XHProf is a hierarchical profiler for PHP. It reports
function-level call counts and inclusive and
exclusive metrics such as wall (elapsed)
time, CPU time and memory usage. A function's profile can be broken
down by callers or callees. The raw data collection component is
implemented in C as a PHP Zend extension called
xhprof. XHProf has a simple HTML based user
interface (written in PHP). The browser based UI for viewing profiler
results makes it easy to view results or to share results with peers.
A callgraph image view is also supported.
XHProf reports can often be helpful in understanding the structure of the code being executed. The hierarchical nature of the reports can be used to determine, for example, what chain of calls led to a particular function getting called.
XHProf supports ability to compare two runs (a.k.a. "diff" reports) or aggregate data from multiple runs. Diff and aggregate reports, much like single run reports, offer "flat" as well as "hierarchical" views of the profile.
XHProf is a light-weight instrumentation based profiler. During the data collection phase, it keeps track of call counts and inclusive metrics for arcs in the dynamic callgraph of a program. It computes exclusive metrics in the reporting/post processing phase. XHProf handles recursive functions by detecting cycles in the callgraph at data collection time itself and avoiding the cycles by giving unique depth qualified names for the recursive invocations.
XHProf's light-weight nature and aggregation capabilities make it well suited for collecting "function-level" performance statistics from production environments. [See additional notes for use in production.]
XHProfLive (not part of the open source kit), for example, is a system-wide performance monitoring system in use at Facebook that is built on top of XHProf. XHProfLive continually gathers function-level profiler data from production tier by running a sample of page requests under XHProf. XHProfLive then aggregates the profile data corresponding to individual requests by various dimensions such as time, page type, and can help answer a variety of questions such as: What is the function-level profile for a specific page? How expensive is function "foo" across all pages, or on a specific page? What functions regressed most in the last hour/day/week? What is the historical trend for execution time of a page/function? and so on.
Originally developed at Facebook, XHProf was open sourced in Mar, 2009.
XHProf provides:
Function-level summary information such as number of calls, inclusive/exclusive wall time, memory usage, and CPU time.
For each function, it provides a breakdown of calls and times per parent (caller) & child (callee), such as:
You may want to compare data from two XHProf runs for various reasons-- to figure out what's causing a regression between one version of the code base to another, to evaluate the performance improvement of a code change you are making, and so on.
A diff report takes two runs as input and provides both flat function-level diff information, and hierarchical information (breakdown of diff by parent/children functions) for each function.
The "flat" view (sample screenshot) in the diff report points out the top regressions & improvements.
Clicking on functions in the "flat" view of the diff report, leads to the "hierarchical" (or parent/child) diff view of a function (sample screenshot). We can get a breakdown of the diff by parent/children functions.
The profile data can also be viewed as a callgraph. The callgraph view highlights the critical path of the program.
XHProf's memory profile mode helps track functions that allocate lots of memory.
It is worth clarifying that that XHProf doesn't strictly track each allocation/free operation. Rather it uses a more simplistic scheme. It tracks the increase/decrease in the amount of memory allocated to PHP between each function's entry and exit. It also tracks increase/decrease in the amount of peak memory allocated to PHP for each function.
include, include_once, require and
require_once operations as if they were functions. The name of
the file being included is used to generate the name for these "fake" functions.
main(): a fictitious function that is at the root of the call graph.
load::<filename>
and run_init::<filename>:
XHProf tracks PHP include/require operations as
function calls.
For example, an include "lib/common.php"; operation will result in two XHProf function entries:
load::lib/common.php - This represents the work done by the
interpreter to compile/load the file. [Note: If you are using a PHP
opcode cache like APC, then the compile only happens on a cache miss
in APC.]
run_init::lib/common.php - This represents
initialization code executed at the file scope as a result of the
include operation.
foo@<n>: Implies that this is a
recursive invocation of foo(), where <n> represents
the recursion depth. The recursion may be direct (such as due to
foo() --> foo()), or indirect (such as
due to foo() --> goo() --> foo()).
True hierarchical profilers keep track of a full call stack at every data gathering point, and are later able to answer questions like: what was the cost of the 3rd invokation of foo()? or what was the cost of bar() when the call stack looked like a()->b()->bar()?
