Lies, Damn Lies, and PHP Benchmarks

I need to get something off my chest. First, I’d like you to examine the following code:

 
class Test
{
    public function output ()
    {
        return 'Hello, world!';
    }
}
 
function test ()
{
    return 'Hello, world!';
}
 
$start = microtime(true);
for ($i = 0; $i < 10000000; $i++) {
    test();
}
printf("%0.2f seconds for 10 million function calls.\n", microtime(true)-$start);
 
$start = microtime(true);
for ($i = 0; $i < 10000000; $i++) {
    $c = new Test();
    $c->output();
}
printf("%0.2f seconds for 10 million class calls.\n", microtime(true)-$start);
 
// Outputs about 4.9 seconds for the first and 7.9 for the second on my system.

How many times have you seen a benchmark like this passed off as proof of some facet of PHP’s behavior, particularly relating to classes versus functions? Next, ask yourself: What does this “benchmark” prove?

The answer: Absolutely nothing. Most developers are already aware (or should be) that instantiation of a class invokes a fair amount of overhead not present with functions in PHP. Yet time and again, whenever I encounter a discussion relating to PHP best practices or performance, I find comments that allude to any of the following:

  • PHP is not Java!
  • ${framework} uses classes, therefore ${framework} is slow!
  • PHP is a templating language. Replicating OOP concepts in PHP is overkill/stupid/slow.

Usually, but not always, one (or more) of the above opinions is presented with an allusion to benchmarks not all that dissimilar from the horrible, horrible sample of code at the beginning of this post. Why?

I don’t have a good answer, but I suspect it’s because PHP is one of the most oft-benchmarked scripting languages of our time, probably because the barrier to entry is so low. The worst part? No one knows how to benchmark. Granted, I’m guilty of the same charge, and I’ve published awful benchmarks in the past somehow “proving” (for some value of proof) that a specific feature is slower than another. So, as penance for my own wrongdoings, I want to make a point, and I want to make it as clear as I possibly can.

Your PHP benchmarks are wrong.

Don’t feel bad, though. Nearly all of them are wrong, including most hardware benchmarks. The reason for this is benchmarks, by their nature, are synthetic, and the nature of a synthetic benchmark is such that it fails to capture real world behavior. If you want to create a realistic benchmark, you’re going to have to put an awful lot of work into emulating a full stack, and the only way to do this correctly is to effectively write a small application. You can’t simply toss a few function calls into a for loop and call it a day. But the problem with this approach is that once you’ve implemented a demonstration application, you’re no longer benchmarking the language–you’re benchmarking your library or application. The only way you can really benchmark a language is to do so in a manner that can be replicated across multiple platforms in a manner that each are roughly equivalent and the benchmark captures the relative performance of each.

If you don’t buy that argument, I’d suggest you look at the TechEmpower framework benchmarks. They’ve put a lot of work into creating a fair, realistic collection of benchmarks for dozens and dozens of frameworks for each of the popular languages. If you’re not willing to invest a similar magnitude of effort into your own benchmarks, you’re not going to prove anything. You won’t prove that a specific language feature is too slow to use, you won’t prove that functions are better than classes, and all you’re going to accomplish is wasting your time–and worse–your readers’ time.

I’ll explain further, but before I delve into greater detail about why looping over a function ten million times proves absolutely nothing, I want to demonstrate by example. Nothing beats a good illustration to broaden one’s horizons, so I’ve fabricated a slightly less terrible benchmark than the code snippet that began this post.

A Slightly Less Terrible Benchmark

This benchmark is intended to illustrate two objectives: 1) That PHP language features don’t differ by much in terms of performance relative to each other and 2) that simple benchmarks are effectively pointless and only prove what the author intends for them to prove. The benchmarks are broken down into six files:

  • out-class-inheritance.php – Uses class inheritance to output a short string.
  • out-class-interface-inheritance.php – Uses an interface to define class methods and inheritance to output a short string.
  • out-class-interface.php – Uses an interface to define class methods to output a short string.
  • out-class.php – Use a simple class to output a short string.
  • out-class-static.php – Uses a static method of a class to output a short string.
  • out-function.php – Uses a single function to output a short string.

