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Caching and tuning fun for high scalability

Caching and tuning fun for high scalability



Caching has been a 'hot' topic for a few years. But caching takes more than merely taking data and putting it in a cache : the right caching techniques can improve performance and reduce load ...

Caching has been a 'hot' topic for a few years. But caching takes more than merely taking data and putting it in a cache : the right caching techniques can improve performance and reduce load significantly. But we'll also look at some major pitfalls, showing that caching the wrong way can bring down your site. If you're looking for a clear explanation about various caching techniques and tools like Memcached, Nginx and Varnish, as well as ways to deploy them in an efficient way, this talk is for you.



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  • Caching serves 3 purposes : - Firstly, to reduce the number of requests or the load at the source of information, which can be a database server, content repository, or anything else.
  • Secondly, you want to improve the response time of each request. If you request a page that takes 50ms to load without caching and you get 10 hits/second, you won't be able to serve those requests with 5 Apache processes. If you could cache some of the data used on the page you might be able to return the page in 20ms. That doesn't just improve user experience, but reduces the load on the webserver, since the number of concurrent connections is a lot lower. → connections closed faster → handle more connections and, as a result, more hits on the same machine. → If you don't : more Apache processes needed → will eat memory, will eat system resources and as a result will also cause context switching.
  • More tuning → Reduce the amount of data that needs to be sent over the network or the Internet - benefits you, as application provider, because you have less traffic on your upstream connection. - also better for the enduser because there will be less data to be downloaded. → Ofcourse part of frontend side, which we'll discuss near the end of the tutorial.
  • The first way to cache in a web application, or actually more commonly a website, is to cache entire pages. This used to be a very popular caching technique, back in the days when pages were very static. It's still popular for things like company websites, blogs, and any site that has pages that don't change a lot. Basically, you just render the page once, put it in a cache, and from the moment onwards, you just retrieve it from the cache.
  • Store part of a page. Probably most common + best way to cache data. - Basically what you do is, take piece of data : - data from the database - result of a calculation - an aggregation of two feeds - parsed data from CSV-file from NFS share located on the other side of the world - could be data that was stored on a USB stick your kid is now chewing on. What I mean is : it doesn't matter where the data came from. Part of a page, usually a block on a page and want save time by not having to get that data from its original source every time again. Instead of saving entire page, where you can have multiple dynamic parts, some of which might not be cached because they are really dynamic, like the current time. So store small block, so that when we render the page, all we do is get small block from cache and place it in dynamic page and output it.
  • Store the output of SQL queries. → Now, who of you know what SQL query caching is, MySQL query cache for example ? → Basically, the MySQL query cache is a cache which stores the output of recently run SQL queries. It's built into MySQL, it's... not enabled by default everywhere, it depends on your distribution. → And it speeds up queries by a huge margin. Disabling it is something I never do, because you gain a lot by having it enabled. → However, there's a few limitations : - First of all, the query cache is limited in size.
  • But, basically one of the big drawbacks of MySQL query cache, is that every time you do an insert, update or delete on a table, the entire query cache for queries referencing that table, is erased. → Another drawback is that you still need to connect to the MySQL server and you still need to go through a lot of the core of MySQL to get your results. → So, storing the output of SQL queries in a separate cache, being Memcache or one of the other tools we're going to see in a moment, is actually not a bad idea. Also because of the fact that, if you have a big Website, you will still get quite a bit load on your MySQL database. So anything that takes the load off the database and takes it to where you have more resources available, is a good idea. → Better : store returned object or group of objects
  • Another caching technique I want to mention is storing the result of complex PHP processes. - You might think about some kind of calculation, but when I mention calculation, people tend to think about getting data from here and there and then summing them. - That's not what I mean. By complex PHP processes I mean things like parsing configuration files, parsing XML files, loading CSV-data in an array, converting mutliple XML-files into an object structure, and so on. - End result of those complex PHP processes can be cached, especially if the data from which we started doesn't change a lot. That way you can save a lot of system resources, which can be used for other things.
  • There's plenty of other types of data to store in cache. The only limit there is your imagination. All you need to think of is : - I have this data - how long did it take me to create it - how often does it change - how often will it be retrieved ? That last bit can be a difficult thing to balance out, but we'll get back to that later.
  • time spent per query pattern how many queries of that query pattern
  • OK, let's talk about where cached data can be stored. I already mentioned MySQL query cache. Turn it on But don't rely on it too heavily especially if you have data that changes often.
  • I said I was going to discuss some do's and don'ts... → This one falls under the category don't → There's a second database mechanism for "caching", at least some people use it for that purpose. It's called database memory tables. → MySQL has such as storage type : it's called a memory or a heap table. And basically it allows you to store data in tables that are stored in memory. → Don't confuse it with a temporary table, which is only valid for your connection. → This is actually a persistent table, well persistent meaning that it will survive after you disconnect, but it won't survive a server reboot, because it's in-memory only. → Advantages of this storage type are that it's faster than disk-based tables and you can join it with disk-based tables. Also, there's a default limit of 16MByte per table and it can be troublesome getting it to work on a master-slave setup. → So my advise is : don't use it.
