA is able to preallocate a device side buffer whilst the other calls merely get the pinned data mapped into a host pointer ready for writing to the device.
First off and if I understand you correctly, clCreateSubBuffer is probably not what you want, as it creates a sub-buffer from an existing OpenCL buffer object. The documentation you linked also tells us that:
The CL_MEM_USE_HOST_PTR, CL_MEM_ALLOC_HOST_PTR and CL_MEM_COPY_HOST_PTR values cannot be specified in flags but are inherited from the corresponding memory access qualifiers associated with buffer.
You said you have a vector on the host and want to send half of it to the device. For this, I would use a regular buffer of half the vector's size (in bytes) on the device.
Then, with a regular buffer, the performance you see is expected.
CL_MEM_ALLOC_HOST_PTR only allocates memory on the host, which does not incur any transfer at all: it is like doing a malloc and not filling the memory.
CL_MEM_COPY_HOST_PTR will allocate a buffer on the device, most probably the RAM on GPUs, and then copy your whole host buffer over to the device memory.
On GPUs, CL_MEM_USE_HOST_PTR most likely allocates so-called page-locked or pinned memory. This kind of memory is the fastest for host->GPU memory transfer and this is the recommended way to do the copy.
To read how to correctly use pinned memory on NVidia devices, refer to chapter 3.1.1 of NVidia's OpenCL best practices guide. Note that if you use too much pinned memory, performance may drop below a host copied memory.
The reason why pinned memory is faster than copied device memory is well-explained in this SO question aswell as this forum thread it points to.
Pompei2, you says CL_MEM_ALLOC_HOST_PTR and CL_MEM_USE_HOST_PTR allocates memory on the device while OpenCL 1.1 Specification says that with CL_MEM_ALLOC_HOST_PTR or CL_MEM_USE_HOST_PTR specified memory will be allocated (in first case) on or will be used from (in second) host memory? Im newble in OpenCL, but want know where is true?)