nccl/ext-profiler/README.md

NCCL Profiler Plugin Documentation

This page describes the NCCL Profiler plugin API and how to implement a profiler plugin for NCCL.

Overview

To allow NCCL to better integrate with DL frameworks, NCCL v2.23 introduced a profiler plugin interface. Any NCCL user can write profiler plugins to extract performance data from NCCL and use it for debugging and analysis.

Similarly to other plugins (e.g., network plugin), the profiler plugins come as a shared library called libnccl-profiler.so. That shared library contains one or more implementations of the NCCL PROFILER API, in the form of versioned structs, filled with pointers to all required functions.

Plugin architecture

Plugin name and supporting multiple profiler plugins

When NCCL is initialized, it will look for a libnccl-profiler.so library and dynamically load it, then look for symbols inside the library.

The NCCL_PROFILER_PLUGIN environment variable allows multiple plugins to coexist. If set, NCCL will look for a library with a name of libnccl-profiler-${NCCL_PROFILER_PLUGIN}.so. It is therefore advised to name the library following that pattern, with a symlink pointing libnccl-profiler.so to libnccl-profiler-${NCCL_PROFILER_PLUGIN}.so. That way, if there are multiple plugins in the path, setting NCCL_PROFILER_PLUGIN will allow users to select the right plugin. Alternatively, the user can also set NCCL_PROFILER_PLUGIN to the pathname of the libnccl-profiler.so library.

Struct versioning

Once a library is found, NCCL will look for a symbol named ncclProfiler_vX, with X increasing over time. The versioning ensures that the plugin and the NCCL core are compatible.

Plugins are encouraged to provide multiple of those symbols, implementing multiple versions of the NCCL PROFILER API, so that the same plugin can be compiled and support a wide range of NCCL versions.

Conversely, and to ease transition, NCCL can choose to support different plugin versions, looking for the latest ncclProfiler struct version, but also looking for older ones so that older plugins would still work.

Headers management

To help users build plugins effortlessly, plugins should copy the ncclProfiler_vX definitions they support to their internal includes. An example is shown in ext-profiler/example where we keep all headers in the nccl/ directory and provide thin layers to implement old version on top of newer ones.

The nccl/ directory is populated with profiler_vX.h files extracting all relevant definitions from old API versions. It also provides error codes in err.h.

API (v2)

Below is the main ncclProfiler_v2 struct. Each function is explained in later sections.

typedef struct {
  const char* name;

  // init - initialize the profiler plugin
  // Input
  //  - context        : opaque profiler context object for separating profiler behavior across comms
  // Output
  //  - eActivationMask: bitmask of active events set by the plugin
  ncclResult_t (*init)(void** context, int* eActivationMask);

  // startEvent - initialize and start a new event for the supplied event descriptor inside the eventset
  // Input
  //  - context: opaque profiler context object
  //  - eDescr : pointer to ncclProfilerEventDescr_t object
  // Output
  //  - eHandle: return event handle for supplied event descriptor object
  ncclResult_t (*startEvent)(void* context, void** eHandle, ncclProfilerEventDescr_v2_t* eDescr);

  // stopEvent - stop/finalize an event inside and event set
  // Input
  //  - eHandle: handle to event object
  ncclResult_t (*stopEvent)(void* eHandle);

  // recordEventState - record event state transitions and event attribute updates
  // Input
  //  - eHandle   : handle to event object created through startEvent
  //  - eStateArgs: optional argument used to capture event attribute updates associated with the state transition
  //  - eState    : event state transition
  ncclResult_t (*recordEventState)(void* eHandle, ncclProfilerEventState_v2_t eState, ncclProfilerEventStateArgs_v2_t* eStateArgs);

  // finalize - finalize the profiler plugin
  // Input
  //  - context: opaque profiler context object
  ncclResult_t (*finalize)(void* context);
} ncclProfiler_v2_t;

Error codes

As rule of thumb, profiler generated errors should not be propagated to NCCL and alter its normal functioning. Nevertheless, the profiler interface returns NCCL error codes, in case any need for them arises in the future. For now, any profiler interface call should only return ncclSuccess. The only exception is init that can return an error so that NCCL can disable the plugin.

Operation overview

NCCL will call the init function first for every new communicator that is initialized. The profiler returns an opaque context handle that is used to isolate profiler instances across communicators. Similarly, NCCL will call finalize to destroy the profiler context, thus freeing resources.

