[PATCH v5 01/15] trace2: Documentation/technical/api-trace2.txt
From: Jeff Hostetler <jeffhost@microsoft.com>
Created design document for Trace2 feature.
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
Documentation/technical/api-trace2.txt | 1347 ++++++++++++++++++++++++
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+= Trace2 API
+
+The Trace2 API can be used to print debug, performance, and telemetry
+information to stderr or a file. The Trace2 feature is inactive unless
+explicitly enabled by enabling one or more Trace2 Targets.
+
+The Trace2 API is intended to replace the existing (Trace1)
+printf-style tracing provided by the existing `GIT_TRACE` and
+`GIT_TRACE_PERFORMANCE` facilities. During initial implementation,
+Trace2 and Trace1 may operate in parallel.
+
+The Trace2 API defines a set of high-level messages with known fields,
+such as (`start`: `argv`) and (`exit`: {`exit-code`, `elapsed-time`}).
+
+Trace2 instrumentation throughout the Git code base sends Trace2
+messages to the enabled Trace2 Targets. Targets transform these
+messages content into purpose-specific formats and write events to
+their data streams. In this manner, the Trace2 API can drive
+many different types of analysis.
+
+Targets are defined using a VTable allowing easy extension to other
+formats in the future. This might be used to define a binary format,
+for example.
+
+== Trace2 Targets
+
+Trace2 defines the following set of Trace2 Targets.
+Format details are given in a later section.
+
+`GIT_TR2` (NORMAL)::
+
+ a simple printf format like GIT_TRACE.
++
+------------
+$ export GIT_TR2=~/log.normal
+$ git version
+git version 2.20.1.155.g426c96fcdb
+------------
++
+------------
+$ cat ~/log.normal
+12:28:42.620009 common-main.c:38 version 2.20.1.155.g426c96fcdb
+12:28:42.620989 common-main.c:39 start git version
+12:28:42.621101 git.c:432 cmd_name version (version)
+12:28:42.621215 git.c:662 exit elapsed:0.001227 code:0
+12:28:42.621250 trace2/tr2_tgt_normal.c:124 atexit elapsed:0.001265 code:0
+------------
+
+`GIT_TR2_PERF` (PERF)::
+
+ a column-based format to replace GIT_TRACE_PERFORMANCE suitable for
+ development and testing, possibly to complement tools like gprof.
++
+------------
+$ export GIT_TR2_PERF=~/log.perf
+$ git version
+git version 2.20.1.155.g426c96fcdb
+------------
++
+------------
+$ cat ~/log.perf
+12:28:42.620675 common-main.c:38 | d0 | main | version | | | | | 2.20.1.155.g426c96fcdb
+12:28:42.621001 common-main.c:39 | d0 | main | start | | | | | git version
+12:28:42.621111 git.c:432 | d0 | main | cmd_name | | | | | version (version)
+12:28:42.621225 git.c:662 | d0 | main | exit | | 0.001227 | | | code:0
+12:28:42.621259 trace2/tr2_tgt_perf.c:211 | d0 | main | atexit | | 0.001265 | | | code:0
+------------
+
+`GIT_TR2_EVENT` (EVENT)::
+
+ a JSON-based format of event data suitable for telemetry analysis.
++
+------------
+$ export GIT_TR2_EVENT=~/log.event
+$ git version
+git version 2.20.1.155.g426c96fcdb
+------------
++
+------------
+$ cat ~/log.event
+{"event":"version","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.620713","file":"common-main.c","line":38,"evt":"1","exe":"2.20.1.155.g426c96fcdb"}
+{"event":"start","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.621027","file":"common-main.c","line":39,"argv":["git","version"]}
+{"event":"cmd_name","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.621122","file":"git.c","line":432,"name":"version","hierarchy":"version"}
+{"event":"exit","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.621236","file":"git.c","line":662,"t_abs":0.001227,"code":0}
+{"event":"atexit","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.621268","file":"trace2/tr2_tgt_event.c","line":163,"t_abs":0.001265,"code":0}
+------------
+
+== Enabling a Target
+
+A Trace2 Target is enabled when the corresponding environment variable
+(`GIT_TR2`, `GIT_TR2_PERF`, or `GIT_TR2_EVENT`) is set. The following
+values are recognized.
+
+`0`::
+`false`::
+
+ Disables the target.
+
+`1`::
+`true`::
+
+ Enables the target and writes stream to `STDERR`.
+
+`[2-9]`::
+
+ Enables the target and writes to the already opened file descriptor.
+
+`<absolute-pathname>`::
+
+ Enables the target, opens and writes to the file in append mode.
+
+`af_unix:<absolute-pathname>`::
+
+ Enables the target, opens and writes to a Unix Domain Socket
+ (on platforms that support them).
+
+== Trace2 API
+
+All public Trace2 functions and macros are defined in `trace2.h` and
+`trace2.c`. All public symbols are prefixed with `trace2_`.
+
+There are no public Trace2 data structures.
+
+The Trace2 code also defines a set of private functions and data types
+in the `trace2/` directory. These symbols are prefixed with `tr2_`
+and should only be used by functions in `trace2.c`.
+
+== Conventions for Public Functions and Macros
+
+The functions defined by the Trace2 API are declared and documented
+in `trace2.h`. It defines the API functions and wrapper macros for
+Trace2.
+
+Some functions have a `_fl()` suffix to indicate that they take `file`
+and `line-number` arguments.
+
+Some functions have a `_va_fl()` suffix to indicate that they also
+take a `va_list` argument.
+
+Some functions have a `_printf_fl()` suffix to indicate that they also
+take a varargs argument.