XHProf keeps track of only 1-level of calling context and is therefore only able to answer questions about a function looking either 1-level up or 1-level down. It turns out that in practice this is sufficient for most use cases.
To make this more concrete, take for instance the following example.
Say you have: 1 call from a() --> c() 1 call from b() --> c() 50 calls from c() --> d()
While XHProf can tell you that d() was called from c() 50 times, it cannot tell you how many of those calls were triggered due to a() vs. b(). [We could speculate that perhaps 25 were due to a() and 25 due to b(), but that's not necessarily true.]
In practice however, this isn't a very big limitation.
The extension lives in the "extension/" sub-directory.
Note: A windows port hasn't been implemented yet. We have
tested xhprof on Linux/FreeBSD so far.
Version 0.9.2 and above of XHProf is also expected to work on Mac OS. [We have tested on Mac OS 10.5.]
Note: XHProf uses the RDTSC instruction (time stamp counter)
to implement a really low overhead timer for elapsed time. So at the
moment xhprof only works on x86 architecture.
Also, since RDTSC values may not be synchronized across CPUs,
xhprof binds the program to a single CPU during the
profiling period.
XHProf's RDTSC based timer functionality doesn't work correctly if SpeedStep technology is turned on. This technology is available on some Intel processors. [Note: Mac desktops and laptops typically have SpeedStep turned on by default. To use XHProf, you'll need to disable SpeedStep.]
The steps below should work for Linux/Unix environments.
% cd <xhprof_source_directory>/extension/ % phpize % ./configure --with-php-config=<path to php-config> % make % make install % make test
php.ini file: You can update your php.ini file to automatically load your extension. Add the following to your php.ini file.
[xhprof] extension=xhprof.so ; ; directory used by default implementation of the iXHProfRuns ; interface (namely, the XHProfRuns_Default class) for storing ; XHProf runs. ; xhprof.output_dir=<directory_for_storing_xhprof_runs>
Test generating raw profiler data using a sample test program like:
foo.php
<?php
function bar($x) {
if ($x > 0) {
bar($x - 1);
}
}
function foo() {
for ($idx = 0; $idx < 2; $idx++) {
bar($idx);
$x = strlen("abc");
}
}
// start profiling
xhprof_enable();
// run program
foo();
// stop profiler
$xhprof_data = xhprof_disable();
// display raw xhprof data for the profiler run
print_r($xhprof_data);
Run the above test program:
% php -dextension=xhprof.so foo.php
You should get an output like:
Array
(
[foo==>bar] => Array
(
[ct] => 2 # 2 calls to bar() from foo()
[wt] => 27 # inclusive time in bar() when called from foo()
)
[foo==>strlen] => Array
(
[ct] => 2
[wt] => 2
)
[bar==>bar@1] => Array # a recursive call to bar()
(
[ct] => 1
[wt] => 2
)
[main()==>foo] => Array
(
[ct] => 1
[wt] => 74
)
[main()==>xhprof_disable] => Array
(
[ct] => 1
[wt] => 0
)
[main()] => Array # fake symbol representing root
(
[ct] => 1
[wt] => 83
)
)
Note: The raw data only contains "inclusive" metrics. For example, the wall time metric in the raw data represents inclusive time in microsecs. Exclusive times for any function are computed during the analysis/reporting phase.
