rfc:dtrace

Request for Comments: DTrace probes for PHP

Introduction

From Wikipedia [1]:

“DTrace is a comprehensive dynamic tracing framework created by Sun Microsystems for troubleshooting kernel and application problems on production systems in real time. […] It is designed to give operational insights that allow users to tune and troubleshoot applications and the OS itself.

Tracing programs (also referred to as scripts) are written using the D programming language (not to be confused with other programming languages named “D”). The language is a subset of C with added functions and variables specific to tracing. D programs resemble awk programs in structure; they consist of a list of one or more probes (instrumentation points), and each probe is associated with an action. Whenever the condition for the probe is met, the associated action is executed (the probe “fires”). A typical probe might fire when a certain file is opened, or a process is started, or a certain line of code is executed. A probe that fires may analyze the run-time situation by accessing the call stack and context variables and evaluating expressions; it can then print out or log some information, record it in a database, or modify context variables. The reading and writing of context variables allows probes to pass information to each other, allowing them to cooperatively analyze the correlation of different events.”

Why DTrace probes for PHP

Applications can be compiled with statically probes to better reflect its underlying structure. This is particularly useful for scripting language to give the user the perspective of the script and not of the underlying executor. This includes that probes are fired at important stages of the execution including the ability to pass useful data to the DTrace consumer.

The proposed patch adds a provider for PHP specific probes to PHP. This will better reflect the execution flow of a PHP script. Former inaccessible information, such as the error message in zend_error are passed to probes. The patch introduces probes to track the compilation of a script, the execution and function call stacks and error/exception handling. Calling a probe doesn't affect the performance until the probe is enabled by DTrace. Disabled probes are replaced by NOPs. If additional information is needed, the information is retrieved and passed to the probes once it is enabled.

Probes Description
request-(startup/shutodnw) These probes are fire when a request starts or shuts down.
compile-file-(entry/return) These probes are fire when the compilation of a script starts and when it finishes.
execute-(entry/return) These probes are fire when a new opcode is executed.
function-(entry/return) These probes are fire when the engine enters or returns from a PHP function or method call.
exception-thrown This probe is fired whenever an exception is thrown
exception-caught This probe is fired whenever an exception is caught
error This probe is fired whenever an error occurs. It doesn't depend on the error_reproting level.

Examples

Display errors

Display all errors, warning and notices independent from the current error_reporting setting or @-operator on all running PHP instances:

#!/usr/sbin/dtrace -s

#pragma D option quiet

php*:::error
{
    printf("Error: %s in line %d (%s)\n", copyinstr(arg0), arg2, copyinstr(arg1));
} 
Count FSTAT calls

The following example counts the fstat calls in a PHP file and add up the time needed for the fstats.

#!/usr/sbin/dtrace -s

#pragma D option quiet

php*:::execute-entry
/!self->start/
{
    self->start = vtimestamp;
}

php*:::execute-entry
/self->start/
{
    self->isInsidePHP++;
    self->filename = copyinstr(arg0);
    self->result[self->filename] = 0;
}

php*:::execute-return
{
    self->isInsidePHP--;
}

syscall::fstat:entry
/self->isInsidePHP > 0/
{
    self->calltime = timestamp;
}

syscall::fstat:return
/self->isInsidePHP > 0/
{
    self->result[self->filename] = timestamp - self->calltime;
}

More about DTrace

Changelog

rfc/dtrace.txt · Last modified: 2011/04/06 12:59 (external edit)