Putting the VM in TVM: The Relay Virtual Machine

Relay, a new program representation, has enabled the representation and optimization of a great breadth of machine learning programs. Unfortunately, by supporting a more expressive set of programs, we have introduced several new execution challenges.

Relay’s interpreter can execute the full language but has notable limitations that make it unsuited for production deployments. It is structured as an inefficient interpreter that performs AST traversal to execute the program. This approach is conceptually simple but inefficient, as the AST traversal heavily relies on indirection.

There are further challenges in compiling dynamic code, such as dynamic scheduling and allocation, fully dynamic tensor shapes, and control flow. The interpreter offers simple solutions for these, but none is sufficiently compelling or optimized.

The second execution mechanism is the existing graph runtime. In order to target Relay programs to this, we compile a small subset of them to the old graph format and execute them on the runtime. Graph runtime provides a fast execution experience but only for a very limited subset of Relay programs.

An alternative but not-standard approach is Relay’s ahead-of-time compiler, which compiles a Relay program into a shared library containing an ahead- of-time implementation. The ahead-of-time compiler provides compelling performance but is difficult to extend and instrument, which can only be done by modifying the code generation and optimization mechanisms.

The Relay virtual machine is intended to be a framework that balances these competing approaches, providing a dynamic execution environment which can be extended, instrumented, and integrated with other approaches like ahead-of-time compilation via a flexible extension mechanism.

The virtual machine is designed to strike a balance between performance and flexibility when deploying and executing Relay programs, without giving up the benefits of TVM.

Virtual machine (VM) design is a well-studied area in programming languages and systems, and there have been various virtual machine designs for both full-fledged and embedded programing languages. Previous language VM designs have been heavily tailored to the execution profile of traditional programs. Traditional programs manipulate small scalar values and consist of a large number of low-level instructions. The sheer quantity of instructions requires instruction execution and dispatch to be extremely efficient. In the context of machine learning we manipulate primarily tensor values, using a (relatively) low number of high level instructions. ML programs’ cost centers are expensive operator invocations, such as GEMM or convolution, over a large input. Due to the execution profile exhibited by ML programs, micro-optimizations present in scalar VMs are dramatically less important.

TVM has provided strong support for vision models, but we want to grow to support a wider variety of models. The graph runtime is able to utilize the fully static nature of the input graphs to perform aggressive optimization such as fully static allocation, and optimal memory reuse. When we introduce models which make use of control flow, recursion, dynamic shapes, and dynamic allocation, we must change how execution works. A virtual machine for Relay is a natural choice.

The rest of this document provides a high-level overview of the Relay virtual machine design and its instruction set.

Design

The VM’s design is focused on simplicity without sacrificing performance. In order to accomplish this we have focused on designing a tensor VM rather than a scalar VM.

In the tensor VM setting, we optimize for cheap “allocation” of objects (by trying to avoid real allocation), reuse of static fragments, and the ability to do dynamic shape (i.e jagged tensors).

Instruction Set

The choices of an instruction set and instruction representation are the most critical design decisions for a VM. The current representation of the instructions is a tagged union containing the op-code and the data payload. An important design decision is the level of abstraction of the instructions (RISC vs. CISC) and how they take their data (fixed-width instruction encoding vs. variable-length encoding). The current version is closer to CISC, with complex instructions like AllocTensor, and is variable-length due to the inclusion of the shape as part of the instruction. The current instruction set is very high-level and corresponds roughly to high-level operations in Relay.

Ret

Arguments:

RegName dst
RegName result

Returns the object in register result to caller’s register dst.

InvokePacked

Arguments:

size_t packed_index
size_t arity
size_t output_size
RegName* packed_args

Invoke the packed function denoted by packed_index. The arity and output_size are used to inform the VM how many inputs and outputs to expect. packed_args stores the list of argument registers.

AllocTensor

Arguments:

RegName dst
RegName shape_register
size_t ndim
DLDataType dtype

Allocate a tensor value of the appropriate shape (stored in shape_register) and dtype. The result is saved to register dst.

AllocDatatype

Arguments:

RegName dst
size_t tag
size_t num_fields
RegName* datatype_fields

Allocate a data type with the tag tag using the num_fields entries from registers datatype_fields. The result is saved to register dst.

AllocClosure

Arguments:

RegName dst
size_t clo_index
size_t num_freevar
RegName* free_vars;

Allocate a closure with the VMFunction at clo_index as its code, and the num_freevar entries from registers in free_vars. The result is saved to register dst.

