Thursday Dec 06, 2012

Adding DTrace Probes to PHP Extensions

The powerful DTrace tracing facility has some PHP-specific probes that can be enabled with --enable-dtrace.

DTrace for Linux is being created by Oracle and is currently in tech preview. Currently it doesn't support userspace tracing so, in the meantime, Systemtap can be used to monitor the probes implemented in PHP. This was recently outlined in David Soria Parra's post Probing PHP with Systemtap on Linux.

My post shows how DTrace probes can be added to PHP extensions and traced on Linux. I was using Oracle Linux 6.3.

Not all Linux kernels are built with Systemtap, since this can impact stability. Check whether your running kernel (or others installed) have Systemtap enabled, and reboot with such a kernel:

# grep CONFIG_UTRACE /boot/config-`uname -r`
# grep CONFIG_UTRACE /boot/config-*

When you install Systemtap itself, the package systemtap-sdt-devel is needed since it provides the sdt.h header file:

# yum install systemtap-sdt-devel

You can now install and build PHP as shown in David's article. Basically the build is with:

$ cd ~/php-src
$ ./configure --disable-all --enable-dtrace
$ make

(For me, running 'make' a second time failed with an error. The workaround is to do 'git checkout Zend/zend_dtrace.d' and then rerun 'make'. See PHP Bug 63704)

David's article shows how to trace the probes already implemented in PHP. You can also use Systemtap to trace things like userspace PHP function calls. For example, create test.php:


$c = oci_connect('hr', 'welcome', 'localhost/orcl');
$s = oci_parse($c, "select dbms_xmlgen.getxml('select * from dual') xml from dual");
$r = oci_execute($s);
$row = oci_fetch_array($s, OCI_NUM);
$x = $row[0]->load();
echo $x;


The normal output of this file is the XML form of Oracle's DUAL table:

$ ./sapi/cli/php ~/test.php
<?xml version="1.0"?>

To trace the PHP function calls, create the tracing file functrace.stp:

probe process("sapi/cli/php").function("zif_*") {
    printf("Started function %s\n", probefunc());

probe process("sapi/cli/php").function("zif_*").return {
    printf("Ended function %s\n", probefunc());

This makes use of the way PHP userspace functions (not builtins) like oci_connect() map to C functions with a "zif_" prefix.

Login as root, and run System tap on the PHP script:

# cd ~cjones/php-src
# stap -c 'sapi/cli/php ~cjones/test.php' ~cjones/functrace.stp
Started function zif_oci_connect
Ended function zif_oci_connect
Started function zif_oci_parse
Ended function zif_oci_parse
Started function zif_oci_execute
Ended function zif_oci_execute
Started function zif_oci_fetch_array
Ended function zif_oci_fetch_array
Started function zif_oci_lob_load
<?xml version="1.0"?>
Ended function zif_oci_lob_load
Started function zif_oci_free_descriptor
Ended function zif_oci_free_descriptor

Each call and return is logged. The Systemtap scripting language allows complex scripts to be built. There are many examples on the web.

To augment this generic capability and the PHP probes in PHP, other extensions can have probes too. Below are the steps I used to add probes to OCI8:

  1. I created a provider file ext/oci8/oci8_dtrace.d, enabling three probes. The first one will accept a parameter that runtime tracing can later display:

    provider php {
    	probe oci8__connect(char *username);
    	probe oci8__nls_start();
    	probe oci8__nls_done();
  2. I updated ext/oci8/config.m4 with the PHP_INIT_DTRACE macro. The patch is at the end of config.m4. The macro takes the provider prototype file, a name of the header file that 'dtrace' will generate, and a list of sources files with probes. When --enable-dtrace is used during PHP configuration, then the outer $PHP_DTRACE check is true and my new probes will be enabled. I've chosen to define an OCI8 specific macro, HAVE_OCI8_DTRACE, which can be used in the OCI8 source code:

    diff --git a/ext/oci8/config.m4 b/ext/oci8/config.m4
    index 34ae76c..f3e583d 100644
    --- a/ext/oci8/config.m4
    +++ b/ext/oci8/config.m4
    @@ -341,4 +341,17 @@ if test "$PHP_OCI8" != "no"; then
    +  if test "$PHP_DTRACE" = "yes"; then
    +     AC_CHECK_HEADERS([sys/sdt.h], [
    +       PHP_INIT_DTRACE([ext/oci8/oci8_dtrace.d],
    +                       [ext/oci8/oci8_dtrace_gen.h],[ext/oci8/oci8.c])
    +         AC_DEFINE(HAVE_OCI8_DTRACE,1,
    +         [Whether to enable DTrace support for OCI8 ])
    +     ], [
    +       AC_MSG_ERROR(
    +         [Cannot find sys/sdt.h which is required for DTrace support])
    +     ])
    +   fi
  3. In ext/oci8/oci8.c, I added the probes at, for this example, semi-arbitrary places:

    diff --git a/ext/oci8/oci8.c b/ext/oci8/oci8.c
    index e2241cf..ffa0168 100644
    --- a/ext/oci8/oci8.c
    +++ b/ext/oci8/oci8.c
    @@ -1811,6 +1811,12 @@ php_oci_connection *php_oci_do_connect_ex(char *username, int username_len, char
    +#ifdef HAVE_OCI8_DTRACE
    +		DTRACE_OCI8_CONNECT(username);
    +	}
     	/* Initialize global handles if they weren't initialized before */
     	if (OCI_G(env) == NULL) {
    @@ -1870,11 +1876,22 @@ php_oci_connection *php_oci_do_connect_ex(char *username, int username_len, char
     		size_t rsize = 0;
     		sword result;
    +#ifdef HAVE_OCI8_DTRACE
    +		}
     		PHP_OCI_CALL_RETURN(result, OCINlsEnvironmentVariableGet, (&charsetid_nls_lang, 0, OCI_NLS_CHARSET_ID, 0, &rsize));
     		if (result != OCI_SUCCESS) {
     			charsetid_nls_lang = 0;
     		smart_str_append_unsigned_ex(&hashed_details, charsetid_nls_lang, 0);
    +#ifdef HAVE_OCI8_DTRACE
    +		}
     	timestamp = time(NULL);

    The oci_connect(), oci_pconnect() and oci_new_connect() calls all use php_oci_do_connect_ex() internally. The first probe simply records that the PHP application made a connection call. I already showed a way to do this without needing a probe, but adding a specific probe lets me record the username. The other two probes can be used to time how long the globalization initialization takes.

    The relationships between the oci8_dtrace.d names like oci8__connect, the probe guards like DTRACE_OCI8_CONNECT_ENABLED() and probe names like DTRACE_OCI8_CONNECT() are obvious after seeing the pattern of all three probes.

    I included the new header that will be automatically created by the dtrace tool when PHP is built. I did this in ext/oci8/php_oci8_int.h:

    diff --git a/ext/oci8/php_oci8_int.h b/ext/oci8/php_oci8_int.h
    index b0d6516..c81fc5a 100644
    --- a/ext/oci8/php_oci8_int.h
    +++ b/ext/oci8/php_oci8_int.h
    @@ -44,6 +44,10 @@
     #  endif
     # endif /* osf alpha */
    +#ifdef HAVE_OCI8_DTRACE
    +#include "oci8_dtrace_gen.h"
     #if defined(min)
     #undef min
  4. Now PHP can be rebuilt:

    $ cd ~/php-src
    $ rm configure && ./buildconf --force
    $ ./configure --disable-all --enable-dtrace \
    $ make

    If 'make' fails, do the 'git checkout Zend/zend_dtrace.d' trick I mentioned.

  5. The new probes can be seen by logging in as root and running:

    # stap -l 'process.provider("php").mark("oci8*")' -c 'sapi/cli/php -i'
  6. To test them out, create a new trace file, oci.stp:

    global numconnects;
    global start;
    global numcharlookups = 0;
    global tottime = 0;
    probe process.provider("php").mark("oci8-connect") {
        printf("Connected as %s\n", user_string($arg1));
        numconnects += 1;
    probe process.provider("php").mark("oci8-nls_start") {
        start = gettimeofday_us();
    probe process.provider("php").mark("oci8-nls_done") {
        tottime += gettimeofday_us() - start;
    probe end {
        printf("Connects: %d, Charset lookups: %ld\n", numconnects, numcharlookups);
        printf("Total NLS charset initalization time: %ld usecs/connect\n", 
                            (numcharlookups > 0 ? tottime/numcharlookups : 0));

    This calculates the average time that the NLS character set lookup takes. It also prints out the username of each connection, as an example of using parameters.

  7. Login as root and run Systemtap over the PHP script:

    # cd ~cjones/php-src
    # stap -c 'sapi/cli/php ~cjones/test.php' ~cjones/oci.stp
    Connected as cj
    <?xml version="1.0"?>
    Connects: 1, Charset lookups: 1
    Total NLS charset initalization time: 164 usecs/connect

    This shows the time penalty of making OCI8 look up the default character set. This time would be zero if a character set had been passed as the fourth argument to oci_connect() in test.php.

Update: To use real (non-SystemTap) DTrace, the extension binary needs to be built slightly differently than shown above in step 2. Instead of modifying config.m4 to reuse PHP_INIT_DTRACE, an OCI8-specific variant of that macro is created. To see how this is done, look at the new OCI8_INIT_DTRACE macro definition and way it is used in PHP OCI8 2.0's ext/oci8/config.m4.

Tuesday Jan 19, 2010

Tracing PHP Oracle Applications, part 1

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