Each benchmark was run three times using Apache Bench (ab) on Arch Linux with a concurrency of 10 across 10000 runs. In keeping with the tradition of poorly designed benchmarks, ab was run on the same system as the web server. It’s a terrible idea, I know, but we’re only measuring approximate performance of individual language features with the intent to prove that there isn’t a substantial difference.

Note that I won’t include the sources to this benchmark here. If you want to examine them, please review this Gist. They’re about as simple as the code fragment at the top of this post and only slightly less stupid.

Performance

Unsurprisingly, for all intents and purposes, each test performed approximately the same. I noticed that tabbing between the shell and my browser introduced sharp drops in performance, so a handful of these had to be re-run from the start. So, I suspect that the reductions in performance as seen for the inheritance and interface tests were likely introduced by system variability. Yes, inheritance should be slightly slower (more overhead) but it’s not substantially slower:

  • out-class-inheritance.php – 15407 req/s, 0.65s total
  • out-class-interface-inheritance.php – 14854 req/s, 0.674s total
  • out-class-interface.php – 14568 req/s, 0.688s total
  • out-class.php – 15367 req/s, 0.651s total
  • out-class-static.php – 15633 req/s, 0.64s total
  • out-function.php – 15146 req/s, 0.66s total

Are these results surprising? They shouldn’t be. This is a benchmark and benchmarks lie. My benchmark lies because it isn’t illustrative of real world use, and as such, these numbers mean absolutely nothing. Well, okay, my results are suggestive that if all things are equal, OOP versus functional design is a meaningless argument. But these results don’t matter because no one in their right mind is going to have an application that consists of a small single class or function. Likewise, no one’s going to run the same function or instantiate a class a few million times in their app and call it good. At least, I’d hope not.

What this does illustrate is that a benchmark can be manipulated to produce desired results, and they can be interpreted with greater variability than most religious texts. While I’d like to believe this benchmark demonstrates that there’s little difference between PHP language features, it isn’t exhaustive enough to measure the impact of specific design decisions. Nor should it, because PHP doesn’t exist in a vacuum and naive designs fail to account for realistic implementations.

The problem, then, is that benchmarks running a small sample of code a million times fail to account for the broader design of a full application, which might be comprised of a few dozen functions, classes, hundreds of queries and so forth. No one class is going to be run a million times for every hit (or it shouldn’t be), and neither will any one function. More importantly, as my benchmarks demonstrate, the real bottleneck is going to be network I/O, followed by disk, and with rare exceptions (and a distant third), the CPU. For dissenters, I would wager that you’re going to encounter network limitations well before any hypothetical “classes are slower than functions” condition is met. Moreover, because the PHP VM is rather slow, it won’t matter a great deal how your application is structured anyway. An application written in Go or C++ is going to perform several orders of magnitude faster than you PHP app. For that matter, an application written using Python, Gevent, and Gunicorn would also render your pet PHP constructs a rather embarrassingly distant last place. Save for frameworks written in C (Phalcon) or new clean-room PHP implementations (like HHVM), the entire debate over classes versus functions is a rather silly one, isn’t it?

Benchmarks are Relative

I can’t emphasize how fascinating and well designed the TechEmpower benchmarks are for illustrative relative performance among different languages and frameworks. That said, I must emphasize again that nothing exists in a vacuum. PHP classes or functions aren’t solitary constructs. They’re running in unison with a web server, a database engine of some sort, and possibly dozens of other platforms, each with a certain amount of overhead or latency, and even network topography can impact service behaviors in suboptimal ways.

