DTrace provider for Apache
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mod_usdt is an Apache2 module that implements an "httpd" DTrace provider, allowing you to trace Apache requests using DTrace. DTrace is a comprehensive dynamic tracing framework available on Illumos-based, BSD, and MacOS systems. For more information about DTrace, see http://en.wikipedia.org/wiki/DTrace.


Use the included examples/http-requests.d to view all HTTP requests as Apache completes them:

# dtrace -s examples/http-requests.d
 time    latency  sta            local                 remote        method uri
1.216   309850us  404  GET    /hello
3.103      646us  404  GET    /world

Or, for busier services, summarize latency over time with a histogram:

# dtrace -s examples/http-requests-bylatency.d
Tracing.  Hit CTRL-C to stop.


       value  ------------- Distribution ------------- count    
          64 |                                         0        
         128 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@       3367     
         256 |@@@@@@                                   578      
         512 |@                                        51       
        1024 |                                         3        
        2048 |                                         0        
        4096 |                                         0        
        8192 |                                         1        
       16384 |                                         0        


First, build the Apache module. You'll need a C compiler and the Apache headers available in some standard location. If you want to build 64-bit, first uncomment the appropriate line in the Makefile first. Then run:

# make

To install:

  1. (optional) Copy build/mod_usdt.so to wherever you keep your Apache module binaries.

  2. Modify the Apache configuration to include the new module by adding this line (specifing the appropriate path):

    LoadModule usdt_module path/to/mod_usdt.so

  3. Restart Apache.

  4. Copy src/httpd.d into /usr/lib/dtrace. You can skip this step, but then every dtrace(1M) invocation must use "-L" to specify the path to the directory containing this file.

4a Using Inside Zones: Due to the current OS limitation of accessing the value curpsinfo->pr_dmodel from within non-global zone; use the correct version of httpd.d (32 or 64-bit): src/httpd-zone-32.d or arc/httpd-zone-64.d respectively. Step #4 above applies.

4b An easy way to determine if the Apache is 32-bit or 64-bit is to execute the following in the global zone:

# dtrace -qn 'syscall::: /execname == "httpd"/ { ab = (curpsinfo->pr_dmodel == PR_MODEL_ILP32) ? "32-bit" : "64-bit" ; exit(0); } END { printf("Apache: %s",ab); }'
Apache: 32-bit

4c Copy the right version and execute the examples to ensure it works in the non-global zone:

# zlogin zone-percona0
[root@zone-percona0 ~/]# cd ~/mod_usdt; mkdir trans32 
[root@zone-percona0 ~/mod_usdt]# cp src/httpd-zone-32.d trans32/. 
[root@zone-percona0 ~/mod_usdt]# dtrace -L trans32/ -s examples/http-requests-bylatency.d 
dtrace: aggregation size lowered to 64k
Tracing.  Hit CTRL-C to stop.
       value  ------------- Distribution ------------- count    
         512 |                                         0        
        1024 |@@@@@@@@@@@@@@@@@@@@                     1        
        2048 |                                         0        
        4096 |                                         0        
        8192 |@@@@@@@@@@@@@@@@@@@@                     1        
       16384 |                                         0 

4c Full details by Dave Pacheco at: https://www.listbox.com/member/archive/184261/2012/01/sort/time_rev/page/1/entry/0:5/20120117154842:A29D5E5A-414C-11E1-A2C5-B34EA897F121/

The module is very simple: it registers hooks for the beginning and end of each request. In each hook function, if the corresponding DTrace probe is enabled, it constructs the appropriate arguments and fires the probe. The module thus has minimal overhead whether the DTrace probes are enabled or not.

Provider details

This provider defines the following probes:

  • httpd$pid:::request-start: request received
  • httpd$pid:::request-start: request completed (response sent)

Both probes are passed two arguments:

  • arg0 (conninfo_t): IP connection details
  • arg1 (httpd_rqinfo_t): HTTP request details

The conninfo_t is the same one used by the NFS, iSCSI, and various other network protocol providers, included here for reference from /usr/lib/dtrace/net.d:

typedef struct conninfo {
	string ci_local;	/* local host address */
	string ci_remote;	/* remote host address */
	string ci_protocol;	/* protocol (ipv4, ipv6, etc) */
} conninfo_t;

The httpd_rqinfo_t structure looks like this:

typedef struct {
	uint64_t	rq_id;		/* unique request identifier */
	uint16_t	rq_status;	/* status code (only on "done" probe) */
	uint16_t	rq_lport;	/* local TCP port */
	uint16_t	rq_rport;	/* remote TCP port */
	string		rq_method;	/* requested method */
	string 		rq_uri;		/* requested URI */
	string 		rq_agent;	/* user agent string */
} httpd_rqinfo_t;

See the source of the example scripts above to see how these structures can be used.


This module is fully functional on 32-bit Apache. 64-bit support is forthcoming. It has only been tested on SmartOS, an Illumos-based (Solaris-based) system. It should be possible to modify this for MacOS and BSD; pull requests welcome.