+
+There are CPP wrapper macros and ifdefs to hide most of these details.
+See `trace2.h` for more details. The following discussion will only
+describe the simplified forms.
+
+== Public API
+
+All Trace2 API functions send a messsage to all of the active
+Trace2 Targets. This section describes the set of available
+messages.
+
+It helps to divide these functions into groups for discussion
+purposes.
+
+=== Basic Command Messages
+
+These are concerned with the lifetime of the overall git process.
+
+`void trace2_initialize()`::
+
+ Determines if any Trace2 Targets should be enabled and
+ initializes the Trace2 facility. This includes starting the
+ elapsed time clocks and thread local storage (TLS).
++
+This function emits a "version" message containing the version of git
+and the Trace2 protocol.
++
+This function should be called from `main()` as early as possible in
+the life of the process.
+
+`int trace2_is_enabled()`::
+
+ Returns 1 if Trace2 is enabled (at least one target is
+ active).
+
+`void trace2_cmd_start(int argc, const char **argv)`::
+
+ Emits a "start" message containing the process command line
+ arguments.
+
+`int trace2_cmd_exit(int exit_code)`::
+
+ Emits an "exit" message containing the process exit-code and
+ elapsed time.
++
+Returns the exit-code.
+
+`void trace2_cmd_error(const char *fmt, va_list ap)`::
+
+ Emits an "error" message containing a formatted error message.
+
+`void trace2_cmd_path(const char *pathname)`::
+
+ Emits a "cmd_path" message with the full pathname of the
+ current process.
+
+=== Command Detail Messages
+
+These are concerned with describing the specific Git command
+after the command line, config, and environment are inspected.
+
+`void trace2_cmd_name(const char *name)`::
+
+ Emits a "cmd_name" message with the canonical name of the
+ command, for example "status" or "checkout".
+
+`void trace2_cmd_mode(const char *mode)`::
+
+ Emits a "cmd_mode" message with a qualifier name to further
+ describe the current git command.
++
+This message is intended to be used with git commands having multiple
+major modes. For example, a "checkout" command can checkout a new
+branch or it can checkout a single file, so the checkout code could
+emit a cmd_mode message of "branch" or "file".
+
+`void trace2_cmd_alias(const char *alias, const char **argv_expansion)`::
+
+ Emits an "alias" message containing the alias used and the
+ argument expansion.
+
+`void trace2_def_param(const char *parameter, const char *value)`::
+
+ Emits a "def_param" message containing a key/value pair.
++
+This message is intended to report some global aspect of the current
+command, such as a configuration setting or command line switch that
+significantly affects program performance or behavior, such as
+`core.abbrev`, `status.showUntrackedFiles`, or `--no-ahead-behind`.
+
+`void trace2_cmd_list_config()`::
+
+ Emits a "def_param" messages for "important" configuration
+ settings.
++
+The environment variable `GIT_TR2_CONFIG_PARAMS` can be set to a
+list of patterns of important configuration settings, for example:
+`core.*,remote.*.url`. This function will iterate over all config
+settings and emit a "def_param" message for each match.
+
+`void trace2_cmd_set_config(const char *key, const char *value)`::
+
+ Emits a "def_param" message for a specific configuration
+ setting IFF it matches the `GIT_TR2_CONFIG_PARAMS` pattern.
++
+This is used to hook into `git_config_set()` and catch any
+configuration changes and update a value previously reported by
+`trace2_cmd_list_config()`.
+
+`void trace2_def_repo(struct repository *repo)`::
+
+ Registers a repository with the Trace2 layer. Assigns a
+ unique "repo-id" to `repo->trace2_repo_id`.
++
+Emits a "worktree" messages containing the repo-id and the worktree
+pathname.
++
+Region and data messages (described later) may refer to this repo-id.
++
+The main/top-level repository will have repo-id value 1 (aka "r1").
++
+The repo-id field is in anticipation of future in-proc submodule
+repositories.
+
+=== Child Process Messages
+
+These are concerned with the various spawned child processes,
+including shell scripts, git commands, editors, pagers, and hooks.
+
+`void trace2_child_start(struct child_process *cmd)`::
+
+ Emits a "child_start" message containing the "child-id",
+ "child-argv", and "child-classification".
++
+Before calling this, set `cmd->trace2_child_class` to a name
+describing the type of child process, for example "editor".
++
+This function assigns a unique "child-id" to `cmd->trace2_child_id`.
+This field is used later during the "child_exit" message to associate
+it with the "child_start" message.
++
+This function should be called before spawning the child process.
+
+`void trace2_child_exit(struct child_proess *cmd, int child_exit_code)`::
+
+ Emits a "child_exit" message containing the "child-id",
+ the child's elapsed time and exit-code.
++
+The reported elapsed time includes the process creation overhead and
+time spend waiting for it to exit, so it may be slightly longer than
+the time reported by the child itself.
++
+This function should be called after reaping the child process.
+
+`int trace2_exec(const char *exe, const char **argv)`::
+
+ Emits a "exec" message containing the "exec-id" and the
+ argv of the new process.
++
+This function should be called before calling one of the `exec()`
+variants, such as `execvp()`.
++
+This function returns a unique "exec-id". This value is used later
+if the exec() fails and a "exec-result" message is necessary.
+
+`void trace2_exec_result(int exec_id, int error_code)`::
+
+ Emits a "exec_result" message containing the "exec-id"
+ and the error code.
++
+On Unix-based systems, `exec()` does not return if successful.
+This message is used to indicate that the `exec()` failed and
+that the current program is continuing.
+
+=== Git Thread Messages
+
+These messages are concerned with Git thread usage.
