Advanced PHP Programming- P10

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  1. 18 Profiling I F YOU PROGRAM PHP PROFESSIONALLY, THERE is little doubt that at some point you will need to improve the performance of an application. If you work on a high-traffic site, this might be a daily or weekly endeavor for you; if your projects are mainly intranet ones, the need may arise less frequently. At some point, though, most applications need to be retuned in order to perform as you want them to. When I’m giving presentations on performance tuning PHP applications, I like to make the distinction between tuning tools and diagnostic techniques. Until now, this book has largely focused on tuning tools: caching methodologies, system-level tunings, database query optimization, and improved algorithm design. I like to think of these techniques as elements of a toolbox, like a hammer, a torque wrench, or a screwdriver are elements of a handyman’s toolbox. Just as you can’t change a tire with a hammer, you can’t address a database issue by improving a set of regular expressions.Without a good toolset, it’s impossible to fix problems; without the ability to apply the right tool to the job, the tools are equally worthless. In automobile maintenance, choosing the right tool is a combination of experience and diagnostic insight. Even simple problems benefit from diagnostic techniques. If I have a flat tire, I may be able to patch it, but I need to know where to apply the patch. More complex problems require deeper diagnostics. If my acceleration is sluggish, I could simply guess at the problem and swap out engine parts until performance is acceptable.That method is costly in both time and materials. A much better solution is to run an engine diagnostic test to determine the malfunctioning part. Software applications are in general much more complex than a car’s engine, yet I often see even experienced developers choosing to make “educated” guesses about the location of performance deficiencies. In spring 2003 the Web sites experienced some extreme slowdowns. Inspection of the Apache Web server logs quickly indicated that the search pages were to blame for the slowdown. However, instead of profiling to find the specific source of the slowdown within those pages, random guessing was used
  2. 430 Chapter 18 Profiling to try to solve the issue.The result was that a problem that should have had a one-hour fix dragged on for days as “solutions” were implemented but did nothing to address the core problem. Thinking that you can spot the critical inefficiency in a large application by intuition alone is almost always pure hubris. Much as I would not trust a mechanic who claims to know what is wrong with my car without running diagnostic tests or a doctor who claims to know the source of my illness without performing tests, I am inherently skepti- cal of any programmer who claims to know the source of an application slowdown but does not profile the code. What Is Needed in a PHP Profiler A profiler needs to satisfy certain requirements to be acceptable for use: n Transparency—Enabling the profiler should not require any code change. Having to change your application to accommodate a profiler is both highly inconvenient (and thus prone to being ignored) and intrinsically dishonest because it would by definition alter the control flow of the script. n Minimal overhead—A profiler needs to impose minimal execution overhead on your scripts. Ideally, the engine should run with no slowdown when a script is not being profiled and almost no slowdown when profiling is enabled. A high over- head means that the profiler cannot be run for production debugging, and it is a large source of internal bias (for example, you need to make sure the profiler is not measuring itself). n Ease of use—This probably goes without saying, but the profiler output needs to be easy to understand. Preferably there should be multiple output formats that you can review offline at your leisure.Tuning often involves a long cycle of introspec- tion and code change. Being able to review old profiles and keep them for later cross-comparison is essential. A Smorgasbord of Profilers As with most features of PHP, a few choices are available for script profilers: n Userspace profilers—An interesting yet fundamentally flawed category of profil- er is the userspace profilers.This is a profiler written in PHP.These profilers are interesting because it is always neat to see utilities for working with PHP written in PHP itself. Unfortunately, userspace profilers are heavily flawed because they require code change (every function call to be profiled needs to be modified to hook the profiler calls), and because the profiler code is PHP, there is a heavy bias generated from the profiler running. I can’t recommend userspace profilers for any operations except timing specific functions on a live application where you cannot install an extension-based profiler. Benchmark_Profiler is an example of a