What burns me the most is that there is this small population of noisy magpie-like developers pushing against efforts to improve PHP development (like the improvements in the PSR series) toward sensible standards. While this population is diminishing, I still see comments from time to time on sites like Hacker News that effectively blame OOP design for PHP’s comparatively poor performance on the web framework benchmark, particularly for frameworks like Symfony. Yet Facebook’s HHVM and, to a lesser extent, the Phalcon PHP framework have demonstrated that this need not be the case. Why is PHP plagued with this sort of nonsense? I have no idea.

Consider for a moment the benefits gleaned from the PSR standards like PSR-0 autoloading and the fairly recent explosion in the number of libraries that, thanks to Composer and Packagist, can be included in any project with little effort. Furthermore, clean design and implementation induces new momentum in the PHP community that will hopefully render spaghetti code like that in projects using archaic practices (vBulletin and IP.Board, I’m looking at you…) a forgotten memory. OOP isn’t a panacea, but when used correctly, it encourages code reuse and generally reduces the time spent on implementation. It’s disappointing that there are musings in the community about whether or not OOP practices have a place in PHP. If you don’t think OOP belongs in PHP, then I suspect you’ve missed that debate by about a decade and a half. Better late than never, right?

Fortunately, as Composer and Packagist have illustrated, such opposition to modern PHP design is constrained to a tiny and ever-dwindling population of the PHP community (I’d actually wager it’s mostly people have have merely dabbled in PHP and don’t currently write much code in it, if any). PHP has its warts, but modern PHP largely abolishes or mitigates some of the worst parts of the language. That isn’t to say PHP is a beautiful language (it’s not), but it doesn’t have to be beautiful to be useful. Perl taught us that lesson years ago.

I think I see an older gentleman in the back with a long gray beard laughing. See? He gets it.

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No Decode Delegates… and Conflicts

It’s not often that we run into unusual conflicts rooted in hard to find places, but those few times that they are encountered–no matter how rare–can be infuriating. I was recently bitten by this one, demonstrated by a particularly bizarre error when attempting to load an image (any type) into the php-imagick ImageMagick extension:

$ php -a 
Interactive shell
 
php > $c = new Imagick('kmix.png');
PHP Warning:  Uncaught exception 'ImagickException' with message 'no decode delegate for this image format `kmix.png' @ error/constitute.c/ReadImage/552' in php shell code:1
Stack trace:
#0 php shell code(1): Imagick->__construct('kmix.png')
#1 {main}
  thrown in php shell code on line 1

Yet, paradoxically, convert -list delegate, convert -list configure, and convert -list format all hinted that PNG and JPEG support were both installed. Worse, searching for “php imagick no decode delegate” resulted in few useful hits, and of those hits most were indicative of configuration problems, custom builds missing the appropriate libraries, and other dilemmas punctuated by the usual PEBKAC retort (though such accusations are sometimes accurate).

Nothing worked, of course. Swapping out to older versions of ImageMagick failed, and I had no intention of trying an earlier version of PHP or php-imagick. At this point, I had no expectations to find the solution online, and the only thing I surmised might work would be to attempt duplicating the problem on another machine. So, I set about installing ImageMagick (v6.8.5-10) and php-imagick (v3.0.1 plus PHP 5.4 patches and v3.1.0RC2) with the current-latest version of PHP available in the Arch repositories (v5.4.16) and quickly discovered that it worked quite well. This was a positive sign: Nothing about the latest builds indicated there was a problem, so it was plausible that the problem lurked elsewhere.

And it did.

I soon noted that the virtual machine I booted was lacking php-gmagick and GraphicsMagick both, though ImageMagick and GraphicsMagick worked quite well on my machine together. Thus, I speculated that perhaps php-gmagick was conflicting somehow with php-imagick. I commented out the gmagick extension, re-ran some unit tests, and presto! php-imagick was now working perfectly fine. Uncommenting the gmagick extension resulted in a return of the problem, and so I was (and am) convinced that the problem is a conflict between both extensions. Possibly, this is due to the definition of symbols or other such nonsense in gmagick that imagick requires (and is very upset when it discovers they already exist).

Is it possible to get them to play nicely together? Maybe.