+
+`void trace2_thread_start(const char *thread_name)`::
+
+ Emits a "thread_start" message.
++
+The `thread_name` field should be a descriptive name, such as the
+unique name of the thread-proc. A unique "thread-id" will be added
+to the name to uniquely identify thread instances.
++
+Region and data messages (described later) may refer to this thread
+name.
++
+This function must be called by the thread-proc of the new thread
+(so that TLS data is properly initialized) and not by the caller
+of `pthread_create()`.
+
+`void trace2_thread_exit()`::
+
+ Emits a "thread_exit" message containing the thread name
+ and the thread elapsed time.
++
+This function must be called by the thread-proc before it returns
+(so that the coorect TLS data is used and cleaned up. It should
+not be called by the caller of `pthread_join()`.
+
+=== Region and Data Messages
+
+These are concerned with recording performance data
+over regions or spans of code.
+
+`void trace2_region_enter(const char *category, const char *label, const struct repository *repo)`::
+
+`void trace2_region_enter_printf(const char *category, const char *label, const struct repository *repo, const char *fmt, ...)`::
+
+`void trace2_region_enter_printf_va(const char *category, const char *label, const struct repository *repo, const char *fmt, va_list ap)`::
+
+ Emits a thread-relative "region_enter" message with optional
+ printf string.
++
+This function pushes a new region nesting stack level on the current
+thread and starts a clock for the new stack frame.
++
+The `category` field is an arbitrary category name used to classify
+regions by feature area, such as "status" or "index". At this time
+it is only just printed along with the rest of the message. It may
+be used in the future to filter messages.
++
+The `label` field is an arbitrary label used to describe the activity
+being started, such as "read_recursive" or "do_read_index".
++
+The `repo` field, if set, will be used to get the "repo-id", so that
+recursive oerations can be attributed to the correct repository.
+
+`void trace2_region_leave(const char *category, const char *label, const struct repository *repo)`::
+
+`void trace2_region_leave_printf(const char *category, const char *label, const struct repository *repo, const char *fmt, ...)`::
+
+`void trace2_region_leave_printf_va(const char *category, const char *label, const struct repository *repo, const char *fmt, va_list ap)`::
+
+ Emits a thread-relative "region_leave" message with optional
+ printf string.
++
+This function pops the region nesting stack on the current thread
+and reports the elapsed time of the stack frame.
++
+The `category`, `label`, and `repo` fields are the same as above.
+The `category` and `label` do not need to match the correpsonding
+"region_enter" message, but it makes the data stream easier to
+understand.
+
+`void trace2_data_string(const char *category, const struct repository *repo, const char *key, const char * value)`::
+
+`void trace2_data_intmax(const char *category, const struct repository *repo, const char *key, intmax value)`::
+
+`void trace2_data_json(const char *category, const struct repository *repo, const char *key, const struct json_writer *jw)`::
+
+ Emits a region- and thread-relative "data" or "data_json" message.
++
+This is a key/value pair message containing information about the
+current thread, region stack, and repository. This could be used
+to print the number of files in a directory during a multi-threaded
+recursive tree walk.
+
+`void trace2_printf(const char *fmt, ...)`::
+
+`void trace2_printf_va(const char *fmt, va_list ap)`::
+
+ Emits a region- and thread-relative "printf" message.
+
+== Trace2 Target Formats
+
+=== NORMAL Format
+
+NORMAL format is enabled when the `GIT_TR2` environment variable is
+set.
+
+Events are written as lines of the form:
+
+------------
+[<time> SP <filename>:<line> SP+] <event-name> [[SP] <event-message>] LF
+------------
+
+`<event-name>`::
+
+ is the event name.
+
+`<event-message>`::
+ is a free-form printf message intended for human consumption.
++
+Note that this may contain embedded LF or CRLF characters that are
+not escaped, so the event may spill across multiple lines.
+
+If `GIT_TR2_BRIEF` is true, the `time`, `filename`, and `line` fields
+are omitted.
+
+This target is intended to be more of a summary (like GIT_TRACE) and
+less detailed than the other targets. It ignores thread, region, and
+data messages, for example.
+
+=== PERF Format
+
+PERF format is enabled when the `GIT_TR2_PERF` environment variable
+is set.
+
+Events are written as lines of the form:
+
+------------
+[<time> SP <filename>:<line> SP+
+ BAR SP] d<depth> SP
+ BAR SP <thread-name> SP+
+ BAR SP <event-name> SP+
+ BAR SP [r<repo-id>] SP+
+ BAR SP [<t_abs>] SP+
+ BAR SP [<t_rel>] SP+
+ BAR SP [<category>] SP+
+ BAR SP DOTS* <perf-event-message>
+ LF
+------------
+
+`<depth>`::
+ is the git process depth. This is the number of parent
+ git processes. A top-level git command has depth value "d0".
+ A child of it has depth value "d1". A second level child
+ has depth value "d2" and so on.
+
+`<thread-name>`::
+ is a unique name for the thread. The primary thread
+ is called "main". Other thread names are of the form "th%d:%s"
+ and include a unique number and the name of the thread-proc.
+
+`<event-name>`::
+ is the event name.
+
+`<repo-id>`::
+ when present, is a number indicating the repository
+ in use. A `def_repo` event is emitted when a repository is
+ opened. This defines the repo-id and associated worktree.
+ Subsequent repo-specific events will reference this repo-id.
++
+Currently, this is always "r1" for the main repository.
+This field is in anticipation of in-proc submodules in the future.
+
+`<t_abs>`::
+ when present, is the absolute time in seconds since the
+ program started.
+
+`<t_rel>`::
+ when present, is time in seconds relative to the start of
+ the current region. For a thread-exit event, it is the elapsed
+ time of the thread.