  • Alright, next. Opcode caching... this is definitely a DO. → There's a few opcode caches out there. → Now what is opcode caching ? Basically, when you run a PHP file, the PHP is converted by the PHP compiler to what is called bytecode. This code is then executed by the PHP engine and that produces the output. → Now, if your PHP code doesn't change a lot, which normally it shouldn't while your application is live, there's no reason for the PHP compiler to convert your source code to bytecode over and over again, because basically it's just doing the same thing, every time. → So an opcode cache caches the bytecode, the compiled version of your source code. That way, it doesn't have to compile the code, unless your source code changes. This can have a huge performance impact.
  • APC is the most popular one and will probably be included in one of the next few releases. Might be 5.4, but there's still a lot of discussion about that. I'm guessing we probably won't see it before 5.5 or who knows 6.0, if that ever comes out. To enable APC, all you have to do is install the module, which can be done using PECL or through your distribution's package management system. Then make sure apc is enabled in php.ini and you're good to go. → The other opcode caches are eAccelerator, which is sort of slightly outdated now, although it does in some cases produce a better performance. But since APC will be included in the PHP core, I'm not sure if it's going to survive for very long anymore. → Then there's Zend Accelerator, which is built into Zend Server. Basically, it's similar to APC in terms of opcode caching functionality, but it's just bundled with the Zend products.
  • → There's also a thing called X-Cache. I must admit I've never tried it. Could be good, but it's pretty hard to find decent information about it. → And there's also a cache for Windows called WinCacheForPhp... has anyone tried it ? → Opcode caching on its own is ofcourse not useful to store specific data, but it will improve your PHP performance. → Also, it reduces memory usage, since compiling the PHP code requires additional memory. → So it's a kind of caching that falls under the tuning category ;-)
  • Slightly better than using local disk is using a local memory disk or a ramdisk. → Advantage : slightly faster, on the other hand if you're using Linux the standard file caching system will cache recently accessed files anyway, so there might not be a big performance impact when comparing to standard disk caching.
  • The biggest downside however is that, just like with disk cache, it stores its data locally, which means it's great if you have only 1 server, but as soon as you move to an architecture with 2 webservers, you can't use it for sessions anymore and you'll have to find a way to keep your cache synchronized, which will in fact cause a lot of overhead. >> empty <<
  • See slide >> replication!<<
  • See slide
  • See slide
  • - Key names must be unique - Prefix/namespace your keys ! → might seem overkill at first, but it's usually necessary after a while, at least for large systems. → Oh, and don't share the same Memcache with multiple projects. Start separate instances for each !) - Be careful with charachters. Use only letters, numbers and underscore ! - Sometimes MD5() is your friend → but : harder to debug - Use clear names. Remember you can't make a list of data in the cache, so you'll need to document them. I know you don't like to write documentation, but you'll simply have to in this case.
  • OK, that sort of covers the basics of how we can use Memcache to cache data for your site. So purely in terms of caching in the code, we've done a lot. → There's still things that you can always add. If you're using Zend Framework or any other major framework, you can cache things like the initialization of the configuration file, creation of the route object (which is a very heavy process if you have a lot of routes). → Things like translation and locale can be cached in Zend Framework using 1 command, so do that ! → But as I said before, the only limit is your imagination... → and your common sense ! → Don't overdo it... make sure that the cache has enough space left for the things you really need to cache.
  • If you're starting a project where the number of hits to the site will be limited at first, but you have no idea on how fast it will grow in the future : - I would suggest to start by using disk-based caching or APC variable caching - You can always move to Memcache later when you deploy a second webserver Keep in mind that your code needs to be ready for this. So you need to use some kind of cache abstraction layer like Zend_Cache
  • So, why don't we switch everything from Apache to nginx ? → Well, it's not THAT easy. There's a lot of Apache modules that Nginx doesn't have, like WebDAV support and many of the authentication modules. → The basic modules are there and they're built into Nginx, which again makes them faster than Apache, because they don't go through some module API whcih causes overhead. → But there are some specific solutions that you can not build using Nginx, although there are some third-party modules out there now, but keep in mind you have to add these by recompiling Nginx. → Now, since we're talking mostly about scaling public websites, chances are we're not going to need any of those modules, so we'll have no trouble at all putting the N in LANMMP.
  • → see slide → And that's all there is to it : it's running. Well, not exactly, we still need to configure it ofcourse.
  • Now, as I mentioned Nginx is very fast and as a first step to using it to scale our website, we're going place it in front of Apache. So, we're going to run it on the same server, but we're going to move Apache to a different port, preferably one that's not accessible from the outside, and we're going to have Nginx forward requests to Apache. → Ofcourse we're not going to send all requests to Apache, 'cause that would be quite stupid, adding overhead. → We're only going to send all dynamic content requests to Apache and serve all static files directly from Nginx.
  • So, we're serving all those extensions directly from disk and forwarding all the rest to the Apache running on port 8080. We're also forwarding the Set-Cookie headers and adding a few headers so Apache can log the original IP if it wants to. → Something to keep in mind here : you will have 2 logfiles now : 1 from Nginx and 1 from Apache. → What you should notice once you start using this type of setup is that your performance from an enduser perspective will remain somewhat the same if your server was not overloaded yet. If it was having issues because of memory problems or too many Apache workers, ... → However, you will suddenly need a lot less Apache workers, which will save you quite a lot of memory. That memory can be used for... Memcache maybe ?
  • OK, what we just did is very nice. → But if you're really not relying on any of the special Apache modules, why would you keep Apache anyway ? → Why not just replace it alltogether ? Well, it depends on what your bottleneck is. → If you're looking for a way to lower your memory usage and you don't mind losing some processing power, this is certainly the way to go. → So let's go for a LNMMP stack. We're going to kick out Apache.