On my machine, PHP extensions and their options are loaded from /etc/php/conf.d. These files are, apparently, loaded alphabetically in order, and since “G” comes before “I,” you might imagine (correctly) then that gmagick gets loaded before imagick which is where the magic stops (my apologies; that pun was awful). By renaming them to something creative, such as “10-imagick.ini” and “20-gmagick.ini”, it is then possible to force imagick to load before gmagick, and they’ll both work cooperatively near as I can tell. Perhaps gmagick defines–but doesn’t use–some interpreter symbols that imagick actually needs, but if they’re already defined gmagick doesn’t care? I don’t know. I can only speculate, because I know next to nothing about PHP internals. (Perhaps now might be a good time to start.)

Although renaming or otherwise moving the extension configs into an order that would suggest imagick.so gets loaded first seems like an adequate solution, the problem is that these two extensions most assuredly do conflict. Between PHP versions, you can’t be guaranteed that the loader mechanism for extensions will remain static and thus the possibility exists that renaming the *.ini files is only an ephemeral solution. Moreover, it’s also possibly an accident that it works on my machine; perhaps the inode order is such that imagick.so now gets loaded before gmagick.so simply due to luck. Or maybe I’ve now reduced it to a race condition? The point is that you shouldn’t rely on this in production. Instead, you should select between GraphicsMagick or ImageMagick, picking the one that best fits your use case. If you’re using a wrapper library to select the best backend available, force it to load a specific one in production mode. Obviously such concerns aren’t much of a bother on a development box, but I’d nevertheless recommend a healthy dose of caution.

If you’ve encountered this issue yourself but never found a reasonable solution, let me know. If this article helps in any measurable way, I’d be happy to hear from you. It’s been a frustrating few hours for me, and I hope that this will at least alleviate you of wasting further time on something that is poorly documented but easy to fix.

2 comments.
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Musing about… Prepared Statements

A spurt of curiosity this evening–more specifically, one of those circumstances we each have from time to time wherein a handful of unrelated thoughts flutter about the conscious mind like a pair of butterflies flitting from flower to flower–consumed me sufficiently that I decided to do a brief Google search on prepared statements. I’m unsure where such a motive originated, but I’m fairly convinced that it was at least tangentially related to some misinformation I’ve heard of late related to web programming advice and also possibly due to my surprise that few commercial PHP bulletin board packages actually use prepared statements.

Before I begin, let’s consider for a moment that last and most disconcerting statement: Few commercial PHP forums use prepared statements. To the uninitiated, this might seem to be a matter of nick-picking unimportant to the real world. To the rest of you, it may come as a sad commentary on the state of modern programming and commercial software (perhaps, fittingly, as a commentary on the average run-of-the-mill PHP programmer). Prepared statements certainly aren’t new, and while they’ve been a part of PHP for a number of years now, it’s infuriating that they hardly see common use.

PHP first shipped PDO with PHP 5.1 (available as a PECL extension for PHP 5.0, circa 2004-2005). Intriguingly, for systems that don’t provide PDO support (or the appropriate drivers for PDO), the MySQLi and PostgreSQL functions and classes have provided prepared statements for quite some time, and the SQLite 3 drivers have provided a prepare() method since PHP 5.3. Commercial bulletin boards, like vBulletin and IPB, have seen many revisions in the years since, and several free/open source packages including phpBB have been part of similarly major overhauls. Yet the overwhelming majority of them still make no use of prepared statements. Humorously, as of this writing, vBulletin does provide a misleadingly-named sql_prepare method in its database class, but it doesn’t emulate prepared statements–it simply provides an escape wrapper with data type introspection and casting.

PDO has been available for nearly 8 years and many RDBMS drivers for PHP have offered prepared statements for at least that long (longer in the case of PostgreSQL if memory serves correctly). Yet every year or two, new major versions of popular PHP message boards are released, and every major release sees the same legacy database code under the hood. Perhaps it’s intentional. Perhaps the developers still want to support PHP 4.x in spite of the fact that it went EOL in 2008. Perhaps they just don’t know any better. Who knows!

