Intel MIC架构下COI框架介绍
开始介绍之前先写一下历史背景，为了最大限度地提高计算速度，单一地提高一个核的主频以提高计算速率的方法已经不再适用。所以向量机、超标量计算机等纷纷出现，并行计算也再度成为了一个热门的方向。现有的并行计算架构主要有两个：GPGPU（通用GPU）以及Intel的MIC（Many Integrated Core）架构。通用GPU加速主要是利用GPU本身具有多线程的特性，将计算密集型任务迁移到GPU上并将计算任务划分到多个线程内同时进行计算，再将计算结果传回已达到提高计算速率的效果。主要的用到的技术就是CUDA。但是CUDA有一个很大的缺点就是：要编写CUDA的代码工作量很大，通常情况下需要将原有的在CPU下可以运行的代码做大量的改动。而这一点正是Intel MIC架构的优势，通常情况下可以在CPU上直接运行的代码也可以在MIC卡上直接运行。如果想了解更多MIC的相关知识可以持续关注我的博客，我将在接下来对MIC进行更详细的介绍。
MIC编程主要有native模式、对称模式以及offload模式。native模式就是程序只在MIC卡上运行，并没有将CPU的运行效能全部运行起来；对称模式就是CPU和MIC卡运行的程序完全相同，可以看成是对等的节点；而offload模式则是以CPU上运行程序为主，但是将一些计算密集型的任务offload到MIC上进行运算。从代码的角度来看，在MIC上运行的并不是一套完整的代码，而只是一些代码片段。但是使用过offload模式进行大规模数值模式程序编写的人应该都有体会，offload的方式有这么两个缺点：1、数据传输会是一个瓶颈，很多时候不是运算不够快而是数据的传输跟不上；2、offload模式通常只适用于一些较为扁平的数据结构的操作。博主手头的程序都是大量使用了面向对象特性的程序，offload模式使用起来很麻烦，对于程序的结构也不能很好的维护。而针对这些问题，本着提高传输效率以及增强编码人员自由度的目的，Intel推出了COI（Coprocessor Offload Infrastructure）。
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COI基本概念
COI是MIC架构下的分载模式的一个库，mic虽然提供了通过简单的编译制导语句（#pragma offload target）的方式来将部fen代码和数据分载到Xeon Phi上进行计算。但这样的方式太过简单，且自由度较小。但如果使用COI，用户可以获取更大的自由度，包括控制CPU和MIC的同步，控制MIC卡上程序的创建和退出；起止端的异步操作；起止端的数据缓冲。为开发更加灵活的MIC程序提供了便利。
利用COI编写的程序在编译的时候就会生成两个可执行文件。一个在CPU上执行，一个在MIC上执行。启动程序只需要调用CPU端的可执行文件即可。系统就会在需要的时候把MIC端可执行文件和所需要的库发送到MIC端并执行MIC端的程序。接下来开始详细介绍COI。
术语说明：因为COI可以实现CPU到MIC的分载，同时也可以实现MIC到CPU的分载，所以在下文中使用source和sink来表示起止端。
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基本概念
Enumeration：COIEngine，COISysinfo
列举出硬件的信息：如MIC卡的数量，线程数，核数，cache等等。示例代码如下：
COIENGINE engine;
COIFUNCTION func[1];
const char* SINK_NAME = "coi_simple_sink_mic";
// Make sure there is an Intel(r) Xeon Phi(tm) device available
//
CHECK_RESULT(
COIEngineGetCount(COI_ISA_MIC, &num_engines));
printf("%u engines available\n", num_engines);
// If there isn't at least one engine, there is something wrong
//
if (num_engines < 1)
{
printf("ERROR: Need at least 1 engine\n");
return -1;
}
Process Management:COIProcess
COIProcess是指source端在sink端创建的一个进程在source端创建的一个进程在source端的一个句柄。source端负责该进程的创建，开启以及销毁等工作。其主要的作用有：
- 抽象sink端的进程的运行
- 提供开启和停止远程进程的各种API已经load动态链接库
- 提供在远端查询函数并执行函数的功能
具体示例代码如下：
COIRESULT result = COI_ERROR;
COIPROCESS proc;
COIENGINE engine;
result = COIProcessCreateFromFile(
engine, // The engine to create the process on.
SINK_NAME, // The local path to the sink side binary to launch.
0, NULL, // argc and argv for the sink process.
false, NULL, // Environment variables to set for the sink process.
true, NULL, // Enable the proxy but don't specify a proxy root path.
0, // The amount of memory to pre-allocate
// and register for use with COIBUFFERs.
NULL, // Path to search for dependencies
&proc // The resulting process handle.