+
+`<category>`::
+ is present on region and data events and is used to
+ indicate a broad category, such as "index" or "status".
+
+`<perf-event-message>`::
+ is a free-form printf message intended for human consumption.
+
+------------
+15:33:33.532712 wt-status.c:2310 | d0 | main | region_enter | r1 | 0.126064 | | status | label:print
+15:33:33.532712 wt-status.c:2331 | d0 | main | region_leave | r1 | 0.127568 | 0.001504 | status | label:print
+------------
+
+If `GIT_TR2_PERF_BRIEF` is true, the `time`, `file`, and `line`
+fields are omitted.
+
+------------
+d0 | main | region_leave | r1 | 0.011717 | 0.009122 | index | label:preload
+------------
+
+The PERF target is intended for interactive performance analysis
+during development and is quite noisy.
+
+=== EVENT Format
+
+EVENT format is enabled when the `GIT_TR2_EVENT` environment
+variable is set.
+
+Each event is a JSON-object containing multiple key/value pairs
+written as a single line and followed by a LF.
+
+------------
+'{' <key> ':' <value> [',' <key> ':' <value>]* '}' LF
+------------
+
+Some key/value pairs are common to all events and some are
+event-specific.
+
+==== Common Key/Value Pairs
+
+The following key/value pairs are common to all events:
+
+------------
+{
+ "event":"version",
+ "sid":"1547659722619736-11614",
+ "thread":"main",
+ "time":"2019-01-16 17:28:42.620713",
+ "file":"common-main.c",
+ "line":38,
+ ...
+}
+------------
+
+`"event":<event>`::
+ is the event name.
+
+`"sid":<sid>`::
+ is the session-id. This is a unique string to identify the
+ process instance to allow all events emitted by a process to
+ be identified. A session-id is used instead of a PID because
+ PIDs are recycled by the OS. For child git processes, the
+ session-id is prepended with the session-id of the parent git
+ process to allow parent-child relationships to be identified
+ during post-processing.
+
+`"thread":<thread>`::
+ is the thread name.
+
+`"time":<time>`::
+ is the UTC time of the event.
+
+`"file":<filename>`::
+ is source file generating the event.
+
+`"line":<line-number>`::
+ is the integer source line number generating the event.
+
+`"repo":<repo-id>`::
+ when present, is the integer repo-id as described previously.
+
+If `GIT_TR2_EVENT_BRIEF` is true, the `file` and `line` fields are omitted
+from all events and the `time` field is only present on the "start" and
+"atexit" events.
+
+==== Event-Specific Key/Value Pairs
+
+`"version"`::
+ This event gives the version of the executable and the EVENT format.
++
+------------
+{
+ "event":"version",
+ ...
+ "evt":"1", # EVENT format version
+ "exe":"2.20.1.155.g426c96fcdb" # git version
+}
+------------
+
+`"start"`::
+ This event contains the complete argv received by main().
++
+------------
+{
+ "event":"start",
+ ...
+ "argv":["git","version"]
+}
+------------
+
+`"exit"`::
+ This event is emitted when git calls `exit()`.
++
+------------
+{
+ "event":"exit",
+ ...
+ "t_abs":0.001227, # elapsed time in seconds
+ "code":0 # exit code
+}
+------------
+
+`"atexit"`::
+ This event is emitted by the Trace2 `atexit` routine during
+ final shutdown. It should be the last event emitted by the
+ process.
++
+(The elapsed time reported here is greater than the time reported in
+the "exit" event because it runs after all other atexit tasks have
+completed.)
++
+------------
+{
+ "event":"atexit",
+ ...
+ "t_abs":0.001227, # elapsed time in seconds
+ "code":0 # exit code
+}
+------------
+
+`"signal"`::
+ This event is emitted when the program is terminated by a user
+ signal. Depending on the platform, the signal event may
+ prevent the "atexit" event from being generated.
++
+------------
+{
+ "event":"signal",
+ ...
+ "t_abs":0.001227, # elapsed time in seconds
+ "signal":13 # SIGTERM, SIGINT, etc.
+}
+------------
+
+`"error"`::
+ This event is emitted when one of the `error()`, `die()`,
+ or `usage()` functions are called.
++
+------------
+{
+ "event":"error",
+ ...
+ "msg":"invalid option: --cahced", # formatted error message
+ "fmt":"invalid option: %s" # error format string
+}
+------------
++
+The error event may be emitted more than once. The format string
+allows post-processors to group errors by type without worrying
+about specific error arguments.
+
+`"cmd_path"`::
+ This event contains the discovered full path of the git
+ executable (on platforms that are configured to resolve it).
++
+------------
+{
+ "event":"cmd_path",
+ ...
+ "path":"C:/work/gfw/git.exe"
+}
+------------
+
+`"cmd_name"`::
+ This event contains the command name for this git process
+ and the hierarchy of commands from parent git processes.
++
+------------
+{
+ "event":"cmd_name",
+ ...
+ "name":"pack-objects",
+ "hierarchy":"push/pack-objects"
+}
+------------
++
+Normally, the "name" field contains the canonical name of the
+command. When a canonical name is not available, one of
+these special values are used:
++
+------------
+"_query_" # "git --html-path"
+"_run_dashed_" # when "git foo" tries to run "git-foo"
+"_run_shell_alias_" # alias expansion to a shell command
+"_run_git_alias_" # alias expansion to a git command
+"_usage_" # usage error
+------------
+
+`"cmd_mode"`::
+ This event, when present, describes the command variant This
+ event may be emitted more than once.
++
+------------
+{
+ "event":"cmd_mode",
+ ...
