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\section{System Model}
\label{sec:model}
\vspace{-0.25em}
This paper focuses on a system composed of a GPU and a multi-core CPU.
GPU applications use a set of the API supported by the system, and
typically take the following steps:
(i) allocate space to the device memory,
(ii) move data to the allocated space on the device memory,
(iii) launch computation on the GPU,
(iv) move resultant data back to the host memory, and
(v) free the allocated space from the device memory.
%This is the most well-known GPU programming model.
We also assume that the GPU is based on NVIDIA's \textit{Fermi}
architecture~\cite{Fermi}.
The concept of Gdev, however, is not limited to Fermi, but is also
applicable to others if the following model is applicable.
\textbf{Command:}
The GPU is operated by commands.
The commands are architecture-specific.
Each GPU context is assigned a FIFO queue into which the CPU programs
submit GPU commands.
When the GPU dispatches these commands, the corresponding GPU context
can execute.
\textbf{Channel:}
Each GPU context is assigned a hardware channel.
Command dispatching and context execution are managed per channel.
In Fermi architecture, multiple channels cannot run simultaneously when
using the same GPU functional unit, while they can when using different
units.
However, they are allowed to coexist, and the GPU switches the channels
automatically in hardware.
\textbf{Address Space:}
Each GPU context runs in separate virtual address space, which is also
associated with the channel.
The device driver is in charge of setting page tables for the memory
management unit on the GPU.
\begin{comment}
\textbf{I/O Register:}
The GPU provides a bunch of memory-mapped I/O registers per context
visible to the device driver through the (PCI) I/O bus.
The device driver needs to manage these registers to send commands and
set up channels and address space.
\end{comment}
\textbf{Compute Unit:}
The GPU maps threads assigned by programmers to cores on the compute unit.
This thread assignment, however, is not visible to the CPU, which
implies that GPU resource management by the system should be
context-based.
Multiple contexts cannot run on the compute unit together, since
more than channel cannot access the same functional unit simultaneously,
though multiple requests spawned from the same context can be processed.
GPU computation is non-preemptive.
\textbf{DMA Unit:}
There are two types of DMA units for data transmission: (i) synchronous
with the compute unit and (ii) asynchronous.
Only the latter type of DMA units can overlap their operations with the
compute unit.
DMA data transmission is also non-preemptive.
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