The NCCL core code is instrumented with calls to startEvent, stopEvent and recordEventState. These are used to start, stop and update events in the profiler, respectively.

API Functions

Initialization

name

The name field should point to a character string with the name of the profiler plugin. This will be used for all logging, especially when NCCL_DEBUG=INFO is set.

init

As soon as NCCL finds the plugin and the correct ncclProfiler symbol, it calls its init function. This allows the plugin to initialize its internal context, used during profiling of NCCL events. If the init function does not return ncclSuccess, NCCL disables the plugin.

finalize

When the profiler is no longer needed, a call to finalize destroys the profiler context and frees up resources.

Profiling

startEvent

When NCCL needs to start profiling a new event it calls startEvent. startEvent takes the profiler context, previously created by init, an event descriptor of type ncclProfilerEventDescr_t and returns an opaque profiler event handle that can be passed to other profiler functions, as discussed later in the document.

The event descriptor contains all the event metadata. Every event type has its own descriptor. Below is the ncclProfilerEventDescr_t struct.

typedef struct {
  uint8_t type;             // event type (e.g., ncclProfileGroup, ncclProfileColl, ...)
  void* parentObj;          // pointer to parent event used to expose the event hierarchy to the profiler
  int rank;                 // rank that generated the event
  union {
    struct {                // collective events metadata
      const char* name;     // string containing name of the communicator
      uint64_t commHash;    // unique hash/id for the communicator
      uint64_t seqNumber;   // sequence number of this collective operation in the communicator
      const char* func;     // string containing name of the collective
      void const* sendBuff; // address of send buffer
      void* recvBuff;       // address of recv buffer
      size_t count;         // data count
      int root;             // root rank
      const char* datatype; // string containing the name of the datatype
      size_t trafficBytes;  // number of transfer bytes
      uint8_t nMaxChannels; // max number of channels for this collective
      uint8_t nWarps;       // number of GPU warps for this collective
      const char* algo;     // string containing name of the algorithm for this collective
      const char* proto;    // string containing name of the protocol for this collective
    } coll;

    struct {                // point-to-point events metadata
      const char* name;
      uint64_t commHash;
      const char* func;
      void* buff;
      const char* datatype;
      size_t count;
      int peer;             // peer rank for this point-to-point
    } p2p;

    struct {                // proxyOp events metadata
      pid_t pid;            // process id that generated the associated `ncclProxyOp` object
      uint8_t channelId;    // id of the channel used by the associated `ncclProxyOp` object
      int peer;             // peer rank
      int nSteps;           // number of network transfers/steps required by the `ncclProxyOp`
      int chunkSize;        // chunk size for this `ncclProxyOp`
      int isSend;           // set to 1 for sends and 0 for recvs
    } proxyOp;

    struct {                // proxyStep events metadata
      int step;             // individual step in `ncclProxyOp`
    } proxyStep;
  };
} ncclProfilerEventDescr_v2_t;

NCCL defines the following events: ncclProfileGroup, ncclProfileColl, ncclProfileP2p, ncclProfileProxyOp, ncclProfileProxyStep, and ncclProfileProxyCtrl.

stopEvent

stopEvent takes the event handle returned by startEvent to stop the event. After the event has been stopped the handle can no longer be used with other profiler calls. Using the event handle after eventStop is undefined behavior.

recordEventState

Some events can only be started and stopped. For example, ncclProfileGroup, ncclProfileColl, ncclProfileP2p, cannot be updated through calls to recordEventState.

ncclProfileProxyOp, ncclProfileProxyStep and ncclProfileProxyCtrl can be updated through calls to recordEventState.

The state of proxy generated events can be updated, along with event attributes, using recordEventState. These events can go through several states during their lifecycle. The list of supported states for the proxy-defined events is reported below.

typedef enum {
  // ncclProfileProxyOp event states
  ncclProfilerProxyOpSendPosted,        // state marks the posting of send buffer to GPU for given network transfer/step
  ncclProfilerProxyOpSendRemFifoWait,   // state marks the waiting of CTS credits from peer rank
  ncclProfilerProxyOpSendTransmitted,   // state marks the sending of network transfer/step to peer rank
  ncclProfilerProxyOpSendDone,          // state marks the ending  of network transfer/step
  ncclProfilerProxyOpRecvPosted,        // state marks the posting of recv to network for given network transfer/step
  ncclProfilerProxyOpRecvReceived,      // state marks the recving of network transfer/step from peer rank
  ncclProfilerProxyOpRecvTransmitted,   // state marks the ending  of the network transfer/step
  ncclProfilerProxyOpRecvDone,          // state marks the consuming of data from GPU