  • If one of the backend webservers goes down, you want all traffic to go to the other one ofcourse. That's where health checks come in
  • >> platforms ! <<
  • >> thing on list <<
  • Indicates how long the file should not be retrieved
  • Split requests across subdomains : - HTTP/1.1 spec advises 2 connections per hostname - To get around that, use multiple subdomains. - Especially put your statics separately → helps when you grow and put them on a CDN - Be careful : don't use too many subdomains → DNS penalty
  • >> in Varnish <<

Caching and tuning fun for high scalability Caching and tuning fun for high scalability Presentation Transcript

  • Caching and tuning fun for high scalability Wim Godden Cu.be Solutions
  • Who am I ? Wim Godden (@wimgtr) Founder of Cu.be Solutions (http://cu.be) Open Source developer since 1997 Developer of OpenX, PHPCompatibility, Nginx SCL, ... Speaker at PHP and Open Source conferences
  • Who are you ? Developers ? System/network engineers ? Managers ? Caching experience ?
  • Goals of this tutorial Everything about caching and tuning A few techniques How-to How-NOT-to → Increase reliability, performance and scalability 5 visitors/day → 5 million visitors/day (Don't expect miracle cure !)
  • LAMP
  • Architecture
  • Test page 3 DB-queries select firstname, lastname, email from user where user_id = 5; select title, createddate, body from article order by createddate desc limit 5; select title, createddate, body from article order by score desc limit 5; Page just outputs result
  • Our base benchmark Apachebench = useful enough Result ? Single webserver Proxy Static PHP Static PHP Apache + PHP 3900 17.5 6700 17.5 Limit : CPU, network or disk Limit : database
  • CachingCaching
  • What is caching ? CACHECACHE
  • What is caching ? x = 5, y = 2 n = 50 Same result CACHECACHE select * from article join user on article.user_id = user.id order by created desc limit 10 Doesn't change all the time
  • Theory of caching DB Cache $data = get('key') false GET /page Page select data from table $data = returned result set('key', $data) if ($data == false)
  • Theory of caching DB Cache HIT
  • Caching goals - 1st goal Reduce # of concurrent request Reduce the load
  • Caching goals - 2nd goal
  • Some figures Pageviews : 5000 (4000 on 10 pages) Avg. loading time : 200ms Cache 10 pages Avg. loading time : 20ms → Total avg. loading time : 56ms Worth it ?
  • Caching goals - 3rd goal Send less data across the network / Internet You benefit → lower bill from upstream provider Users benefit → faster page load Wait a second... that's a frontend problem ! True, but remember : the backend is transmitting it !
  • Caching techniques #1 : Store entire pages Company Websites Blogs Full pages that don't change Render → Store in cache → retrieve from cache
  • Caching techniques #2 : Store parts of a page Most common technique Usually a small block in a page Best effect : reused on lots of pages Can be inserted on dynamic pages
  • Caching techniques #3 : Store SQL queries ↔ SQL query cache Limited in size
  • Caching techniques #3 : Store SQL queries ↔ SQL query cache Limited in size Resets on every insert/update/delete Server and connection overhead Goal : not to get rid of DB free up DB resources for more hits ! Better : store processed data instead of raw data store group of objects
  • Caching techniques #4 : Store complex PHP results Not just calculations CPU intensive tasks : Config file parsing XML file parsing Loading CSV in an array Save resources → more resources available
  • Caching techniques #xx : Your call Only limited by your imagination ! When you have data, think : Creating time ? Modification frequency ? Retrieval frequency ?
  • How to find cacheable data New projects : start from 'cache everything' Existing projects : Check page loading times Look at MySQL slow query log Make a complete query log (don't forget to turn it off !) → Use Percona Toolkit (pt-query-digest)
  • Databases - pt-query-digest # Profile # Rank Query ID Response time Calls R/Call Apdx V/M Item # ==== ================== ================ ===== ======= ==== ===== ========== # 1 0x543FB322AE4330FF 16526.2542 62.0% 1208 13.6806 1.00 0.00 SELECT output_option # 2 0xE78FEA32E3AA3221 0.8312 10.3% 6412 0.0001 1.00 0.00 SELECT poller_output poller_item # 3 0x211901BF2E1C351E 0.6811 8.4% 6416 0.0001 1.00 0.00 SELECT poller_time # 4 0xA766EE8F7AB39063 0.2805 3.5% 149 0.0019 1.00 0.00 SELECT wp_terms wp_term_taxonomy wp_term_relationships # 5 0xA3EEB63EFBA42E9B 0.1999 2.5% 51 0.0039 1.00 0.00 SELECT UNION wp_pp_daily_summary wp_pp_hourly_summary # 6 0x94350EA2AB8AAC34 0.1956 2.4% 89 0.0022 1.00 0.01 UPDATE wp_options # MISC 0xMISC 0.8137 10.0% 3853 0.0002 NS 0.0 <147 ITEMS>