+ "name":"branch"
+}
+------------
++
+The "name" field is an arbitrary string to describe the command mode.
+For example, checkout can checkout a branch or an individual file.
+And these variations typically have different performance
+characteristics that are not comparable.
+
+`"alias"`::
+ This event is present when an alias is expanded.
++
+------------
+{
+ "event":"alias",
+ ...
+ "alias":"l", # registered alias
+ "argv":["log","--graph"] # alias expansion
+}
+------------
+
+`"child_start"`::
+ This event describes a child process that is about to be
+ spawned.
++
+------------
+{
+ "event":"child_start",
+ ...
+ "child_id":2,
+ "child_class":"?",
+ "use_shell":false,
+ "argv":["git","rev-list","--objects","--stdin","--not","--all","--quiet"]
+
+ "hook_name":"<hook_name>" # present when child_class is "hook"
+ "cd":"<path>" # present when cd is required
+}
+------------
++
+The "child_id" field can be used to match this child_start with the
+corresponding child_exit event.
++
+The "child_class" field is a rough classification, such as "editor",
+"pager", "transport/*", and "hook". Unclassified children are classified
+with "?".
+
+`"child_exit"`::
+ This event is generated after the current process has returned
+ from the waitpid() and collected the exit information from the
+ child.
++
+------------
+{
+ "event":"child_exit",
+ ...
+ "child_id":2,
+ "pid":14708, # child PID
+ "code":0, # child exit-code
+ "t_rel":0.110605 # observed run-time of child process
+}
+------------
++
+Note that the session-id of the child process is not available to
+the current/spawning process, so the child's PID is reported here as
+a hint for post-processing. (But it is only a hint because the child
+proces may be a shell script which doesn't have a session-id.)
++
+Note that the `t_rel` field contains the observed run time in seconds
+for the child process (starting before the fork/exec/spawn and
+stopping after the waitpid() and includes OS process creation overhead).
+So this time will be slightly larger than the atexit time reported by
+the child process itself.
+
+`"exec"`::
+ This event is generated before git attempts to `exec()`
+ another command rather than starting a child process.
++
+------------
+{
+ "event":"exec",
+ ...
+ "exec_id":0,
+ "exe":"git",
+ "argv":["foo", "bar"]
+}
+------------
++
+The "exec_id" field is a command-unique id and is only useful if the
+`exec()` fails and a corresponding exec_result event is generated.
+
+`"exec_result"`::
+ This event is generated if the `exec()` fails and control
+ returns to the current git command.
++
+------------
+{
+ "event":"exec_result",
+ ...
+ "exec_id":0,
+ "code":1 # error code (errno) from exec()
+}
+------------
+
+`"thread_start"`::
+ This event is generated when a thread is started. It is
+ generated from *within* the new thread's thread-proc (for TLS
+ reasons).
++
+------------
+{
+ "event":"thread_start",
+ ...
+ "thread":"th02:preload_thread" # thread name
+}
+------------
+
+`"thread_exit"`::
+ This event is generated when a thread exits. It is generated
+ from *within* the thread's thread-proc (for TLS reasons).
++
+------------
+{
+ "event":"thread_exit",
+ ...
+ "thread":"th02:preload_thread", # thread name
+ "t_rel":0.007328 # thread elapsed time
+}
+------------
+
+`"def_param"`::
+ This event is generated to log a global parameter.
++
+------------
+{
+ "event":"def_param",
+ ...
+ "param":"core.abbrev",
+ "value":"7"
+}
+------------
+
+`"def_repo"`::
+ This event defines a repo-id and associates it with the root
+ of the worktree.
++
+------------
+{
+ "event":"def_repo",
+ ...
+ "repo":1,
+ "worktree":"/Users/jeffhost/work/gfw"
+}
+------------
++
+As stated earlier, the repo-id is currently always 1, so there will
+only be one def_repo event. Later, if in-proc submodules are
+supported, a def_repo event should be emitted for each submodule
+visited.
+
+`"region_enter"`::
+ This event is generated when entering a region.
++
+------------
+{
+ "event":"region_enter",
+ ...
+ "repo":1, # optional
+ "nesting":1, # current region stack depth
+ "category":"index", # optional
+ "label":"do_read_index", # optional
+ "msg":".git/index" # optional
+}
+------------
++
+The `category` field may be used in a future enhancement to
+do category-based filtering.
++
+The `GIT_TR2_EVENT_NESTING` environment variable can be used to
+filter deeply nested regions and data events. It defaults to "2".
+
+`"region_leave"`::
+ This event is generated when leaving a region.
++
+------------
+{
+ "event":"region_leave",
+ ...
+ "repo":1, # optional
+ "t_rel":0.002876, # time spent in region in seconds
+ "nesting":1, # region stack depth
+ "category":"index", # optional
+ "label":"do_read_index", # optional
+ "msg":".git/index" # optional
+}
+------------
+
+`"data"`::
+ This event is generated to log a thread- and region-local
+ key/value pair.
++
+------------
+{
+ "event":"data",
+ ...
+ "repo":1, # optional
+ "t_abs":0.024107, # absolute elapsed time
+ "t_rel":0.001031, # elapsed time in region/thread
+ "nesting":2, # region stack depth
+ "category":"index",
+ "key":"read/cache_nr",
+ "value":"3552"
+}
+------------
++
+The "value" field may be an integer or a string.
+
+`"data-json"`::
+ This event is generated to log a pre-formatted JSON string
+ containing structured data.
++
+------------
+{
+ "event":"data_json",
+ ...
