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NAME
dyntrace - Interface to dynamic tracing
DESCRIPTION
This module implements interfaces to dynamic tracing, should such be compiled into the
virtual machine. For a standard and/or commercial build, no dynamic tracing is available,
in which case none of the functions in this module is usable or give any effect.
Should dynamic tracing be enabled in the current build, either by configuring with
./configure --with-dynamic-trace=dtrace or with ./configure --with-dynamic-
trace=systemtap, the module can be used for two things:
* Trigger the user-probe user_trace_i4s4 in the NIF library dyntrace.so by calling
dyntrace:p/{1,2,3,4,5,6,7,8}.
* Set a user specified tag that will be present in the trace messages of both the
efile_drv and the user-probe mentioned above.
Both building with dynamic trace probes and using them is experimental and unsupported by
Erlang/OTP. It is included as an option for the developer to trace and debug performance
issues in their systems.
The original implementation is mostly done by Scott Lystiger Fritchie as an Open Source
Contribution and it should be viewed as such even though the source for dynamic tracing as
well as this module is included in the main distribution. However, the ability to use
dynamic tracing of the virtual machine is a very valuable contribution which OTP has every
intention to maintain as a tool for the developer.
How to write d programs or systemtap scripts can be learned from books and from a lot of
pages on the Internet. This manual page does not include any documentation about using the
dynamic trace tools of respective platform. The examples directory of the runtime_tools
application however contains comprehensive examples of both d and systemtap programs that
will help you get started. Another source of information is the dtrace and systemtap
chapters in the Runtime Tools Users' Guide.
EXPORTS
available() -> boolean()
This function uses the NIF library to determine if dynamic tracing is available.
Usually calling erlang:system_info/1 is a better indicator of the availability of
dynamic tracing.
The function will throw an exception if the dyntrace NIF library could not be
loaded by the on_load function of this module.
p() -> true | false | error | badarg
Calling this function will trigger the "user" trace probe user_trace_i4s4 in the
dyntrace NIF module, sending a trace message only containing the user tag and
zeroes/empty strings in all other fields.
p(integer() | string()) -> true | false | error | badarg
Calling this function will trigger the "user" trace probe user_trace_i4s4 in the
dyntrace NIF module, sending a trace message containing the user tag and the
integer or string parameter in the first integer/string field.
p(integer() | string(), integer() | string()) -> true | false | error | badarg
Calling this function will trigger the "user" trace probe user_trace_i4s4 in the
dyntrace NIF module, sending a trace message containing the user tag and the
integer() or string() parameters as the first fields of respective type. integer()
parameters should be put before any string() parameters. I.e. p(1,"Hello") is ok,
as is p(1,1) and p("Hello","Again"), but not p("Hello",1).
p(integer() | string(), integer() | string(), integer() | string()) -> true | false |
error | badarg
Calling this function will trigger the "user" trace probe user_trace_i4s4 in the
dyntrace NIF module, sending a trace message containing the user tag and the
integer() or string() parameters as the first fields of respective type. integer()
parameters should be put before any string() parameters, as in p/2.
p(integer() | string(), integer() | string(), integer() | string(), integer() | string())
-> true | false | error | badarg
Calling this function will trigger the "user" trace probe user_trace_i4s4 in the
dyntrace NIF module, sending a trace message containing the user tag and the
integer() or string() parameters as the first fields of respective type. integer()
parameters should be put before any string() parameters, as in p/2.
p(integer(), integer() | string(), integer() | string(), integer() | string(), string())
-> true | false | error | badarg
Calling this function will trigger the "user" trace probe user_trace_i4s4 in the
dyntrace NIF module, sending a trace message containing the user tag and the
integer() or string() parameters as the first fields of respective type. integer()
parameters should be put before any string() parameters, as in p/2.
There can be no more than four parameters of any type (integer() or string()), so
the first parameter has to be an integer() and the last a string().
p(integer(), integer(), integer() | string(), integer() | string(), string(), string()) ->
true | false | error | badarg
Calling this function will trigger the "user" trace probe user_trace_i4s4 in the
dyntrace NIF module, sending a trace message containing the user tag and the
integer() or string() parameters as the first fields of respective type. integer()
parameters should be put before any string() parameters, as in p/2.