  • Databases - pt-query-digest # Query 2: 0.26 QPS, 0.00x concurrency, ID 0x92F3B1B361FB0E5B at byte 14081299 # This item is included in the report because it matches --limit. # Scores: Apdex = 1.00 [1.0], V/M = 0.00 # Query_time sparkline: | _^ | # Time range: 2011-12-28 18:42:47 to 19:03:10 # Attribute pct total min max avg 95% stddev median # ============ === ======= ======= ======= ======= ======= ======= ======= # Count 1 312 # Exec time 50 4s 5ms 25ms 13ms 20ms 4ms 12ms # Lock time 3 32ms 43us 163us 103us 131us 19us 98us # Rows sent 59 62.41k 203 231 204.82 202.40 3.99 202.40 # Rows examine 13 73.63k 238 296 241.67 246.02 10.15 234.30 # Rows affecte 0 0 0 0 0 0 0 0 # Rows read 59 62.41k 203 231 204.82 202.40 3.99 202.40 # Bytes sent 53 24.85M 46.52k 84.36k 81.56k 83.83k 7.31k 79.83k # Merge passes 0 0 0 0 0 0 0 0 # Tmp tables 0 0 0 0 0 0 0 0 # Tmp disk tbl 0 0 0 0 0 0 0 0 # Tmp tbl size 0 0 0 0 0 0 0 0 # Query size 0 21.63k 71 71 71 71 0 71 # InnoDB: # IO r bytes 0 0 0 0 0 0 0 0 # IO r ops 0 0 0 0 0 0 0 0 # IO r wait 0 0 0 0 0 0 0 0 # pages distin 40 11.77k 34 44 38.62 38.53 1.87 38.53 # queue wait 0 0 0 0 0 0 0 0 # rec lock wai 0 0 0 0 0 0 0 0 # Boolean: # Full scan 100% yes, 0% no # String: # Databases wp_blog_one (264/84%), wp_blog_tw… (36/11%)... 1 more # Hosts # InnoDB trxID 86B40B (1/0%), 86B430 (1/0%), 86B44A (1/0%)... 309 more # Last errno 0 # Users wp_blog_one (264/84%), wp_blog_two (36/11%)... 1 more # Query_time distribution # 1us # 10us # 100us # 1ms # 10ms ################################################################ # 100ms # 1s # 10s+ # Tables # SHOW TABLE STATUS FROM `wp_blog_one ` LIKE 'wp_options'G # SHOW CREATE TABLE `wp_blog_one `.`wp_options`G # EXPLAIN /*!50100 PARTITIONS*/ SELECT option_name, option_value FROM wp_options WHERE autoload = 'yes'G
  • Caching storage - MySQL query cache Use it Don't rely on it Good if you have : lots of reads few different queries Bad if you have : lots of insert/update/delete lots of different queries
  • The problem with SQL query caching select id, name from someTable where x = 5; ← uncached select id, name from someTable where x = 5; ← cached update someTable set name="Jim" where x = 10; select id, name from someTable where x = 5; ← uncached Imagine : 500 select/sec, 10 updates/min → 10 cache purges per min 50 select/sec, 10 update/sec → 10 cache purge per sec
  • Caching storage - Database memory tables Tables stored in memory In MySQL : memory/heap table ↔ temporary table : memory tables are persistent temporary tables are session-specific Faster than disk-based tables Can be joined with disk-based tables But : default 16MByte limit master-slave = trouble if you don't need join → overhead of DB software So : don't use it unless you need to join
  • Caching storage - Opcode caching DO !
  • Caching storage - Opcode caching APC De-facto standard Will be in PHP core in 5.4 ? 5.5 ? 6.0 ? PECL or packages eAccelerator Zend Accelerator X-Cache WinCacheForPhp
  • Caching storage - Opcode caching APC De-facto standard until 5.4 PECL or packages Zend Optimizer+ Built-in with PHP 5.5 eAccelerator PHP PHP + APC 42.18 req/sec 206.20 req/sec
  • Caching storage - Disk Data with few updates : good Caching SQL queries : preferably not DON'T use NFS or other network file systems high latency possible problem for sessions : locking issues !
  • Caching storage - Memory disk (ramdisk) Usually faster than physical disk But : OS file caching makes difference minimal (if you have enough memory)
  • Caching storage - Disk / ramdisk Overhead : filesystem Limited number of files per directory → Subdirectories Local 5 Webservers → 5 local caches How will you keep them synchronized ? → Don't say NFS or rsync !
  • Caching storage - APC variable cache More than an opcode cache (PHP 5.5 → use APCu) Store user data in memory apc_add / apc_store to add/update apc_fetch to retrieve apc_delete Fast → huge performance impact Session support ! Downside : local storage → hard to scale restart Apache → cache = empty
  • Caching storage - Memcache(d) Facebook, Twitter, YouTube, … → need we say more ? Distributed memory caching system Multiple machines ↔ 1 big memory-based hash-table Key-value storage system Keys - max. 250bytes Values - max. 1Mbyte
  • Caching storage - Memcache(d) Facebook, Twitter, YouTube, … → need we say more ? Distributed memory caching system Multiple machines ↔ 1 big memory-based hash-table Key-value storage system Keys - max. 250bytes Values - max. 1Mbyte Extremely fast... non-blocking, UDP (!)
  • Memcache - where to install
  • Memcache - where to install
  • Memcache - installation & running it Installation Distribution package PECL Windows : binaries Running No config-files memcached -d -m <mem> -l <ip> -p <port> ex. : memcached -d -m 2048 -l -p 11211
  • Caching storage - Memcache - some notes Not fault-tolerant It's a cache ! Lose session data Lose shopping cart data ...
  • Caching storage - Memcache - some notes Not fault-tolerant It's a cache ! Lose session data Lose shopping cart data … Different libraries Original : libmemcache New : libmemcached (consistent hashing, UDP, binary protocol, …) Firewall your Memcache port !