  3. Installing and Using APD 431 userspace profiler in PEAR, and is available at Benchmark. n Advanced PHP Debugger (APD)—APD was developed by Daniel Cowgill and me. APD is a PHP extension-based profiler that overrides the execution calls in the Zend Engine to provide high-accuracy timings. Naturally, I am a little biased in its favor, but I think that APD provides the most robust and configurable profiling capabilities of any of the candidates. It creates trace files that are machine readable so they can be postprocessed in a number of different ways. It also pro- vides user-level hooks for output formatting so that you can send profiling results to the browser, to XML, or using any format you wanted. It also provides a step- ping, interactive debugger, which us not covered here. APD is available from PEAR’s PECL repository at n DBG—DBG is a Zend extension-based debugger and profiler that is available both in a free version and as a commercial product bundled with the commercial PHPEd code editor. DBG has good debugger support but lacks the robust profil- ing support of APD. DBG is available at n Xdebug—Xdebug is a Zend extension-based profiler debugger written by Derick Rethans. Xdebug is currently the best debugger of the three extension-based solu- tions, featuring multiple debugger interfaces and a robust feature set. Its profiling capabilities are still behind APD’s, however, especially in the ability to reprocess an existing trace in multiple ways. Xdebug is available from The rest of this chapter focuses on using APD to profile scripts. If you are attached to another profiler (and by all means, you should always try out all the options), you should be able to apply these lessons to any of the other profilers.The strategies covered here are independent of any particular profiler; only the output examples differ from one pro- filer to another. Installing and Using APD APD is part of PECL and can thus be installed with the PEAR installer: # pear install apd After ADP is installed, you should enable it by setting the following in your php.ini file: zend_extension=/path/to/ apd.dumpdir=/tmp/traces APD works by dumping trace files that can be postprocessed with the bundled pprofp trace-processing tool.These traces are dumped into apd.dumpdir, under the name, where pid is the process ID of the process that dumped the trace.
  4. 432 Chapter 18 Profiling To cause a script to be traced, you simply need to call this when you want tracing to start (usually at the top of the script): apd_set_pprof_trace(); APD works by logging the following events while a script runs: nWhen a function is entered. nWhen a function is exited. nWhen a file is included or required. Also, whenever a function return is registered, APD checkpoints a set of internal coun- ters and notes how much they have advanced since the previous checkpoint.Three counters are tracked: n Real Time (a.k.a. wall-clock time)—The actual amount of real time passed. n User Time—The amount of time spent executing user code on the CPU. n System Time—The amount of time spent in operating system kernel-level calls. Accuracy of Internal Timers APD’s profiling is only as accurate as the systems-level resource measurement tools it has available to it. On FreeBSD, all three of the counters are measured with microsecond accuracy. On Linux (at least as of version 2.4), the User Time and System Time counters are only accurate to the centisecond. After a trace file has been generated, you analyze it with the pprofp script. pprofp implements a number of sorting and display options that allow you to look at a script’s behavior in a number of different ways through a single trace file. Here is the list of options to pprofp: pprofp Sort options -a Sort by alphabetic names of subroutines. -l Sort by number of calls to subroutines -r Sort by real time spent in subroutines. -R Sort by real time spent in subroutines (inclusive of child calls). -s Sort by system time spent in subroutines. -S Sort by system time spent in subroutines (inclusive of child calls). -u Sort by user time spent in subroutines. -U Sort by user time spent in subroutines (inclusive of child calls). -v Sort by average amount of time spent in subroutines. -z Sort by user+system time spent in subroutines. (default) Display options -c Display Real time elapsed alongside call tree. -i Suppress reporting for php built-in functions
  5. A Tracing Example 433 -m Display file/line locations in traces. -O Specifies maximum number of subroutines to display. (default 15) -t Display compressed call tree. -T Display uncompressed call tree. Of particular interest are the -t and -T options, which allow you to display a call tree for the script and the entire field of sort options. As indicated, the sort options allow for functions to be sorted either based on the time spent in that function exclusively (that is, not including any time spent in any child function calls) or on time spent, inclusive of function calls. In general, sorting on real elapsed time (using -r and -R) is most useful because it is the amount of time a visitor to the page actually experiences.This measurement includes time spent idling in database access calls waiting for responses and time spent in any other blocking operations. Although identifying these bottlenecks is useful, you might also want to evaluate the performance of your raw code without counting time spent in input/output (I/O) waiting. For this, the -z and -Z options are useful because they sort only on time spent on the CPU. A Tracing Example To see exactly what APD generates, you can run it on the following simple script: Figure 18.1 shows the results of running this profiling with -r.The results are not sur- prising of course: sleep(1); takes roughly 1 second to complete. (Actually slightly longer than 1 second, this inaccuracy is typical of the sleep function in many languages; you should use usleep() if you need finer-grain accuracy.) hello() and goodbye() are both quite fast. All the functions were executed a single time, and the total script execu- tion time was 1.0214 seconds.
