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 a newbie to Pin, and basically, I would like to use a Pintool to initialize a memory region, which can be read/write by the user process later during run time.
I would like to make the memory region on heap, as the data structure I want to initialize is relatively large. After reading the Pin manual, I am aware of Pin safecopy which can be used for memory copy.
However, I don't know how to allocate memory on the heap, which, should be accessible by the attached client process.
Am I clear enough? Could anyone give me some help on it? Thank you!
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. 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.
I have a buffer created with CL_MEM_USE_HOST_PTR | CL_MEM_READ_WRITE flags. I have used this in one kernel and then downloaded (queue.enqueueReadBuffer(...)) the data back to the host memory set when the buffer was created. I have modified these data on CPU and now I'd like to use them in another kernel.
When I have uploaded (queue.enqueueWriteBuffer) the data manually using non-blocking write and then enqueued kernel with this buffer as argument, it returned the CL_OUT_OF_RESOURCES error. Blocking write was just fine.
Why did this happen? I thought that the blocking/non-blocking version only controls if I can work with the memory on CPU after the enqueueWriteBuffer call returns, with in-order queue there should be no difference for the kernel.
Second question is whether I have to upload it manually at all - does the CL_MEM_USE_HOST_PTR mean that the data has to be uploaded from host to device in for every time some kernel uses the buffer as argument? As I have to download the data manually when I require them, has the above mentioned flag any pros?
Thanks
I can't be sure of the specific problem for your CL_OUT_OF_RESOURCES error. This error seems to be raised as kind of a catch-all for problems in the system, so the actual error you're getting might be caused by something else in your program (maybe the kernel).
In regards to using the CL_MEM_USE_HOST_PTR, you still still have to manually upload the data. The OpenCL specification states:
This flag is valid only if host_ptr is not NULL. If specified, it
indicates that the application wants the OpenCL implementation to use
memory referenced by host_ptr as the storage bits for the memory
object. 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.
For some devices the data will be cached on the device memory. In order to sync your data you would have to use some clEnqueueReadBuffer / clEnqueueWriteBuffer or clEnqueueMapBuffer / clEnqueueUnmapBuffer. For discrete CPU+GPU combinations (i.e. seperate GPU card), I'm not sure what benefit there would be to doing CL_MEM_USE_HOST_PTR, since the data will be cached anyway.
Upon reading the specification, there might be some performance benefit for using clEnqueueMapBuffer / clEnqueueUnmapBuffer instead of clEnqueueReadBuffer / clEnqueueWriteBuffer, but I haven't tested this for any real devices.
Best of luck!