  • Memcache in code <?php $memcache = new Memcache(); $memcache->addServer('', 11211); $memcache->addServer('', 11211); $myData = $memcache->get('myKey'); if ($myData === false) { $myData = GetMyDataFromDB(); // Put it in Memcache as 'myKey', without compression, with no expiration $memcache->set('myKey', $myData, false, 0); } echo $myData;
  • Memcache in code <?php $memcache = new Memcache(); $memcache->addServer('', 11211); $memcache->addServer('', 11211); $myData = $memcache->get('myKey'); if ($memcache->getResultCode() == Memcached::RES_NOTSTORED) { $myData = GetMyDataFromDB(); // Put it in Memcache as 'myKey', without compression, with no expiration $memcache->set('myKey', $myData, false, 0); } echo $myData;
  • Benchmark with Memcache Single webserver Proxy Static PHP Static PHP Apache + PHP 3900 17.5 6700 17.5 Apache + PHP + MC 3900 55 6700 108
  • Where's the data ? Memcache client decides (!) 2 hashing algorithms : Traditional Server failure → all data must be rehashed Consistent Server failure → 1/x of data must be rehashed (x = # of servers) No replication !
  • Memcache slabs (or why Memcache says it's full when it's not) Multiple slabs of different sizes : Slab 1 : 40 bytes Slab 2 : 50 bytes (40 * 1.25) Slab 3 : 63 bytes (63 * 1.25) (and so on...) Multiplier (1.25 by default) can be configured Store a lot of objects of different sizes → Certain slabs : full → Other slabs : Mostly empty → Eviction of data !
  • Memcache - Is it working ? Connect to it using telnet "stats" command → Use Cacti or other monitoring tools STAT pid 2941 STAT uptime 10878 STAT time 1296074240 STAT version 1.4.5 STAT pointer_size 64 STAT rusage_user 20.089945 STAT rusage_system 58.499106 STAT curr_connections 16 STAT total_connections 276950 STAT connection_structures 96 STAT cmd_get 276931 STAT cmd_set 584148 STAT cmd_flush 0 STAT get_hits 211106 STAT get_misses 65825 STAT delete_misses 101 STAT delete_hits 276829 STAT incr_misses 0 STAT incr_hits 0 STAT decr_misses 0 STAT decr_hits 0 STAT cas_misses 0 STAT cas_hits 0 STAT cas_badval 0 STAT auth_cmds 0 STAT auth_errors 0 STAT bytes_read 613193860 STAT bytes_written 553991373 STAT limit_maxbytes 268435456 STAT accepting_conns 1 STAT listen_disabled_num 0 STAT threads 4 STAT conn_yields 0 STAT bytes 20418140 STAT curr_items 65826 STAT total_items 553856 STAT evictions 0 STAT reclaimed 0
  • Memcache - backing up
  • Memcache - deleting <?php $memcache = new Memcache(); $memcache->delete('myKey');
  • Memcache - caching a page <?php $output = $memcache->get('page_' . $page_id); if ($output === false) { ob_start(); GetMyPageInRegularWay($page_id); $output = ob_get_contents(); ob_end_clean(); $memcache->set('page_' . $page_id, $output, false, 600); // Cache 10 mins } echo $output;
  • Memcache - tip Page with multiple blocks ? → use Memcached::getMulti() But : what if you get some hits and some misses ? getMulti($array) Hashing algorithm
  • Naming your keys Key names must be unique Prefix / namespace your keys ! Only letters, numbers and underscore Why ? → Change caching layer md5() is useful → BUT : harder to debug Use clear names Document your key names !
  • Updating data
  • Updating data LCD_Popular_Product_List
  • Adding/updating data $memcache->delete('ArticleDetails__Toshiba_32C100U_32_Inch'); $memcache->delete('LCD_Popular_Product_List');
  • Adding/updating data
  • Adding/updating data - Why it crashed
  • Adding/updating data - Why it crashed
  • Adding/updating data - Why it crashed
  • Cache stampeding
  • Cache stampeding
  • Memcache code ? DB Visitor interface Admin interface Memcache code
  • Standard caching code public function getArticle($id) { $cache = Zend_Registry::get('Zend_Cache'); if (!$articleList = $cache->load('article_' . $id)) { $select = $this->db->select() ->from('article', array('id', 'title', 'body', 'created')) ->join('user', 'user.id = article.user_id', array('username')) ->where('article.id = ?', $id); $articleList = $db->fetchRow($select, $id); $cache->save($articleList, 'article_' . $id); } return $articleList; }
  • Standard caching code public function getArticleUncached($id) { $select = $this->db->select() ->from('article', array('id', 'title', 'body', 'created')) ->join('user', 'user.id = article.user_id', array('username')) ->where('article.id = ?', $id); return $db->fetchRow($select, $id); } public function getArticle($id) { $cache = Zend_Registry::get('Zend_Cache'); if (!$articleList = $cache->load('article_' . $id)) { $articleList = $this->getArticleUncached($id); $cache->save($articleList, 'article_' . $id); } return $articleList; } public function updateArticleCache($id) { $cache->save( $this->getArticleUncached($id), 'article_' . $id ); }
  • Cache warmup scripts Used to fill your cache when it's empty Run it before starting Webserver ! 2 ways : Visit all URLs Error-prone Hard to maintain Call all cache-updating methods Make sure you have a warmup script !
  • Cache stampeding - what about locking ? Seems like a nice idea, but... While lock in place What if the process that created the lock fails ?