Why Prepared Statements?

A prepared statement or parameterized statement, as it is occasionally known by in DBA parlance, is a specially-formatted SQL string that utilizes placeholders, either question marks (?), special named parameters (such as “:name”), or other database-specific strings, to indicate to the database or the driver where data is to be inserted. This has the benefit that, in theory at least, any data managed by a prepared statement is unlikely to serve as a vector for SQL injection attacks. The reason this works is because most drivers dispatch the prepared statement and its data separately on the wire and process them independently providing a certain degree of isolation. But wait, there’s more! Because of the implementation nature of prepared statements on most platforms, the query planner can often optimize and partially compile the statement such that, if it runs again, much of the legwork has already been completed and the query can run faster. Software like forums or blogs often execute the same query multiple times–with different data–so one might think it would be a natural fit. If it’s such a good thing, why do so many popular packages forgo such a benefit?

While I can’t answer for many developers, I think I know what at least part of the answer might be. First, for enormous code bases like vBulletin (and phpBB to a lesser extent), virtually no effort is made to separate the application logic from the underlying model. I’ll be fair in my distinction: The presentation layer is thankfully separated from the mess in the form of templates, but the remaining code is a bowl of spaghetti not unlike that of many of the very first PHP applications (and Perl!) that first graced the Internet over a decade ago. Because the model (and, by extension, the SQL) is so deeply entrenched in the functional logic of the application, reworking it to use prepared statements–and consider, also, that many of these queries are generated programmatically–would be a tremendous undertaking of many man-hours. Cleaning up the code properly such that it is more of a structurally sound framework (think MVC) is most certainly out of the picture. It isn’t impossible, of course, but when you consider that some functions in many of these software packages have persisted since the dawn of time, such refactoring becomes the thing of fairy tales.

To illustrate some of my displeasure, vBulletin version 4.2 still provides an iif function which is little more than a wrapper for the ternary operator (?:) in PHP. The ternary operator has been around since at least PHP4, yet there it is, in all its glory, a legacy function still available from the early days of PHP3 when such a beast didn’t exist!

One might think that it would simply be a matter of adding some logging code to old function calls, tracing the source that called them, and then reworking the culprit code to use built in language features. It might even take less than an afternoon.

The Drawbacks Programmer Mistakes

While prepared statements (parameterized queries for those of you who are embarrassingly excited by more elaborate verbiage) aren’t a panacea (I did it again) for all things SQL injection-like, they’re a good mitigation strategy, but it’s important to use them with caution. As Jason Lam states on the ISC Diary, “I still remember 4-5 years ago when SQL injection just started to become popular, the common mitigation suggested [was] to use prepared statement [sic] as if [they were] a magic bullet. As we [now] understand the SQL injection problem better, we realize that even prepared statement can be vulnerable to SQL injection as well.”

Well, yeah. This is where I smack my forehead. Maybe I’m being overly critical as I re-read a post from 5 years ago, because I’ve had the tremendously good fortune of witnessing some magnificently terrible code in my time as a web application developer.

Mr. Lam goes on to explain the insertion of unchecked user input, but I can’t shake the feeling that there is an implicit overtone in the article that it is somehow the fault of prepared statements. Perhaps more accurately, the article is faulting most of us for having championed prepared statements as a welcome solution to a very common and widespread problem. Realistically, though, it’s not an issue with prepared statements–they work just fine. It’s an issue with developers inappropriately using the tools at their disposal and doing so in a manner that simply transfers the vulnerability from query() to prepare() by forgetting to properly manage incoming data. Though, I should say that I’m inclined to suggest that programmatically assembling a prepared statement is somewhat counter-productive. More on this later.