  // ncclProfileProxyStep event states
  ncclProfilerProxyStepSendGPUWait,     // state marks the waiting of send data from GPU for given network transfer/step
  ncclProfilerProxyStepSendWait,        // state marks the waiting of send data from network for given network transfer/step
  ncclProfilerProxyStepRecvWait,        // state marks the waiting of recv data from network for given network transfer/step
  ncclProfilerProxyStepRecvFlushWait,   // state marks the waiting of recv data flush to GPU for given network transfer/step
  ncclProfilerProxyStepRecvGPUWait,     // state marks the waiting of recv data consumption from GPU for given network transfer/step

  // ncclProfileProxyCtrl event states
  ncclProfilerProxyCtrlIdle,            // state marks proxy progress thread idle
  ncclProfilerProxyCtrlActive,          // state marks proxy progress thread active
  ncclProfilerProxyCtrlSleep,           // state marks proxy progress thread sleeping
  ncclProfilerProxyCtrlWakeup,          // state marks proxy progress thread waking up
  ncclProfilerProxyCtrlAppend,          // state marks append of new network work item begin
  ncclProfilerProxyCtrlAppendEnd,       // state marks append of new network work item end
} ncclProfilerEventState_v2_t;

ncclProfileProxyOp events are generated by the proxy progress thread while it is processing network requests for the GPU kernel. ProxyOp events are generated for every active channel and provide a summary of the activity of the proxy progress thread for that channel.

ncclProfileProxyStep events are generated by the proxy progress thread while it is processing network requests for the GPU kernel. ProxyStep events describe individual network transfer in the channel. Thus, they provide a more fine-grained view w.r.t. ProxyOp events.

ncclProfileProxyCtrl events are generated by the proxy progress thread while it is not processing network requests for the GPU kernel. This includes everything else that the proxy thread might be doing, including appending new ncclProxyOp objects to the list of work elements to process.

State transitions for the events described can also come with event attribute updates. For this reason the profiler defines the ncclProfilerEventStateArgs_t struct, reported below.

typedef union {
  struct {                // attributes to update for ncclProfileProxyOp events
    size_t transSize;     // data transferred thus far
    int steps;            // network transfer/steps processed thus far
  } proxyOp;

  struct {                // attributes to update for ncclProfileProxyCtrl
    int appendedProxyOps; // number of appended proxy ops thus far
  } proxyCtrl;
} ncclProfilerEventStateArgs_v2_t;

The example profiler in ext-profiler/example contains details on how to capture and use the events above.

Event hierarchy

NCCL core events (reported above) are organized into a hierarchy as reported below:

Group event
   |
   +- Collective event
   |  |
   |  +- ProxyOp event
   |     |
   |     +- ProxyStep event
   |
   +- Point-to-point event
      |
      +- ProxyOp event
         |
         +- ProxyStep event

ProxyCtrl event

Profiler instrumentation and logging

Profiling of collective and p2p operations

The NCCL code is instrumented with profiler callbacks at different levels to capture start/stop of groups, collective and point-to-point operations, as well as proxy progress activity. Due to the asynchronous nature of NCCL operations, events associated to collective and point-to-point operations are not easy to delimit precisely. For example, without both proxy and/or kernel activity it is impossible for the profiler to figure out when a collective operation completes. Therefore, stopEvent for collectives simply indicates to the profiler that the collective has been enqueued. The profiler can leverage proxy event information, if these are enabled, to estimate when the collective ends. In this case, the profiler can look at the stopEvent call of the last ncclProfileProxyOp event to mark the completion of the associated collective event. This can be achieved by reference counting the collective event and letting calls to startEvent and stopEvent increment and decrement the reference counter, respectively.

PXN

PXN causes some proxy operations to be processed in a remote proxy thread that differs from the one that generated the operation. When this happens, the event hierarchy reported above breaks. Because the profiler can use the hierarchy information, provided by NCCL in the event descriptor, to dereference the parent event during startEvent, the remote proxy thread must be in the same address space of the proxy thread originating the operation. To avoid the profiler instance in the remote proxy address space to dereference a pointer from another address space the event descriptor includes the PID of the originator. The profiler plugin needs to check that the originator PID matches the local PID before dereferencing the parent event.