);
if (result != COI_SUCCESS)
{
printf("COIProcessCreateFromFile result %s\n",
COIResultGetName(result));
return -1;
}
printf("Sink process created, press enter to destroy it.\n");
getchar();
// Destroy the process
//
result = COIProcessDestroy(
proc, // Process handle to be destroyed
-1, // Wait indefinitely until main() (on sink side) returns
false, // Don't force to exit. Let it finish executing
// functions enqueued and exit gracefully
&sink_return, // Don't care about the exit result.
&exit_reason
);
if (result != COI_SUCCESS)
{
printf("COIProcessDestroy result %s\n", COIResultGetName(result));
return -1;
}
执行流：COIPipeline
COIPipeline类似于RPC（Remote Procedure Call）的机制，可以讲一系列指令序列插入到COIPipeline中，这些指令可以顺序地在sink端执行，这里的指令序列主要是要在远程调用的函数序列。其主要有以下几个重要的性质：
- 在COIPipeline中插入的函数会在sink端按序执行。
- COIPipeline就是一种远程调用的机制。因为可以在远端调用完整函数，所以有了比单一offload更多的自由度。
- COIPipeline在插入函数时除了插入该函数需要的参数外，还可以传递一块buffer
- 从source到sink端的数据传输使用的是SCIF
示例代码如下：（source端）
CHECK_RESULT(
COIProcessCreateFromFile(
engine, // The engine to create the process on.
SINK_NAME, // The local path to the sink side binary to launch.
0, NULL, // argc and argv for the sink process.
false, NULL, // Environment variables to set for the sink
// process.
true, NULL, // Enable the proxy but don't specify a proxy root
// path.
0, // The amount of memory to pre-allocate
// and register for use with COIBUFFERs.
NULL, // Path to search for dependencies
&proc // The resulting process handle.
));
printf("Created sink process %s\n", SINK_NAME);
// Pipeline:
// After a sink side process is created, multiple pipelines can be created
// to that process. Pipelines are queues where functions(represented by
// COIFUNCTION) to be executed on sink side can be enqueued.
// The following call creates a pipeline associated with process created
// earlier.
CHECK_RESULT(
COIPipelineCreate(
proc, // Process to associate the pipeline with
NULL, // Do not set any sink thread affinity for the pipeline
0, // Use the default stack size for the pipeline thread
&pipeline // Handle to the new pipeline
));
printf("Created pipeline\n");
// Retrieve handle to function belonging to sink side process
const char* func_name = "Foo";
CHECK_RESULT(
COIProcessGetFunctionHandles(
proc, // Process to query for the function
1, // The number of functions to look up
&func_name, // The name of the function to look up
func // A handle to the function
));
printf("Got handle to sink function %s\n", func_name);
const char *misc_data = "Hello COI";
int strlength = (int)strlen(misc_data) + 1;
// Enough to hold the return value
char* return_value = (char*) malloc(strlength);
if (return_value == NULL) {
fprintf(stderr, "failed to allocate return value\n");
return -1;
}
// Enqueue the function for execution
// Pass in misc_data and return value pointer to run function
// Get an event to wait on until the run function completion
CHECK_RESULT(
COIPipelineRunFunction(
pipeline, func[0], // Pipeline handle and function handle
0, NULL, NULL, // Buffers and access flags
0, NULL, // Input dependencies
misc_data, strlength, // Misc Data to pass to the function
return_value, strlength, // Return values that will be passed back
&completion_event // Event to signal when it completes
));
printf("Called sink function %s(\"%s\" [%d bytes])\n",
func_name, misc_data, strlength);
// Now wait indefinitely for the function to complete
CHECK_RESULT(
COIEventWait(
1, // Number of events to wait for
&completion_event, // Event handles
-1, // Wait indefinitely
true, // Wait for all events
NULL, NULL // Number of events signaled
// and their indices
));
printf("Function returned \"%s\"\n", return_value);
sink端：
// main is automatically called whenever the source creates a process.
// However, once main exits, the process that was created exits.
int main(int argc, char** argv)
{
UNUSED_ATTR COIRESULT result;
UNREFERENCED_PARAM (argc);
UNREFERENCED_PARAM (argv);
// Functions enqueued on the sink side will not start executing until
// you call COIPipelineStartExecutingRunFunctions(). This call is to
// synchronize any initialization required on the sink side
result = COIPipelineStartExecutingRunFunctions();
assert(result == COI_SUCCESS);
// This call will wait until COIProcessDestroy() gets called on the source
// side. If COIProcessDestroy is called without force flag set, this call
// will make sure all the functions enqueued are executed and does all
// clean up required to exit gracefully.
COIProcessWaitForShutdown();
return 0;
}
// Prototype of run function that can be retrieved on the source side.
// Copies misc data to return pointer.