+ "repo":1, # optional
+ "t_abs":0.015905,
+ "t_rel":0.015905,
+ "nesting":1,
+ "category":"process",
+ "key":"windows/ancestry",
+ "value":["bash.exe","bash.exe"]
+}
+------------
+
+== Example Trace2 API Usage
+
+Here is a hypothetical usage of the Trace2 API showing the intended
+usage (without worrying about the actual Git details).
+
+Initialization::
+
+ Initialization happens in `main()`. Behind the scenes, an
+ `atexit` and `signal` handler are registered.
++
+----------------
+int main(int argc, const char **argv)
+{
+ int exit_code;
+
+ trace2_initialize();
+ trace2_cmd_start(argv);
+
+ exit_code = cmd_main(argc, argv);
+
+ trace2_cmd_exit(exit_code);
+
+ return exit_code;
+}
+----------------
+
+Command Details::
+
+ After the basics are established, additional command
+ information can be sent to Trace2 as it is discovered.
++
+----------------
+int cmd_checkout(int argc, const char **argv)
+{
+ trace2_cmd_name("checkout");
+ trace2_cmd_mode("branch");
+ trace2_def_repo(the_repository);
+
+ // emit "def_param" messages for "interesting" config settings.
+ trace2_cmd_list_config();
+
+ if (do_something())
+ trace2_cmd_error("Path '%s': cannot do something", path);
+
+ return 0;
+}
+----------------
+
+Child Processes::
+
+ Wrap code spawning child processes.
++
+----------------
+void run_child(...)
+{
+ int child_exit_code;
+ struct child_process cmd = CHILD_PROCESS_INIT;
+ ...
+ cmd.trace2_child_class = "editor";
+
+ trace2_child_start(&cmd);
+ child_exit_code = spawn_child_and_wait_for_it();
+ trace2_child_exit(&cmd, child_exit_code);
+}
+----------------
++
+For example, the following fetch command spawned ssh, index-pack,
+rev-list, and gc. This example also shows that fetch took
+5.199 seconds and of that 4.932 was in ssh.
++
+----------------
+$ export GIT_TR2_BRIEF=1
+$ export GIT_TR2=~/log.normal
+$ git fetch origin
+...
+----------------
++
+----------------
+$ cat ~/log.normal
+version 2.20.1.vfs.1.1.47.g534dbe1ad1
+start git fetch origin
+worktree /Users/jeffhost/work/gfw
+cmd_name fetch (fetch)
+child_start[0] ssh git@github.com ...
+child_start[1] git index-pack ...
+... (Trace2 events from child processes omitted)
+child_exit[1] pid:14707 code:0 elapsed:0.076353
+child_exit[0] pid:14706 code:0 elapsed:4.931869
+child_start[2] git rev-list ...
+... (Trace2 events from child process omitted)
+child_exit[2] pid:14708 code:0 elapsed:0.110605
+child_start[3] git gc --auto
+... (Trace2 events from child process omitted)
+child_exit[3] pid:14709 code:0 elapsed:0.006240
+exit elapsed:5.198503 code:0
+atexit elapsed:5.198541 code:0
+----------------
++
+When a git process is a (direct or indirect) child of another
+git process, it inherits Trace2 context information. This
+allows the child to print the command hierarchy. This example
+shows gc as child[3] of fetch. When the gc process reports
+its name as "gc", it also reports the hierarchy as "fetch/gc".
+(In this example, trace2 messages from the child process is
+indented for clarity.)
++
+----------------
+$ export GIT_TR2_BRIEF=1
+$ export GIT_TR2=~/log.normal
+$ git fetch origin
+...
+----------------
++
+----------------
+$ cat ~/log.normal
+version 2.20.1.160.g5676107ecd.dirty
+start git fetch official
+worktree /Users/jeffhost/work/gfw
+cmd_name fetch (fetch)
+...
+child_start[3] git gc --auto
+ version 2.20.1.160.g5676107ecd.dirty
+ start /Users/jeffhost/work/gfw/git gc --auto
+ worktree /Users/jeffhost/work/gfw
+ cmd_name gc (fetch/gc)
+ exit elapsed:0.001959 code:0
+ atexit elapsed:0.001997 code:0
+child_exit[3] pid:20303 code:0 elapsed:0.007564
+exit elapsed:3.868938 code:0
+atexit elapsed:3.868970 code:0
+----------------
+
+Regions::
+
+ Regions can be use to time an interesting section of code.
++
+----------------
+void wt_status_collect(struct wt_status *s)
+{
+ trace2_region_enter("status", "worktrees", s->repo);
+ wt_status_collect_changes_worktree(s);
+ trace2_region_leave("status", "worktrees", s->repo);
+
+ trace2_region_enter("status", "index", s->repo);
+ wt_status_collect_changes_index(s);
+ trace2_region_leave("status", "index", s->repo);
+
+ trace2_region_enter("status", "untracked", s->repo);
+ wt_status_collect_untracked(s);
+ trace2_region_leave("status", "untracked", s->repo);
+}
+
+void wt_status_print(struct wt_status *s)
+{
+ trace2_region_enter("status", "print", s->repo);
+ switch (s->status_format) {
+ ...
+ }
+ trace2_region_leave("status", "print", s->repo);
+}
+----------------
++
+In this example, scanning for untracked files ran from +0.012568 to
++0.027149 (since the process started) and took 0.014581 seconds.
++
+----------------
+$ export GIT_TR2_PERF_BRIEF=1
+$ export GIT_TR2_PERF=~/log.perf
+$ git status
+...
+
+$ cat ~/log.perf
+d0 | main | version | | | | | 2.20.1.160.g5676107ecd.dirty
+d0 | main | start | | | | | git status
+d0 | main | def_repo | r1 | | | | worktree:/Users/jeffhost/work/gfw
+d0 | main | cmd_name | | | | | status (status)
+...