  6. 434 Chapter 18 Profiling Figure 18.1 Profiling results for a simple script. To generate a full call tree, you can run pprofp with the -Tcm options.This generates a full call tree, with cumulative times and file/line locations for each function call. Figure 18.2 shows the output from running this script. Note that in the call tree, sleep is indented because it is a child call of hello(). Figure 18.2 A full call tree for a simple script.
  7. Profiling a Larger Application 435 Profiling a Larger Application Now that you understand the basics of using APD, let’s employ it on a larger project. Serendipity is open-source Web log software written entirely in PHP. Although it is most commonly used for private individuals’Web logs, Serendipity was designed with large, multiuser environments in mind, and it supports an unlimited number of authors. In this sense, Serendipity is an ideal starting point for a community-based Web site to offer Web logs to its users. As far as features go, Serendipity is ready for that sort of high- volume environment, but the code should first be audited to make sure it will be able to scale well. A profiler is perfect for this sort of analysis. One of the great things about profiling tools is that they give you easy insight into any code base, even one you might be unfamiliar with. By identifying bottlenecks and pinpointing their locations in code, APD allows you to quickly focus your attention on trouble spots. A good place to start is profiling the front page of the Web log.To do this, the index.php file is changed to a dump trace. Because the Web log is live, you do not gen- erate a slew of trace files by profiling every page hit, so you can wrap the profile call to make sure it is called only if you manually pass PROFILE=1 on the URL line:
  8. 436 Chapter 18 Profiling Figure 18.3 Initial profiling results for the Serendipity index page. Figure 18.4 An exclusive call summary for the Serendipity index page. What you are seeing here is the cost of compiling all the Serendipity includes. Remember the discussion of compiler caches in Chapter 9, “External Performance Tunings,” that one of the major costs associated with executing PHP scripts is the time spent parsing and compiling them into intermediate code. Because include files are all parsed and compiled at runtime, you can directly see this cost in the example shown in Figure 18.4.You can immediately optimize away this overhead by using a compiler cache. Figure 18.5 shows the effect of installing APC and rerunning the profiles. include_once() is still at the top of inclusive times (which is normal because it includes a large amount of the page logic), but its exclusive time has dropped completely out of the top five calls. Also, script execution time has almost been cut in half.