COINATIVELIBEXPORT
void Foo (uint32_t in_BufferCount,
void** in_ppBufferPointers,
uint64_t* in_pBufferLengths,
void* in_pMiscData,
uint16_t in_MiscDataLength,
void* in_pReturnValue,
uint16_t in_ReturnValueLength)
{
UNREFERENCED_PARAM(in_BufferCount);
UNREFERENCED_PARAM(in_ppBufferPointers);
UNREFERENCED_PARAM(in_pBufferLengths);
UNREFERENCED_PARAM(in_pMiscData);
UNREFERENCED_PARAM(in_MiscDataLength);
assert (in_MiscDataLength>=in_ReturnValueLength);
if(in_pMiscData!=NULL && in_pReturnValue!=NULL)
{
memcpy(in_pReturnValue, in_pMiscData, in_ReturnValueLength);
}
}
source端的COIEventWait暂时可以不关心，接下来会详细介绍。通过上述代码可以发现，在source端，将对应的函数插入COIPipeline队列之后，该函数将会在sink端执行。但前提是该函数必须在sink端的代码中被申明。不过被插入到在COIPipeline中的函数并不是会立即在sink端执行。首先是要在sink端调用COIPipelineStartExecutingRunFunctions（）之后才能被执行。另外如果被插入的执行函数有相关的buffer，那么函数也必须在buffer可用的时候才能执行，此外通过COIEvent也可以来控制函数的执行。更多的内容会在后面讲到。
COIBuffer
COIBuffer用于管理在远程设备上的数据。
Buffer可以通过传入执行函数给远程端点，也可以直接使用读写API。
COI的runtime（运行时环境）来管理buffer
- Buffer用于在source和sink端的数据传输，buffer可以使source和sink端的读写异步，隐藏掉通信延迟。
- Buffer可能是位于device也可能是位于host端的物理内存中。
- Buffer实际是利用SCIF的内存窗口实现的
- 数据的传输实际利用的是readfrom/writeto API
- map操作可以访问到buffer对应的区域，而不需要将其数据移到host上
COIBuffer的操作比较复杂，更多的细节会在后续的博客中涉及到。
COIEvent
COIEvent可以用来创建依赖关系，从而使source和sink端进行同步，可以通过创建事件然后等待事件来达到同步，有点类似于MPI中的MPI_Barrier函数。一个函数的执行可以有一个先导COIEvent，只有当该event被消费之后，位于COIPipeline中的对应函数才能被执行（该函数此时必须位于COIPipeline队列的首位）。同时，当该函数被执行结束之后，也可以产生一个事件，远端的程序可以通过等待对应的时候来进行同步操作。具体看示例代码：
source端：
// This tutorial demonstrates:
// 1. Registering a User event
// 2. Pass the event to the sink side
// 3. Signaling the event from the sink side and using it
// to synchronize on the source side.
// It first enqueues a run function with a registered user event (which
// is not signaled) as input dependency. Then a second function is enqueued
// on a different pipeline that signals the user event.
// User events are one shot events. Once they are signaled they
// can't be signaled again. You have to register them again to enable
// signaling.
...
//Create two pipelines
CHECK_RESULT(
COIPipelineCreate(
proc, // Process to associate the pipeline with
NULL, // Do not set any sink thread affinity for the pipeline
0, // Use the default stack size for the pipeline thread
&pipeline[0] // Handle to the new pipeline
));
CHECK_RESULT(
COIPipelineCreate(
proc, // Process to associate the pipeline with
NULL, // Do not set any sink thread affinity for the pipeline
0, // Use the default stack size for the pipeline thread
&pipeline[1] // Handle to the new pipeline
));
printf("Created sink process %s and two pipelines\n", SINK_NAME);
// Retrieve handle to functions belonging to sink side process
const char* names[] = {"Return2","SignalUserEvent"};
CHECK_RESULT(
COIProcessGetFunctionHandles(
proc, // Process to query for the function
2, // The number of functions to query
names, // The name of the function
func // A handle to the function
));
printf("Got handles to functions %s and %s\n", names[0], names[1]);
uint64_t return_value = 0;
COIEVENT user_event;
// Register this event so that it can be signaled
CHECK_RESULT(
COIEventRegisterUserEvent(&user_event));
printf("Registered user event\n");
// Now pass this registered user event as an input dependency to the run
// function. This run function will not be started until the user event
// is signaled.