There can be no more than four parameters of any type (integer() or string()), so
the first two parameters has to be integer()'s and the last two string()'s.
p(integer(), integer(), integer(), integer() | string(), string(), string(), string()) ->
true | false | error | badarg
Calling this function will trigger the "user" trace probe user_trace_i4s4 in the
dyntrace NIF module, sending a trace message containing the user tag and the
integer() or string() parameters as the first fields of respective type. integer()
parameters should be put before any string() parameters, as in p/2.
There can be no more than four parameters of any type (integer() or string()), so
the first three parameters has to be integer()'s and the last three string()'s.
p(integer(), integer(), integer(), integer(), string(), string(), string(), string()) ->
true | false | error | badarg
Calling this function will trigger the "user" trace probe user_trace_i4s4 in the
dyntrace NIF module, sending a trace message containing all the integer()'s and
string()'s provided, as well as any user tag set in the current process.
get_tag() -> binary() | undefined
This function returns the user tag set in the current process. If no tag is set or
dynamic tracing is not available, it returns undefined
get_tag() -> binary() | undefined
This function returns the user tag set in the current process or, if no user tag is
present, the last user tag sent to the process together with a message (in the same
way as sequential trace tokens are spread to other processes together with
messages. For an explanation of how user tags can be spread together with messages,
see spread_tag/1. If no tag is found or dynamic tracing is not available, it
returns undefined
put_tag(Item) -> binary() | undefined
Types:
Item = iodata()
This function sets the user tag of the current process. The user tag is a binary(),
but can be specified as any iodata(), which is automatically converted to a binary
by this function.
The user tag is provided to the user probes triggered by calls top
dyntrace:p/{1,2,3,4,5,6,7,8} as well as probes in the efile_driver. In the future,
user tags might be added to more probes.
The old user tag (if any) is returned, or undefined if no user tag was present or
dynamic tracing is not enabled.
spread_tag(boolean()) -> TagData
Types:
TagData = opaque data that can be used as parameter to restore_tag/1
This function controls if user tags are to be spread to other processes with the
next message. Spreading of user tags work like spreading of sequential trace
tokens, so that a received user tag will be active in the process until the next
message arrives (if that message does not also contain the user tag.
This functionality is used when a client process communicates with a file i/o-
server to spread the user tag to the I/O-server and then down to the efile_drv
driver. By using spread_tag/1 and restore_tag/1, one can enable or disable
spreading of user tags to other processes and then restore the previous state of
the user tag. The TagData returned from this call contains all previous information
so the state (including any previously spread user tags) will be completely
restored by a later call to restore_tag/1.
The file module already spread's tags, so there is noo need to manually call these
function to get user tags spread to the efile driver through that module.
The most use of this function would be if one for example uses the io module to
communicate with an I/O-server for a regular file, like in the following example:
f() ->
{ok, F} = file:open("test.tst",[write]),
Saved = dyntrace:spread_tag(true),
io:format(F,"Hello world!",[]),
dyntrace:restore_tag(Saved),
file:close(F).
In this example, any user tag set in the calling process will be spread to the I/O-
server when the io:format call is done.
restore_tag(TagData) -> true
Types:
TagData = opaque data returned by spread_tag/1
Restores the previous state of user tags and their spreading as it was before a
call to spread_tag/1. Note that the restoring is not limited to the same process,
one can utilize this to turn off spreding in one process and restore it in a newly
created, the one that actually is going to send messages:
f() ->
TagData=dyntrace:spread_tag(false),
spawn(fun() ->
dyntrace:restore_tag(TagData),
do_something()
end),
do_something_else(),
dyntrace:restore_tag(TagData).
Correctly handling user tags and their spreading might take some effort, as Erlang
programs tend to send and receive messages so that sometimes the user tag gets lost
due to various things, like double receives or communication with a port (ports do
not handle user tags, in the same way as they do not handle regular sequential
trace tokens).