Ironically, while doing some research for this article, I ran across a couple of posts on Stack Overflow that presented this problem of unchecked user input as one of the primary drawbacks of prepared statements. Really? Drawbacks? If you’re not using named parameters or placeholders for your query data, you’re probably not using prepared statements correctly! But drawbacks? Gee, maybe we were a little too vigilant in telling people to use prepared statements–so much so that they did a find/replace for query and swapped it with prepare. (I’m being facetious; so, to head off any comments to the contrary, it’s not at all possible to simply swap some text, because prepared statements do require a little more work.)

The problem I have with labeling unchecked user input as a drawback of prepared statements is that it is no longer a “real” prepared statement whenever such data is concatenated with the resulting query. Yes, it is still a prepared statement, insofar as calling prepare() on the driver’s end, but it’s no longer being used like a prepared statement. Here’s a hint to new developers, particularly PHP developers since a huge percentage of them are guilty of doing stupid things like this: Never concatenate unchecked input in any query–prepared or otherwise. If you’re using a prepared statement, use it like a damned prepared statement. The moment you start piping data into the query string itself, it’s no longer going to have the benefits of a prepared statement. (I’ll give you a special exception if you’re using LIMIT and concatenating integers with your queries since not all of you may be running MySQL 5.0.7 or later.)

Will the Real Prepared Statement Please Step Forward?

In my mind, and trust me, it’s a very strange place in here, a prepared statement is one that may contain parameters and is “prepared” ahead of time for reuse (that is, compiled) by the driver or the RDBMS (usually the RDBMS). Nothing more, nothing less. The instant some unfiltered data is slapped on to the end of the query, it’s no longer a pure prepared statement; instead, it becomes a mistake. Again: Prepared statements are parameterized queries that are usually compiled by the backend for a little extra speed. A query can contain anything else that the programmer adds into it, but fundamentally, a prepared statement is something that dictates a very specific structure. It certainly cannot overcome the mistakes of a naive developer who, believing that a prepared statement will magically fix all of their (singular they, sorry linguistic purists) security-related woes, use such a tool in addition to dangerous techniques like concatenating unchecked input. Another way to look at it is thus: If prepared statements are prepared (that is, compiled) by the database for reuse, and the developer is concatenating a dynamic value to the statement, the entire benefit of preparing (compiling) that statement is immediately lost, because the RDBMS has to re-compile the statement every single time it’s sent along the wire. Please, don’t do this.

Of course, there may be reasons not to use prepared statements all the time. For one, prepared statements in MySQL versions prior to 5.1 can no longer be managed by the query cache which may impact performance (High Performance MySQL, 2nd ed., p229). DBMSs that don’t support prepared statements can, in PDO at least, can have them emulated by the PDO driver at the cost of some pre-processing performance, and using older PHP functions like the popular-but-now-deprecated mysql_* ones just outright don’t support anything but basic queries (they also don’t use the binary interface, making them somewhat slower). If you’re using only a single query with absolutely no intention of reusing it, prepared statements may incur some overhead since the query must be compiled. Furthermore, for MySQL at least, if you’re not using stored procedures, the database has no way to share compiled prepared statements among multiple connections. Yet, while a prepared statement is no substitute for caution–particularly with programmatically-generated queries–it is a useful tool in the developer’s arsenal to protect against attacks like SQL injection. Plus, if you make it a habit to use PDO (you should), not only do you get emulated prepared statements for databases that don’t support them, you also get to use the modern MySQL APIs under the hood and some consistency, which says a lot in the PHP world.

Tangentially, this is also why it boggles my mind that many sites (banks, news agencies, airlines, and even some game companies) limit what characters the user can enter for their password, and how so many companies with an online presence often have draconian limits of less than 16 characters, inclusive. Seriously: If you’re correctly storing a secure hash of the password (HMAC, bcrypt, scrypt, or at least SHA256 or similar), you don’t need to store the password directly, nor does it matter if the password is 5 or 500 characters. It’s going to be comprised of a fixed length of some limited set of ASCII characters representing hexadecimal numbers which can be stored without much fuss. The 1990s were over two decades away. I think it’s time we stopped writing code like Y2K is still a viable problem.

Also, let’s start using prepared statements a little more often.

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