  9. Profiling a Larger Application 437 Figure 18.5 A Serendipity index profile running with an APC compiler cache. If you look at the calls that remain, you can see that these are the three biggest offenders: n serendipity_plugin_api::generate_plugins n serendipity_db_query n mysql_db_query You might expect database queries to be slow. Database accesses are commonly the bot- tleneck in many applications. Spotting and tuning slow SQL queries is covered in Chapter 12, “Interacting with Databases,” so this chapter does not go into detail about that. As predicted earlier, the high real-time cost of the database queries is matched with no user and system time costs because the time that is spent in these queries is exclusive- ly spent on waiting for a response from the database server. The generate_plugins() function is a different story. Serendipity allows custom user plug-ins for side navigation bar items and comes with a few bundled examples, including a calendar, referrer tracking, and archive search plug-ins. It seems unnecessary for this plug-in generation to be so expensive. To investigate further, you can generate a complete call tree with this: > pprofp -tcm /tmp/pprof.28986 Figure 18.6 shows a segment of the call tree that is focused on the beginning of the first call to serendipity_plugin_api::generate_plugins().The first 20 lines or so show what seems to be normal lead-up work. A database query is run (via serendipity_db_query()), and some string formatting is performed. About midway down the page, in the serendipity_drawcalendar() function, the trace starts to look
  10. 438 Chapter 18 Profiling very suspicious. Calling mktime() and date() repeatedly seems strange. In fact, date() is called 217 times in this function. Looking back up to the exclusive trace in Figure 18.5, you can see that the date() function is called 240 times in total and accounts for 14.8% of the script’s execution time, so this might be a good place to optimize. Figure 18.6 A call tree for the Serendipity index page. Fortunately, the call tree tells you exactly where to look:, lines 245–261. Here is the offending code: 227 print (“”); 228 for ($y=0; $y0 || $y>=$firstDayWeekDay) && $currDay 1) $cellProp.=’Active’; 239 print(“”);
  11. Profiling a Larger Application 439 240 241 // Print day 242 if ($serendipity[“rewrite”]==true) 243 $link = $serendipity[“serendipityHTTPPath”].”archives/”. 244 date(“Ymd”, mktime(0,0,0, $month, $currDay, $year)). 245 “.html”; 246 else 247 $link = $serendipity[“serendipityHTTPPath”];; 248 if (date(“m”) == $month && 249 date(“Y”) == $year && 250 date(“j”) == currDay) { 251 echo “”; 252 } 253 if ($activeDays[$currDay] > 1) { 254 print (“”); 255 } 256 print ($currDay); 257 if ($activeDays[$currDay] > 1) print (“”); 258 if (date(“m”) == $month && 259 date(“Y”) == $year && 260 date(“j”) == $currDay) { 261 echo “”; 262 } 263 print(“”); 264 $currDay++; 265 } 266 else { 267 print “”; 268 print “ ”; 269 } 270 } 271 print (“”); This is a piece of the serendipity_drawcalendar() function, which draws the calendar in the navigation bar. Looking at line 244, you can see that the date() call is dependent on $month, $currDay, and $year. $currDay is incremented on every iteration through the loop, so you cannot cleanly avoid this call.You can, however, replace it: date(“Ymd”, mktime(0,0,0, $month, $currDay, $year)) This line makes a date string from $month, $currDay, and $year.You can avoid the date() and mktime() functions by simply formatting the string yourself: sprintf(“%4d%02d%02d:, $year, $month, $currDay) However, the date calls on lines 248, 249, 250, 258, 259, and 260 are not dependent on any variables, so you can pull their calculation to outside the loop.When you do this, the top of the loop should precalculate the three date() results needed:
  12. 440 Chapter 18 Profiling 227 $date_m = date(“m”); 228 $date_Y = date(“Y”); 229 $date_j = date(“j”); 230 print (“”); 231 for ($y=0; $y
  13. Spotting General Inefficiencies 441 credentials, the users’ cookie would be decrypted and used for both authentication and as a basic cache of their personal data. User sessions were to be timed out, so the cookie contained a timestamp that was reset on every request and used to ensure that the ses- sion was still valid. This code had been in use for three years and was authored in the days of PHP3, when non-binary-safe data (for example, data containing nulls) was not correctly han- dled in the PHP cookie handling code—and before rawurlencode() was binary safe. The functions looked something like this: function hexencode($data) { $ascii = unpack(“C*”, $data); $retval = ‘’; foreach ($ascii as $v) { $retval .= sprintf(“%02x”, $v); } return $retval; } function hexdecode($data) { $len = strlen($data); $retval = ‘’; for($i=0; $i < $len; $i+= 2) { $retval .= pack(“C”, hexdec( substr($data, $i, 2) ) ); } return $retval; } On encoding, a string of binary data was broken down into its component characters with unpack().The component characters were then converted to their hexadecimal values and reassembled. Decoding affected the reverse. On the surface, these functions are pretty efficient—or at least as efficient as they can be when written in PHP. When I was testing APD, I discovered to my dismay that these two functions con- sumed almost 30% of the execution time of every page on the site.The problem was that the user cookies were not small—they were about 1KB on average—and looping through an array of that size, appending to a string, is extremely slow in PHP. Because the functions were relatively optimal from a PHP perspective, we had a couple choices: n Fix the cookie encoding inside PHP itself to be binary safe. n Use a built-in function that achieves a result similar to what we were looking for (for example, base64_encode()). We ended up choosing the former option, and current releases of PHP have binary-safe cookie handling. However, the second option would have been just as good.