  • Quick word about expiration General rule : don't let things expire Exception to the rule : things that have an end date (calendar items)
  • So... DON'T DELETE FROM CACHE & DON'T EXPIRE FROM CACHE (unless you know you'll never store it again)
  • Quick-tip Start small → disk or APC Move to Memcached/Redis/... later But : is your code ready ? → Use a component like Zend_Cache to switch easily !
  • Time for... a break (15 min) After the break : Byebye Apache Reverse proxying The importance of frontend ...
  • Nginx Web server Reverse proxy Lightweight, fast 12.89% of all Websites
  • Nginx No threads, event-driven Uses epoll / kqueue Low memory footprint 10000 active connections = normal
  • Nginx - a true alternative to Apache ? Not all Apache modules mod_auth_* mod_dav* … Basic modules are available Some 3rd party modules (needs recompilation !)
  • Nginx - Installation Packages Win32 binaries → Not for production ! Build from source (./configure; make; make install)
  • Nginx - Configuration server { listen 80; server_name www.domain.ext *.domain.ext; index index.html; root /home/domain.ext/www; } server { listen 80; server_name photo.domain.ext; index index.html; root /home/domain.ext/photo; }
  • Nginx - phase 1 Move Apache to a different port (8080) Put Nginx at port 80 Nginx serves all statics (images, css, js, …) Forward dynamic requests to Apache
  • Nginx for static files only server { listen 80; server_name www.domain.ext; location ~* ^.*.(jpg|jpeg|gif|png|ico|css|zip|tgz|gz|rar|bz2|doc|xls|pdf|ppt|txt|tar|rtf|js)$ { expires 30d; root /home/www.domain.ext; } location / { proxy_pass http://www.domain.ext:8080; proxy_pass_header Set-Cookie; proxy_set_header X-Real-IP $remote_addr; proxy_set_header Host $host; proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for; } }
  • Nginx for PHP ? Bottleneck = PHP ? Keep it in Apache Bottleneck = memory ? Go for it ! LANMMP to... LNMPP (ok, this is getting ridiculous)
  • Nginx with PHP-FPM Since PHP 5.3.3 Runs on port 9000 Nginx connects using fastcgi method location / { fastcgi_pass; fastcgi_index index.php; include fastcgi_params; fastcgi_param SCRIPT_NAME $fastcgi_script_name; fastcgi_param SCRIPT_FILENAME /home/www.domain.ext/$fastcgi_script_name; fastcgi_param SERVER_NAME $host; fastcgi_intercept_errors on; }
  • Nginx + PHP-FPM features Graceful upgrade Spawn new processes under high load Chroot Slow request log !
  • Nginx + PHP-FPM features Graceful upgrade Spawn new processes under high load Chroot Slow request log ! fastcgi_finish_request() → offline processing
  • Nginx + PHP-FPM - performance ? Single webserver Proxy Static PHP Static PHP Apache + PHP 3900 17.5 6700 17.5 Apache + PHP + MC 3900 55 6700 108 Nginx + PHP-FPM + MC 11700 57 11200 112 Limit : single-threaded Apachebench
  • Nginx + PHP-FPM - performance ? Single webserver Proxy Static PHP Static PHP Apache + PHP 3900 17.5 6700 17.5 Apache + PHP + MC 3900 55 6700 108 Nginx + PHP-FPM + MC 11700 57 11200 112 Apache (tuned) + PHP/MC 10600 55 11400 108 Limit : single-threaded Apachebench
  • Reverse proxy time...
  • Varnish Not just a load balancer Reverse proxy cache / http accelerator / … Caches (parts of) pages in memory Careful : uses threads (like Apache) Nginx usually scales better (but doesn't have VCL)
  • Varnish - Installation & configuration Installation Packages Source : ./configure && make && make install Configuration /etc/default/varnish /etc/varnish/*.vcl
  • Varnish - backends + load balancing backend server1 { .host = ""; } backend server2 { .host = ""; } director example_director round-robin { { .backend = server1; } { .backend = server2; } }
  • Varnish - backends + load balancing backend server1 { .host = ""; .probe = { .url = "/"; .interval = 5s; .timeout = 1 s; .window = 5; .threshold = 3; } }
  • Varnish - VCL Varnish Configuration Language DSL (Domain Specific Language) → compiled to C Hooks into each request Defines : Backends (web servers) ACLs Load balancing strategy Can be reloaded while running
  • Varnish - whatever you want Real-time statistics (varnishtop, varnishhist, ...) ESI
  • Article content page Article content (TTL : 15 min) /article/732 Varnish - ESI Header (TTL : 60 min) /top Latest news (TTL : 2 min) /news Navigation (TTL : 60 min) /nav
  • Going to /page/id/732 <esi:include src="/top"/> <esi:include src="/nav"/> <esi:include src="/news"/> <esi:include src="/article/732"/>
  • Article content page <esi:include src="/article/732"/> Varnish - ESI Perfect for caching pages <esi:include src="/top"/> <esi:include src="/news"/> <esi:include src="/nav"/> In your Varnish config : sub vcl_fetch { if (req.url == "/news") { esi; /* Do ESI processing */ set obj.ttl = 2m; } elseif (req.url == "/nav") { esi; set obj.ttl = 1m; } elseif …. …. }
  • Varnish with ESI - hold on tight ! Single webserver Proxy Static PHP Static PHP Apache + PHP 3900 17.5 6700 17.5 Apache + PHP + MC 3900 55 6700 108 Nginx + PHP-FPM + MC 11700 57 11200 112 Varnish - - 11200 4200
  • Varnish - what can/can't be cached ? Can : Static pages Images, js, css Pages or parts of pages that don't change often (ESI) Can't : POST requests Very large files (it's not a file server !) Requests with Set-Cookie User-specific content
  • ESI → no caching on user-specific content ? Logged in as : Wim Godden 5 messages TTL = 5minTTL=1h TTL = 0s ?