+d0 | main | region_enter | r1 | 0.010988 | | status | label:worktrees
+d0 | main | region_leave | r1 | 0.011236 | 0.000248 | status | label:worktrees
+d0 | main | region_enter | r1 | 0.011260 | | status | label:index
+d0 | main | region_leave | r1 | 0.012542 | 0.001282 | status | label:index
+d0 | main | region_enter | r1 | 0.012568 | | status | label:untracked
+d0 | main | region_leave | r1 | 0.027149 | 0.014581 | status | label:untracked
+d0 | main | region_enter | r1 | 0.027411 | | status | label:print
+d0 | main | region_leave | r1 | 0.028741 | 0.001330 | status | label:print
+d0 | main | exit | | 0.028778 | | | code:0
+d0 | main | atexit | | 0.028809 | | | code:0
+----------------
++
+Regions may be nested. This causes messages to be indented in the
+PERF target, for example.
+Elapsed times are relative to the start of the correpsonding nesting
+level as expected. For example, if we add region message to:
++
+----------------
+static enum path_treatment read_directory_recursive(struct dir_struct *dir,
+ struct index_state *istate, const char *base, int baselen,
+ struct untracked_cache_dir *untracked, int check_only,
+ int stop_at_first_file, const struct pathspec *pathspec)
+{
+ enum path_treatment state, subdir_state, dir_state = path_none;
+
+ trace2_region_enter_printf("dir", "read_recursive", NULL, "%.*s", baselen, base);
+ ...
+ trace2_region_leave_printf("dir", "read_recursive", NULL, "%.*s", baselen, base);
+ return dir_state;
+}
+----------------
++
+We can further investigate the time spent scanning for untracked files.
++
+----------------
+$ export GIT_TR2_PERF_BRIEF=1
+$ export GIT_TR2_PERF=~/log.perf
+$ git status
+...
+$ cat ~/log.perf
+d0 | main | version | | | | | 2.20.1.162.gb4ccea44db.dirty
+d0 | main | start | | | | | git status
+d0 | main | def_repo | r1 | | | | worktree:/Users/jeffhost/work/gfw
+d0 | main | cmd_name | | | | | status (status)
+...
+d0 | main | region_enter | r1 | 0.015047 | | status | label:untracked
+d0 | main | region_enter | | 0.015132 | | dir | ..label:read_recursive
+d0 | main | region_enter | | 0.016341 | | dir | ....label:read_recursive vcs-svn/
+d0 | main | region_leave | | 0.016422 | 0.000081 | dir | ....label:read_recursive vcs-svn/
+d0 | main | region_enter | | 0.016446 | | dir | ....label:read_recursive xdiff/
+d0 | main | region_leave | | 0.016522 | 0.000076 | dir | ....label:read_recursive xdiff/
+d0 | main | region_enter | | 0.016612 | | dir | ....label:read_recursive git-gui/
+d0 | main | region_enter | | 0.016698 | | dir | ......label:read_recursive git-gui/po/
+d0 | main | region_enter | | 0.016810 | | dir | ........label:read_recursive git-gui/po/glossary/
+d0 | main | region_leave | | 0.016863 | 0.000053 | dir | ........label:read_recursive git-gui/po/glossary/
+...
+d0 | main | region_enter | | 0.031876 | | dir | ....label:read_recursive builtin/
+d0 | main | region_leave | | 0.032270 | 0.000394 | dir | ....label:read_recursive builtin/
+d0 | main | region_leave | | 0.032414 | 0.017282 | dir | ..label:read_recursive
+d0 | main | region_leave | r1 | 0.032454 | 0.017407 | status | label:untracked
+...
+d0 | main | exit | | 0.034279 | | | code:0
+d0 | main | atexit | | 0.034322 | | | code:0
+----------------
++
+Trace2 regions are similar to the existing trace_performance_enter()
+and trace_performance_leave() routines, but are thread safe and
+maintain per-thread stacks of timers.
+
+Data Messages::
+
+ Data messages added to a region.
++
+----------------
+int read_index_from(struct index_state *istate, const char *path,
+ const char *gitdir)
+{
+ trace2_region_enter_printf("index", "do_read_index", the_repository, "%s", path);
+
+ ...
+
+ trace2_data_intmax("index", the_repository, "read/version", istate->version);
+ trace2_data_intmax("index", the_repository, "read/cache_nr", istate->cache_nr);
+
+ trace2_region_leave_printf("index", "do_read_index", the_repository, "%s", path);
+}
+----------------
++
+This example shows that the index contained 3552 entries.
++
+----------------
+$ export GIT_TR2_PERF_BRIEF=1
+$ export GIT_TR2_PERF=~/log.perf
+$ git status
+...
+$ cat ~/log.perf
+d0 | main | version | | | | | 2.20.1.156.gf9916ae094.dirty
+d0 | main | start | | | | | git status
+d0 | main | def_repo | r1 | | | | worktree:/Users/jeffhost/work/gfw
+d0 | main | cmd_name | | | | | status (status)
+d0 | main | region_enter | r1 | 0.001791 | | index | label:do_read_index .git/index
+d0 | main | data | r1 | 0.002494 | 0.000703 | index | ..read/version:2
+d0 | main | data | r1 | 0.002520 | 0.000729 | index | ..read/cache_nr:3552
+d0 | main | region_leave | r1 | 0.002539 | 0.000748 | index | label:do_read_index .git/index
+...
+----------------
+
+Thread Events::
+
+ Thread messages added to a thread-proc.