  14. 442 Chapter 18 Profiling A simple fix resulted in a significant speedup.This was not a single script speedup, but a capacity increase of 30% across the board. As with all technical problems that have sim- ple answers, the question from on top was “How did this happen?”The answer is multi- faceted but simple, and the reason all high-traffic scripts should be profiled regularly: n The data had changed—When the code had been written (years before), user cookies had been much smaller (less than 100 bytes), and so the overhead was much lower. n It didn’t actually break anything—A 30% slowdown since inception is inher- ently hard to track.The difference between 100ms and 130ms is impossible to spot with the human eye.When machines are running below capacity (as is common in many projects), these cumulative slowdowns do not affect traffic levels. n It looked efficient—The encoding functions are efficient, for code written in PHP.With more than 2,000 internal functions in PHP’s standard library, it is not hard to imagine failing to find base64_encode() when you are looking for a built-in hex-encoding function. n The code base was huge—With nearly a million lines of PHP code, the appli- cation code base was so large that a manual inspection of all the code was impossi- ble.Worse still, with PHP lacking a hexencode() internal function, you need to have specific information about the context in which the userspace function is being used to suggest that base64_encode() will provide equivalent functionality. Without a profiler, this issue would never have been caught.The code was too old and buried too deep to ever be found otherwise. Note There is an additional inefficiency in this cookie strategy. Resetting the user’s cookie on every access could guarantee that a user session was expired after exactly 15 minutes, but it required the cookie to be re- encrypted and reset on every access. By changing the time expiration time window to a fuzzy one—between 15 and 20 minutes for expiration—you can change the cookie setting strategy so that it is reset only if it is already more than 5 minutes old. This will buy you a significant speedup as well. Removing Superfluous Functionality After you have identified and addressed any obvious bottlenecks that have transparent changes, you can also use APD to gather a list of features that are intrinsically expensive. Cutting the fat from an application is more common in adopted projects (for example, when you want to integrate a free Web log or Web mail system into a large application) than it is in projects that are completely home-grown, although even in the latter case, you occasionally need to remove bloat (for example, if you need to repurpose the appli- cation into a higher-traffic role).
  15. Removing Superfluous Functionality 443 There are two ways to go about culling features.You can systematically go through a product’s feature list and remove those you do not want or need. (I like to think of this as top-down culling.) Or you can profile the code, identify features that are expensive, and then decide whether you want or need them (bottom-up culling).Top-down culling certainly has an advantage: It ensures that you do a thorough job of removing all the fea- tures you do not want.The bottom-up methodology has some benefits as well: n It identifies features. In many projects, certain features are undocumented. n It provides incentive to determine which features are nice and which are necessary. n It supplies data for prioritizing pruning. In general, I prefer using the bottom-up method when I am trying to gut a third-party application for use in a production setting, where I do not have a specific list of features I want to remove but am simply trying to improve its performance as much as necessary. Let’s return to the Serendipity example.You can look for bloat by sorting a trace by inclusive times. Figure 18.7 shows a new trace (after the optimizations you made earlier), sorted by exclusive real time. In this trace, two things jump out: the define() functions and the preg_replace() calls. Figure 18.7 A postoptimization profile.