  • Coming soon... Based on Nginx Reduces load by 50 – 95% Requires code changes ! Well-built project → few changes Effect on webservers and database servers
  • ESI on Nginx Logged in as : Wim Godden 5 messages NEWSMenu
  • ESI on Nginx Logged in as : Wim Godden 5 messages NEWSMenu
  • SCL on Nginx + Memcached <scl:include key="news" src="/news" ttl="5m" /> <scl:include key="menu" src="/menu" ttl="1h" /> <scl:include key="top" src="/top" session="true" ttl="1h" />
  • Requesting /page (1st time) Nginx Shared memory 1 2 3 4 /page /page
  • Requesting /page ESI subrequests (1st time) Nginx 1 2 3 /menu /news /top (in SCL session)
  • Requesting /page (next time) Nginx Shared memory 1 2 /page /menu /news /top (in SCL session) /page
  • New message is sent... POST /send DB insert into... set(...) top (in SCL session)
  • Advantages No repeated GET hits to webserver anymore ! At login : POST → warm up the cache ! No repeated hits for user-specific content Not even for non-specific content
  • First release : ESI Part of the ESI 1.0 spec Only relevant features implemented Extension for dynamic session support But : unavailable for copyright reasons
  • Rebuilt from scratch : SCL Session-specific Caching Language Ideas for a better name ? Language details : Control structures : if/else, switch/case, foreach Variable handling Strings : concatenation, substring, ...
  • SCL code samples You are logged in as : <scl:session_var("person_name") /> You are logged in as : <@s("person_name") />
  • SCL code samples <scl:switch var="session_var('isAdmin')"> <scl:case value="1"> <scl:include key="admin-buttons" src="/admin-buttons.php" /> </scl:case> <scl:default> <div id="just-a-user"> <scl:include key="user-buttons" src="/user-buttons.php" /> </div> </scl:default> </scl:switch>
  • What's the result ?
  • What's the result ?
  • Figures 2nd customer : No. of web servers : 72 → 8 No. of db servers : 15 → 4 Total : 87 → 12 (86% reduction !) Last customer : No. of total servers : +/- 1350 Expected reduction : 1350 → 300 Expected savings : €1.6 Million per year
  • A real example : vBulletin DB Server Load Web Server Load Max Requests/sec (1 = 282) 0 5 10 15 20 25 30 35 Standard install With Memcached Nginx + SCL + memcached
  • Why is it so much faster ?
  • Availability Good news : It will become Open Source It's solid : ESI version stable at 4 customers Bad news : First customer holds copyrights Total rebuild → Open Source release No current projects, so spare time project Beta : Dec 2013 Final : Q1-Q2 2014 (on Github !)
  • Time to tune...
  • Apache - tuning tips Disable unused modules → fixes 10% of performance issues Set AllowOverride to None. Enable only where needed ! Disable SymLinksIfOwnerMatch. Enable only where needed ! MinSpareServers, MaxSpareServers, StartServers, MaxClients, MPM selection → a whole session of its own ;-) Don't mod_proxy → use Nginx or Varnish ! High load on an SSL-site ? → put SSL on a reverse proxy
  • PHP speed - some tips Upgrade PHP - every minor release has 5-15% speed gain ! Use an opcode cache (Zend O+, APC, eAccelerator, XCache) Profile your code XHProf Xdebug But : turn off profilers on acceptance/production platforms !
  • KCachegrind is your friend
  • PHP speed - some tips Most performance issues are in DB queries → look there first ! Log PHP errors and review those logs ! Shorter code != faster code → keep your code readable ! Hardware cost < Manpower cost → 1 more server < 30 mandays of labor Keep micro-optimizations in code = last thing on list
  • DB speed - some tips Avoid dynamic functions Ex. : select id from calendar where startDate > curdate() Better : select id from calendar where startDate > "2013-05-14" Use same types for joins i.e. don't join decimal with int RAND() is evil ! count(*) is evil in InnoDB without a where clause ! Persistent connect is sort-of evil Index, index, index ! → But only on fields that are used in where, order by, group by !
  • Caching & Tuning @ frontend http://www.websiteoptimization.com/speed/tweak/average-web-page/
  • Caching in the browser HTTP 304 (Not modified) Expires/Cache-Control header 2 notes : Don't use POST if you want to cache Don't cache user-specific pages in browser (security !)
  • HTTP 304 Browser Server No header Last Modified: Fri 28 Jan 2011 08:31:01 GMT If-Modified-Since: Fri 28 Jan 2011 08:31:01 GMT 200 OK / 304 Not Modified First request Next requests
  • HTTP 304 with ETag Browser Server No header Etag: 8a53321-4b-43f0b6dd972c0 If-None-Match: 8a53321-4b-43f0b6dd972c0 200 OK / 304 Not Modified First request Next requests
  • Expires/Cache-control header Cache-Control HTTP/1.1 Seconds to expiry Used by browsers Browser Server No header Expires: Fri 29 Nov 2011 12:11:08 GMT Cache-Control: max-age=86400 First request Next requests No requests until item expires Expires HTTP/1.0 Date to expire on Used by old proxies Requires clock to be accurate !