Note: By default only call counts & elapsed time is profiled. You can optionally also profile CPU time and/or memory usage. Replace,
xhprof_enable();in the above program with, for example:
xhprof_enable(XHPROF_FLAGS_CPU + XHPROF_FLAGS_MEMORY);
You should now get an output like:
Array
(
[foo==>bar] => Array
(
[ct] => 2 # number of calls to bar() from foo()
[wt] => 37 # time in bar() when called from foo()
[cpu] => 0 # cpu time in bar() when called from foo()
[mu] => 2208 # change in PHP memory usage in bar() when called from foo()
[pmu] => 0 # change in PHP peak memory usage in bar() when called from foo()
)
[foo==>strlen] => Array
(
[ct] => 2
[wt] => 3
[cpu] => 0
[mu] => 624
[pmu] => 0
)
[bar==>bar@1] => Array
(
[ct] => 1
[wt] => 2
[cpu] => 0
[mu] => 856
[pmu] => 0
)
[main()==>foo] => Array
(
[ct] => 1
[wt] => 104
[cpu] => 0
[mu] => 4168
[pmu] => 0
)
[main()==>xhprof_disable] => Array
(
[ct] => 1
[wt] => 1
[cpu] => 0
[mu] => 344
[pmu] => 0
)
[main()] => Array
(
[ct] => 1
[wt] => 139
[cpu] => 0
[mu] => 5936
[pmu] => 0
)
)
Skipping builtin functions during profiling
By default PHP builtin functions (such as strlen) are
profiled. If you do not want to profile builtin functions (to either
reduce the overhead of profiling further or size of generated raw
data), you can use the XHPROF_FLAGS_NO_BUILTINS
flag as in for example:
// do not profile builtin functions xhprof_enable(XHPROF_FLAGS_NO_BUILTINS);
Ignoring specific functions during profiling (0.9.2 or higher)
Starting with release 0.9.2 of xhprof, you can tell XHProf to
ignore a specified list of functions during profiling. This allows you
to ignore, for example, functions used for indirect function calls
such as call_user_func and
call_user_func_array. These intermediate functions
unnecessarily complicate the call hierarchy and make the XHProf
reports harder to interpret since they muddle the parent-child
relationship for functions called indirectly.
To specify the list of functions to be ignored during profiling
use the 2nd (optional) argument to xhprof_enable.
For example,
// elapsed time profiling; ignore call_user_func* during profiling
xhprof_enable(0,
array('ignored_functions' => array('call_user_func',
'call_user_func_array')));
or,
// elapsed time + memory profiling; ignore call_user_func* during profiling
xhprof_enable(XHPROF_FLAGS_MEMORY,
array('ignored_functions' => array('call_user_func',
'call_user_func_array')));
The XHProf UI is implemented in PHP. The code resides in two
subdirectories, xhprof_html/ and xhprof_lib/.
The xhprof_html directory contains the 3 top-level PHP pages.
index.php: For viewing a single run or diff report.
callgraph.php: For viewing a callgraph of a XHProf run as an image.
typeahead.php: Used implicitly for the function typeahead form
on a XHProf report.
The xhprof_lib directory contains supporting code for
display as well as analysis (computing flat profile info, computing
diffs, aggregating data from multiple runs, etc.).
Web server config: You'll need to make sure that the
xhprof_html/ directory is accessible from your web server, and that
your web server is setup to serve PHP scripts.
Managing XHProf Runs
Clients have flexibility in how they save the XHProf raw data obtained from an XHProf run. The XHProf UI layer exposes an interface iXHProfRuns (see xhprof_lib/utils/xhprof_runs.php) that clients can implement. This allows the clients to tell the UI layer how to fetch the data corresponding to a XHProf run.
The XHProf UI libaries come with a default file based implementation of the iXHProfRuns interface, namely "XHProfRuns_Default" (also in xhprof_lib/utils/xhprof_runs.php). This default implementation stores runs in the directory specified by xhprof.output_dir INI parameter.
A XHProf run must be uniquely identified by a namespace and a run id.
a) Saving XHProf data persistently:
Assuming you are using the default implementation
XHProfRuns_Default of the
iXHProfRuns interface, a typical XHProf run
followed by the save step might look something like:
// start profiling
xhprof_enable();
// run program
....
// stop profiler
$xhprof_data = xhprof_disable();
//
// Saving the XHProf run
// using the default implementation of iXHProfRuns.
//
include_once $XHPROF_ROOT . "/xhprof_lib/utils/xhprof_lib.php";
include_once $XHPROF_ROOT . "/xhprof_lib/utils/xhprof_runs.php";
$xhprof_runs = new XHProfRuns_Default();
// Save the run under a namespace "xhprof_foo".
//
// **NOTE**:
// By default save_run() will automatically generate a unique
// run id for you. [You can override that behavior by passing
// a run id (optional arg) to the save_run() method instead.]
//
$run_id = $xhprof_runs->save_run($xhprof_data, "xhprof_foo");
echo "---------------\n".
"Assuming you have set up the http based UI for \n".
"XHProf at some address, you can view run at \n".
"http://<xhprof-ui-address>/index.php?run=$run_id&source=xhprof_foo\n".