  16. 444 Chapter 18 Profiling In general, I think it is unwise to make any statements about the efficiency of define(). The usual alternative to using define() is to utilize a global variable. Global variable declarations are part of the language syntax (as opposed to define(), which is a func- tion), so the overhead of their declaration is not as easily visible through APD.The solu- tion I would recommend is to implement constants by using const class constants. If you are running a compiler cache, these will be cached in the class definition, so they will not need to be reinstantiated on every request. The preg_replace() calls demand more attention. By using a call tree (so you can be certain to find the instances of preg_replace() that are actually being called), you can narrow down the majority of the occurrences to this function: function serendipity_emoticate($str) { global $serendipity; foreach ($serendipity[“smiles”] as $key => $value) { $str = preg_replace(“/([\t\ ]?)”.preg_quote($key,”/”). “([\t\ \!\.\)]?)/m”, “$1$2”, $str); } return $str; } where $serendipity[‘smiles’] is defined as $serendipity[“smiles”] = array(“:’(“ => $serendipity[“serendipityHTTPPath”].”pixel/cry_smile.gif”, “:-)” => $serendipity[“serendipityHTTPPath”].”pixel/regular_smile.gif”, “:-O” => $serendipity[“serendipityHTTPPath”].”pixel/embaressed_smile.gif”, “:O” => $serendipity[“serendipityHTTPPath”].”pixel/embaressed_smile.gif”, “:-(“ => $serendipity[“serendipityHTTPPath”].”pixel/sad_smile.gif”, “:(“ => $serendipity[“serendipityHTTPPath”].”pixel/sad_smile.gif”, “:)” => $serendipity[“serendipityHTTPPath”].”pixel/regular_smile.gif”, “8-)” => $serendipity[“serendipityHTTPPath”].”pixel/shades_smile.gif”, “:-D” => $serendipity[“serendipityHTTPPath”].”pixel/teeth_smile.gif”, “:D” => $serendipity[“serendipityHTTPPath”].”pixel/teeth_smile.gif”, “8)” => $serendipity[“serendipityHTTPPath”].”pixel/shades_smile.gif”, “:-P” => $serendipity[“serendipityHTTPPath”].”pixel/tounge_smile.gif”, “;-)” => $serendipity[“serendipityHTTPPath”].”pixel/wink_smile.gif”, “;)” => $serendipity[“serendipityHTTPPath”].”pixel/wink_smile.gif”, “:P” => $serendipity[“serendipityHTTPPath”].”pixel/tounge_smile.gif”, ); and here is the function that actually applies the markup, substituting images for the emoticons and allowing other shortcut markups: function serendipity_markup_text($str, $entry_id = 0) { global $serendipity;
  17. Removing Superfluous Functionality 445 $ret = $str; $ret = str_replace(‘\_’, chr(1), $ret); $ret = preg_replace(‘/#([[:alnum:]]+?)#/’,’&\1;’,$ret); $ret = preg_replace(‘/\b_([\S ]+?)_\b/’,’\1’,$ret); $ret = str_replace(chr(1), ‘\_’, $ret); //bold $ret = str_replace(‘\*’,chr(1),$ret); $ret = str_replace(‘**’,chr(2),$ret); $ret = preg_replace(‘/(\S)\*(\S)/’,’\1’ . chr(1) . ‘\2’,$ret); $ret = preg_replace(‘/\B\*([^*]+)\*\B/’,’\1’,$ret); $ret = str_replace(chr(2),’**’,$ret); $ret = str_replace(chr(1),’\*’,$ret); // monospace font $ret = str_replace(‘\%’,chr(1),$ret); $ret = preg_replace_callback(‘/%([\S ]+?)%/’, ‘serendipity_format_tt’, $ret); $ret = str_replace(chr(1),’%’,$ret) ; $ret = preg_replace(‘/\|([0-9a-fA-F]+?)\|([\S ]+?)\|/’, ‘\2’,$ret); $ret = preg_replace(‘/\^([[:alnum:]]+?)\^/’,’\1’,$ret); $ret = preg_replace(‘/\@([[:alnum:]]+?)\@/’,’\1’,$ret); $ret = preg_replace(‘/([\\\])([*#_|^@%])/’, ‘\2’, $ret); if ($serendipity[‘track_exits’]) { $serendipity[‘encodeExitsCallback_entry_id’] = $entry_id; $ret = preg_replace_callback( “#
  18. 446 Chapter 18 Profiling The second function, serendipity_markup_text(), implements certain common text typesetting conventions.This phrase: *hello* is replaced with this: hello Other similar replacements are made as well. Again, this is performed at display time so that you can add new text markups later without having to manually alter existing entries.This function runs nine preg_replace() and eight str_replace() calls on every entry. Although these features are certainly neat, they can become expensive as traffic increases. Even with a single small entry, these calls constitute almost 15% of the script’s runtime. On my personal Web log, the speed increases I have garnered so far are already more than