Multi-Process Support

Support for Multi-Process kernel execution is added in the 2018.2 release as a preview feature and supported as Early Access feature in 2018.3.

Requirements

Multiple processes can share access to the same device provided each process uses the same xclbin. Attempting to load different xclbins via different processes concurrently will result in only one process being successfull in loading its xclbin. The other processes will get error code -EBUSY or -EPERM.

Usage

Processes share access to all device resources; as of 2018.3, there is no support for exclusive access to resources by any process.

If two or more processes execute the same kernel, then these processes will acquire the kernel’s compute units per the xocl kernel driver compute unit scheduler, which is first-come first-serve. All processes have the same priority in XRT.

To enable multi-process support, add the following entry to sdaccel.ini in the same directory as all the executable(s):

[Runtime]
multiprocess=true

Known problems

Debug and Profile will not function correctly when multi-process has been enabled. Emulation flow does not have support for multi-process yet.

Implementation Details For Curious

Since 2018.3 downloading an xclbin to the device does not guarantee a lock on the device for the downloading process. Application is required to create explicit context for each Compute Unit (CU) it wants to use. OCL applications automatically handle context creation without user needing to change any code. XRT native applications should create context on a CU with xclOpenContext() API which requires xclbin UUID and CU index. This information can be obtained from the xclbin binary. xclOpenContext() increments the xclbin UUID which prevents that xclbin from being unloaded. A corresponding xclCloseContext() releases the reference count. xclbins can only be swapped if the reference count is zero. If an application dies or exits without explicitly releasing the contexts it had opened before the driver would automatically release the stale contexts.

The following diagram shows a possibility with 7 processes concurrently using a device. The processes in green are successful but processes in red fail at diffrent stages with appropriate error codes. Processes P0, P1, P2, P3, P4 and P6 are each trying to use xclbin with UUID_X, process P5 is attempting to use UUID_Y. Processes P0, P1, P3, P4, and P6 are trying to use CU_0 in UUID_X. Process P2 is trying to use CU_1 in UUID_X and Process P5 is trying to use CU_0 in UUID_Y. The diagram shows timeline view with all 7 processes running concurrently.