++
+For example, the multithreaded preload-index code can be
+instrumented with a region around the thread pool and then
+per-thread start and exit events within the threadproc.
++
+----------------
+static void *preload_thread(void *_data)
+{
+ // start the per-thread clock and emit a message.
+ trace2_thread_start("preload_thread");
+
+ // report which chunk of the array this thread was assigned.
+ trace2_data_intmax("index", the_repository, "offset", p->offset);
+ trace2_data_intmax("index", the_repository, "count", nr);
+
+ do {
+ ...
+ } while (--nr > 0);
+ ...
+
+ // report elapsed time taken by this thread.
+ trace2_thread_exit();
+ return NULL;
+}
+
+void preload_index(struct index_state *index,
+ const struct pathspec *pathspec,
+ unsigned int refresh_flags)
+{
+ trace2_region_enter("index", "preload", the_repository);
+
+ for (i = 0; i < threads; i++) {
+ ... /* create thread */
+ }
+
+ for (i = 0; i < threads; i++) {
+ ... /* join thread */
+ }
+
+ trace2_region_leave("index", "preload", the_repository);
+}
+----------------
++
+In this example preload_index() was executed by the `main` thread
+and started the `preload` region. Seven threads, named
+`th01:preload_thread` through `th07:preload_thread`, were started.
+Events from each thread are atomically appended to the shared target
+stream as they occur so they may appear in random order with respect
+other threads. Finally, the main thread waits for the threads to
+finish and leaves the region.
++
+Data events are tagged with the active thread name. They are used
+to report the per-thread parameters.
++
+----------------
+$ export GIT_TR2_PERF_BRIEF=1
+$ export GIT_TR2_PERF=~/log.perf
+$ git status
+...
+$ cat ~/log.perf
+...
+d0 | main | region_enter | r1 | 0.002595 | | index | label:preload
+d0 | th01:preload_thread | thread_start | | 0.002699 | | |
+d0 | th02:preload_thread | thread_start | | 0.002721 | | |
+d0 | th01:preload_thread | data | r1 | 0.002736 | 0.000037 | index | offset:0
+d0 | th02:preload_thread | data | r1 | 0.002751 | 0.000030 | index | offset:2032
+d0 | th03:preload_thread | thread_start | | 0.002711 | | |
+d0 | th06:preload_thread | thread_start | | 0.002739 | | |
+d0 | th01:preload_thread | data | r1 | 0.002766 | 0.000067 | index | count:508
+d0 | th06:preload_thread | data | r1 | 0.002856 | 0.000117 | index | offset:2540
+d0 | th03:preload_thread | data | r1 | 0.002824 | 0.000113 | index | offset:1016
+d0 | th04:preload_thread | thread_start | | 0.002710 | | |
+d0 | th02:preload_thread | data | r1 | 0.002779 | 0.000058 | index | count:508
+d0 | th06:preload_thread | data | r1 | 0.002966 | 0.000227 | index | count:508
+d0 | th07:preload_thread | thread_start | | 0.002741 | | |
+d0 | th07:preload_thread | data | r1 | 0.003017 | 0.000276 | index | offset:3048
+d0 | th05:preload_thread | thread_start | | 0.002712 | | |
+d0 | th05:preload_thread | data | r1 | 0.003067 | 0.000355 | index | offset:1524
+d0 | th05:preload_thread | data | r1 | 0.003090 | 0.000378 | index | count:508
+d0 | th07:preload_thread | data | r1 | 0.003037 | 0.000296 | index | count:504
+d0 | th03:preload_thread | data | r1 | 0.002971 | 0.000260 | index | count:508
+d0 | th04:preload_thread | data | r1 | 0.002983 | 0.000273 | index | offset:508
+d0 | th04:preload_thread | data | r1 | 0.007311 | 0.004601 | index | count:508
+d0 | th05:preload_thread | thread_exit | | 0.008781 | 0.006069 | |
+d0 | th01:preload_thread | thread_exit | | 0.009561 | 0.006862 | |
+d0 | th03:preload_thread | thread_exit | | 0.009742 | 0.007031 | |
+d0 | th06:preload_thread | thread_exit | | 0.009820 | 0.007081 | |
+d0 | th02:preload_thread | thread_exit | | 0.010274 | 0.007553 | |
+d0 | th07:preload_thread | thread_exit | | 0.010477 | 0.007736 | |
+d0 | th04:preload_thread | thread_exit | | 0.011657 | 0.008947 | |
+d0 | main | region_leave | r1 | 0.011717 | 0.009122 | index | label:preload
+...
+d0 | main | exit | | 0.029996 | | | code:0
+d0 | main | atexit | | 0.030027 | | | code:0
+----------------
++
+In this example, the preload region took 0.009122 seconds. The 7 threads
+took between 0.006069 and 0.008947 seconds to work on their portion of
+the index. Thread "th01" worked on 508 items at offset 0. Thread "th02"
+worked on 508 items at offset 2032. Thread "th04" worked on 508 itemts
+at offset 508.
++
+This example also shows that thread names are assigned in a racy manner
+as each thread starts and allocates TLS storage.
+
+== Future Work
+
+=== Relationship to the Existing Trace Api (api-trace.txt)
+
+There are a few issues to resolve before we can completely
+switch to Trace2.
+
+* Updating existing tests that assume GIT_TRACE format messages.
+
+* How to best handle custom GIT_TRACE_<key> messages?
+
+** The GIT_TRACE_<key> mechanism allows each <key> to write to a
+different file (in addition to just stderr).
+
+** Do we want to maintain that ability or simply write to the existing
+Trace2 targets (and convert <key> to a "category").
+
--
gitgitgadget