the log will probably ever need. But if you were adapting this to be a service to users on a high-traffic Web site, removing this overhead might be critical. You have two choices for reducing the impact of these calls.The first is to simply remove them altogether. Emoticon support can be implemented with a JavaScript entry editor that knows ahead of time what the emoticons are and lets the user select from a menu.The text markup can also be removed, requiring users to write their text markup in HTML. A second choice is to retain both of the functions but apply them to entries before they are saved so that the overhead is experienced only when the entry is created. Both of these methods remove the ability to change markups after the fact without modifying existing entries, which means you should only consider removing them if you need to. A Third Method for Handling Expensive Markup I once worked on a site where there was a library of regular expressions to remove profanity and malicious JavaScript/CSS from user-uploaded content (to prevent cross-site scripting attacks). Because users can be extremely…creative…in their slurs, the profanity list was a constantly evolving entity as new and unusual foul language was discovered by the customer service folks. The site was extremely high traffic, which meant that the sanitizing process could not be effectively applied at request time (it was simply too expen- sive), but the dynamic nature of the profanity list meant that we needed to be able to reapply new filter rules to existing entries. Unfortunately, the user population was large enough that actively applying the fil- ter to all user records was not feasible either. The solution we devised was to use two content tables and a cache-on-demand system. An unmodified copy of a user’s entry was stored in a master table. The first time it was requested, the current filter set was applied to it, and the result was stored in a cache table. When subsequent requests for a page came in, they checked the cache table first, and only on failure did they re-cache the entry. When the filter set was updated, the cache table was truncated, removing all its data. Any new page requests would immediately be re-cached—this time with the new filter. This caching table could easily have been replaced with a network file system if we had so desired.
  19. Further Reading 447 The two-tier method provided almost all the performance gain of the modify-on-upload semantics. There was still a significant hit immediately after the rule-set was updated, but there was all the convenience of modify-on-request. The only downside to the method was that it required double the storage necessary to implement either of the straightforward methods (because the original and cached copies are stored sepa- rately). In this case, this was an excellent tradeoff. Further Reading There is not an abundance of information on profiling tools in PHP.The individual pro- filers mentioned in this chapter all have some information on their respective Web sites but there is no comprehensive discussion on the art of profiling. In addition to PHP-level profilers, there are a plethora of lower-level profilers you can use to profile a system.These tools are extremely useful if you are trying to improve the performance of the PHP language itself, but they’re not terribly useful for improving an application’s performance.The problem is that it is almost impossible to directly connect lower-level (that is, engine-internal) C function calls or kernel system calls to actions you take in PHP code. Here are some excellent C profiling tools: n gprof is the GNU profiler and is available on almost any system. It profiles C code well, but it can be difficult to interpret. n valgrind, combined with its companion GUI kcachegrind, is an incredible memory debugger and profiler for Linux. If you write C code on Linux, you should learn to use valgrind. n ooprofile is a kernel-level profiler for Linux. If you are doing low-level debug- ging where you need to profile an application’s system calls, ooprofile is a good tool for the job.
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