"---------------\n";
The above should save the run as a file in the directory specified
by the xhprof.output_dir INI parameter. The file's
name might be something like
49bafaa3a3f66.xhprof_foo; the two parts being the
run id ("49bafaa3a3f66") and the namespace ("xhprof_foo"). [If you
want to create/assign run ids yourself (such as a database sequence
number, or a timestamp), you can explicitly pass in the run id to the
save_run method.
b) Using your own implementation of iXHProfRuns
If you decide you want your XHProf runs to be stored differently (either in a compressed format, in an alternate place such as DB, etc.) database, you'll need to implement a class that implements the iXHProfRuns() interface.
You'll also need to modify the 3 main PHP entry pages (index.php,
callgraph.php, typeahead.php) in the "xhprof_html/" directory to use
the new class instead of the default class XHProfRuns_Default.
Change this line in the 3 files.
$xhprof_runs_impl = new XHProfRuns_Default();
You'll also need to "include" the file that implements your class in the above files.
Accessing runs from UI
a) Viewing a Single Run Report
To view the report for run id say <run_id> and namespace <namespace> use a URL of the form:
http://<xhprof-ui-address>/index.php?run=<run_id>&source=<namespace>
For example,
http://<xhprof-ui-address>/index.php?run=49bafaa3a3f66&source=xhprof_foo
b) Viewing a Diff Report
To view the report for run ids say <run_id1> and <run_id2> in namespace <namespace> use a URL of the form:
http://<xhprof-ui-address>/index.php?run1=<run_id1>&run2=<run_id2>&source=<namespace>
c) Aggregate Report
You can also specify a set of run ids for which you want an aggregated view/report.
Say you have three XHProf runs with ids 1, 2 & 3 in namespace "benchmark". To view an aggregate report of these runs:
http://<xhprof-ui-address>/index.php?run=1,2,3&source=benchmark
Weighted aggregations: Further suppose that the above three runs correspond to three types of programs p1.php, p2.php and p3.php that typically occur in a mix of 20%, 30%, 50% respectively. To view an aggregate report that corresponds to a weighted average of these runs using:
http://<xhprof-ui-address>/index.php?run=1,2,3&wts=20,30,50&source=benchmark
Some observations/guidelines. Your mileage may vary:
We recommend using elapsed time + memory profiling mode in production. [Note: The additional overhead of memory profiling mode is really low.]
// elapsed time profiling (default) + memory profiling xhprof_enable(XHPROF_FLAGS_MEMORY);
To profile say 1/10000 of your requests, instrument the beginning of your request processing with something along the lines of:
if (mt_rand(1, 10000) == 1) {
xhprof_enable(XHPROF_FLAGS_MEMORY);
$xhprof_on = true;
}
At the end of the request (or in a request shutdown function), you might then do something like:
if ($xhprof_on) {
// stop profiler
$xhprof_data = xhprof_disable();
// save $xhprof_data somewhere (say a central DB)
...
}
You can then rollup/aggregate these individual profiles by time
(e.g., 5 minutely/hourly/daily basis), page/request type,or other
dimensions using xhprof_aggregate_runs().
The xhprof extension also provides a very light weight sampling mode. The sampling interval is 0.1 secs. Samples record the full function call stack. The sampling mode can be useful if an extremely low overhead means of doing performance monitoring and diagnostics is desired.
The relevant functions exposed by the extension for using the
sampling mode are xhprof_sample_enable() and
xhprof_sample_disable().
[TBD: more detailed documentation on sampling mode.]
The xhprof_lib/utils/xhprof_lib.php file contains
additional library functions that can be used for manipulating/
aggregating XHProf runs.
For example:
xhprof_aggregate_runs():
can be used to aggregate multiple XHProf runs into a single run. This
can be helpful for building a system-wide "function-level" performance
monitoring tool using XHProf. [For example, you might to roll up
XHProf runs sampled from production periodically to generate hourly,
daily, reports.]
xhprof_prune_run(): Aggregating large number of
XHProf runs (especially if they correspond to different types of
programs) can result in the callgraph size becoming too large. You can
use xhprof_prune_run function to prune the callgraph data
by editing out subtrees that account for a very small portion of the
total time.
xhprof_html/jquery subdirectory.
The HTML-based navigational interface for browsing profiler results is inspired by that of a similar tool that exists for Oracle's stored procedure language, PL/SQL. But that's where the similarity ends; the internals of the profiler itself are quite different.