CHECK_RESULT(
COIPipelineRunFunction(
pipeline[0], func[0], // Pipeline handle and function
// handle
0, NULL, NULL, // Buffers and access flags to
// pass to the function
1, &user_event, // Input dependencies
NULL, 0, // Misc data to pass to
// the function
&return_value, sizeof(return_value), // Return value passed back
// from the function
&completion_event // Event to signal when
// the function is complete
));
printf("Enqueued sink function %s depending on user event\n", names[0]);
// Sleep for 2 sec which is enough for run function to be started on sink
// side
#ifndef _WIN32
sleep(2);
#else
Sleep(2000);
#endif
// Now try waiting for the completion_event. It should return
// COI_TIME_OUT_REACHED (as the event isn't signaled)
if(COIEventWait(1, &completion_event, 0, true, NULL, NULL) !=
COI_TIME_OUT_REACHED)
{
printf("Error: Did not execute as expected\n");
return -1;
}
printf("As expected, event wait timed out\n");
// User event handles can be passed down to run function as misc
// data (or via buffers) and on sink side can be type-casted back to
// COIEVENT object to signal them.
CHECK_RESULT(
COIPipelineRunFunction(
pipeline[1], func[1], // Pipeline handle and function
// handle
0, NULL, NULL, // Buffers and access flags to
// pass to the function
0, NULL, // Input dependencies
&user_event, sizeof(user_event), // Misc data to pass to
// the function
NULL,0, // Return value passed back
// from the function
NULL // Event to signal when
// the function is complete
));
printf("Enqueued sink function %s passing user event as misc arg\n",
names[1]);
// Wait until the user event is signaled
CHECK_RESULT(
COIEventWait(
1, // Number of events to wait for
&user_event, // Event handles
-1, // Wait indefinitely
true, // Wait for all events
NULL, NULL // Number of events signaled
// and their indices
));
printf("Successfully waited for user event (signaled sink side)\n");
// Once a user event is signaled the first run function will be able to
// proceed. Wait until the function finishes (-1 wait indefinite)
CHECK_RESULT(
COIEventWait(
1, // Number of events to wait for
&completion_event, // Event handles
-1, // Wait indefinitely
true, // Wait for all events
NULL, NULL // Number of events signaled
// and their indices
));
printf("Sink function %s completed since user event signaled\n", names[0]);
// Unregister the event to cleanup
CHECK_RESULT(
COIEventUnregisterUserEvent(user_event));
MIC端：
// main is automatically called whenever the source creates a process.
// However, once main exits, the process that was created exits.
int main(int argc, char** argv)
{
UNUSED_ATTR COIRESULT result;
UNREFERENCED_PARAM (argc);
UNREFERENCED_PARAM (argv);
// Functions enqueued on the sink side will not start executing until
// you call COIPipelineStartExecutingRunFunctions()
// This call is to synchronize any initialization required on the sink side
result = COIPipelineStartExecutingRunFunctions();
assert(result == COI_SUCCESS);
// This call will wait until COIProcessDestroy() gets called on the source
// side. If COIProcessDestroy is called without force flag set, this call
// will make sure all the functions enqueued are executed and does all
// clean up required to exit gracefully.
COIProcessWaitForShutdown();
return 0;
}
// Prototype of run functions that can be retrieved on the sink side
// This Function just returns 2
COINATIVELIBEXPORT
void Return2(uint32_t in_BufferCount,
void** in_ppBufferPointers,
uint64_t* in_pBufferLengths,
void* in_pMiscData,
uint16_t in_MiscDataLength,
void* in_pReturnValue,
uint16_t in_ReturnValueLength)
{
UNREFERENCED_PARAM(in_BufferCount);
UNREFERENCED_PARAM(in_ppBufferPointers);
UNREFERENCED_PARAM(in_pBufferLengths);
UNREFERENCED_PARAM(in_pMiscData);
UNREFERENCED_PARAM(in_MiscDataLength);
if (sizeof(uint64_t) <= in_ReturnValueLength)
{
*(uint64_t*)(in_pReturnValue) = 2;
}
}
//Assumes a user_event is passed as Misc_data and signals it
COINATIVELIBEXPORT
void SignalUserEvent(uint32_t in_BufferCount,
void** in_ppBufferPointers,
uint64_t* in_pBufferLengths,
void* in_pMiscData,
uint16_t in_MiscDataLength,
void* in_pReturnValue,
uint16_t in_ReturnValueLength)
{
UNREFERENCED_PARAM(in_BufferCount);
UNREFERENCED_PARAM(in_ppBufferPointers);
UNREFERENCED_PARAM(in_pBufferLengths);
UNREFERENCED_PARAM(in_MiscDataLength);
UNREFERENCED_PARAM(in_pReturnValue);
UNREFERENCED_PARAM(in_ReturnValueLength);
COIEVENT user_event;
assert(in_pMiscData != NULL);
assert(in_MiscDataLength >= sizeof(user_event));
memcpy(&user_event, in_pMiscData, sizeof(user_event));
COIEventSignalUserEvent(user_event);
}
可以看到，虽然在source端的代码中func[0]被先于func1[1]插入COIPipeline中，但是func[0]存在一个输入依赖——user_event,而user_event被当作func[1]的参数传输到sink端，而只有当user_event在sink端被消费（singnaled）之后，func[0]方可以执行。