GetField

Arguments:

RegName dst
RegName object
size_t field_index

Get the field value with index field_index from object. And saves the result to register dst.

If

Arguments:

RegName test
RegName target
size_t true_offset
size_t false_offset

Check if the object at register test is equal to target. If equal, relative jump by true_offset, else relative jump by false_offset.

GetTagi

Arguments:

RegName object
RegName dst

Get the object tag for Datatype object in register object. And saves the reult to register dst.

Fatal

Fail the virtual machine execution.

Goto

Arguments:

size_t pc_offset

Relative unconditional jump by pc_offset.

Invoke

Arguments:

size_t func_index

Invoke function at func_index, consumes the number of arguments contained in the VMFunction’s arity field.

InvokeClosure

Arguments:

RegName closure
size_t num_closure_args
RegName* closure_args

Invokes closure, consuming the number of arguments declared in the closure’s VMFunction.

LoadConst

Arguments:

RegName dst
size_t const_index

Load the constant at const_index from the constant pool. The result is saved to register dst.

LoadConsti

Arguments:

size_t val
RegName dst

Load the constant integer val to register dst. The result is a 0-rank tensor.

Object Representation

We use a simple object representation that uses shared pointers and tagging. There is a huge space of possible object representations trade-offs, but we believe micro-optimizing this code has little to no effect on the end-to-end performance.

struct ObjectCell {
  ObjectTag tag;
  ...
};

struct Object {
  std::shared_ptr<ObjectCell> ptr;
  ...
}

See include/tvm/runtime/vm.h for more details.

Currently, we support 3 types of objects: tensors, data types, and closures.

VMObject VMTensor(const tvm::runtime::NDArray& data);
VMObject VMDatatype(size_t tag, const std::vector<VMObject>& fields);
VMObject VMClosure(size_t func_index, std::vector<VMObject> free_vars);

Stack and State

The Relay VM maintains a stack frame, which contains information about how to resume the previous call. Registers are allocated in a continuous space (virtual register file) for each function.

We keep track of a set of Relay functions we have called, a pointer into its bytecode, an offset into the byte code (known as the program counter).

struct VirtualMachine {
  ...
  std::vector<VMFrame> frames;
  ...
  // Current function.
  size_t func_index;
  // Pointer into the current function's instructions.
  const Instruction* code;
  // Current program counter relative to the code pointer.
  size_t pc;
  ...
};

Dispatch Loop

A critical piece of a VM is the dispatch loop. The dispatch loop usually dominates the execution time of a virtual machine, but we have experimentally found this not to be the case for Relay. We have just implemented a simple switch/goto dispatch loop which dispatches based on instruction op code.

This loop is implemented by VirtualMachine::Run().

VM Compiler

An important part of this infrastructure is a compiler from Relay’s full IR into a sequence of bytecode. The VM compiler transforms a tvm::relay::Module into a tvm::relay::vm::VirtualMachine. The virtual machine contains a set of compiled functions, the compiled functions are contained in tvm::relay::vm::Function. The functions contain metadata about the the function as well as its compiled bytecode. For full definitions of the data structures see vm.h.

Optimizations

There are quite a few optimizations required by the VM compiler.

We have implemented them in the old pass style, but plan to port them to the new pass manager (#2546) before merging.

Optimizations marked with TODO are not implemented yet.

  • A-Normal Form

  • Lambda Lift (see src/relay/vm/lambda_lift.cc)

  • Inline Primitives (see src/relay/vm/inline_primitives.cc)

  • Inliner (see src/relay/pass/inliner.cc)

  • Constant Pool Layout (see src/relay/backend/vm/compiler.cc)

  • ADT Tag Allocation (see src/relay/backend/vm/compiler.cc)

  • Tail Call Optimization (TODO)

  • Liveness Analysis (TODO)

Serialization

A final and yet-to-be-implemented part of the VM design is serialization. The accompanying PR will introduce both the bytecode and its serialization, as well as VM-level serialization. The design premise is that a VM can be efficiently stored to disk and resumed at a later time. This would also allow us to efficiently schedule many models on to a single machine in order to obtain good utilization.

Unresolved Questions

How do we handle dynamic shapes?

TODO

How can we modify the VM to support JIT compilation of certain code paths?

In the code generation space there are still many tradeoffs to be analyzed and the VM is designed to be very flexible so we can modify it for future experiments.

How do we support heterogenous execution?

Heterogenous execution should work out of the box assuming we have annotated the appropriate device copies. In order to do this properly we need to run the device annotation and copying passes.