  • Pragma: no-cache = evil "Pragma: no cache" doesn't make it uncacheable Don't want caching on a page ? HTTP/1.0 : "Expires : Fri, 30 Oct 1998 00:00:00 GMT" (in the past) HTTP/1.1 : "Cache-Control: no-store"
  • Frontend tuning 1. You optimize backend 2. Frontend engineers messes up → havoc on backend 3. Don't forget : frontend sends requests to backend ! SO... Care about frontend Test frontend Check what requests frontend sends to backend
  • Tuning frontend Minimize requests Combine CSS/JavaScript files Use inline images in CSS/XHTML (not supported on all browsers yet)
  • Frontend tuning - inline CSS/XHTML images #navbar span { width: 31px; height: 31px; display: inline; float: left; margin-right: 4px; } .home { background-image: url(data:image/gif;base64,R0lGODlhHwAfAPcAAAAAAIxKAKVjCLW1tb29tcbGvc7OxtZ7ANbWztbW1tbe1t7e1uelMefn1ufn3ufn5+fv3u +MAO/v5+/v7/fGCPf35/f37//nY//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////// ////////////////////////////////////////////////........MEl0nGVUC6tObNnPceSFBaQVMJAxC4lo3gNOrUaFnTHoAxNm3XVxPfRq139e8BEGAjWD5bgI ALw287T8AcAXLly2kjOACdc17higXSIKDO/Lpv7Qq4bw7APgBq8eOzX69InrZ6xe3dbxZffyTGkb8tdx8F+b0Xn2sFsCSBAgTM5lp63RH YnoHUudZgRgkGOGCB+43nGk4OGcQTabKx5dyJKJ7ImoUNCaRRAZYN1ppsrT3Y2gIwyjSQBAtUpABml/0IJGYd6VjQUDH9uBFkGx Gm5I8dPQaRUAQUMBdhhBV25ZYUJZBcSAtSJBddWZZ5UAGPOTXlgkNVOSZdBxEwIkYu7VhYnAol5GaadRqF0Uaz0TgXnX2umV FyGakJUUAAADs=); margin-left: 4px; } <img border=0 src="data:image/gif;base64,R0lGODlhHwAfAPcAAAAAAIxKAKVjCLW1tb29tcbGvc7OxtZ7ANbWztbW1tbe1t7e1uelMefn1ufn3ufn5+fv 3u+MAO/v5+/v7/fGCPf35/f37//nY/......Uaz0TgXnX2umVFyGakJUUAAADs=">
  • Tuning frontend Minimize requests Combine CSS/JavaScript files Use inline images in CSS/XHTML (not supported on all browsers yet) Use CSS Sprites
  • CSS Sprites
  • Tuning content - CSS sprites
  • Tuning content - CSS sprites 11 images 11 HTTP requests 24KByte 1 image 1 HTTP requests 14KByte
  • Tuning frontend Minimize requests Combine CSS/JavaScript files Use inline images in CSS/XHTML (not supported on all browsers yet) Use CSS Sprites (horizontally if possible) Put CSS at top Put JavaScript at bottom Max. no connections Especially if JavaScript does Ajax (advertising-scripts, …) ! Avoid iFrames Again : max no. of connections Don't scale images in HTML Have a favicon.ico (don't 404 it !) → see my blog
  • Tuning frontend Don't use inline CSS/JavaScript CSS/JavaScript need to be external files (minified, merged) Why ? → Cacheable by browser / reverse proxy Use GET for Ajax retrieval requests (and cache them !) Optimize images (average 50-60% !) Split requests across subdomains Put statics on a separate subdomain (without cookies !) www.phpbenelux.eu Max. 2 requests www.phpbenelux.eu Max. 2 requests Max. 2 requests images.phpbenelux.eu
  • Tuning miscellaneous Avoid DNS lookups Frontend : don't use too many subdomains (2 = ideal) Backend : Turn off DNS resolution in Apache : HostnameLookups Off If your app uses external data Run a local DNS cache (timeout danger !) Make sure you can trust DNS servers (preferable run your own) Compress non-binary content (GZIP) mod_deflate in Apache HttpGzipModule in Nginx (HttpGzipStaticModule for pre-zipped statics !) No native support in Varnish
  • What else can kill your site ? Redirect loops Multiple requests More load on Webserver More PHP to process Additional latency for visitor Try to avoid redirects anyway → In ZF : use $this->_forward instead of $this->_redirect Watch your logs, but equally important... Watch the logging process → Logging = disk I/O → can kill your server !
  • Above all else... be prepared ! Have a monitoring system Use a cache abstraction layer (disk → Memcache) Don't install for the worst → prepare for the worst Have a test-setup Have fallbacks → Turn off non-critical functionality
  • So... Cache But : never delete, always push ! Have a warmup script Monitor your cache Have an abstraction layer Apache = fine, Nginx = better Static pages ? Use Varnish Tune your frontend → impact on backend !
  • Questions ?
  • Questions ?
  • Contact Twitter @wimgtr Web http://techblog.wimgodden.be Slides http://www.slideshare.net/wimg E-mail wim.godden@cu.be Please... Rate my talk : http://joind.in/9044
  • Thanks ! Please... Rate my talk : http://joind.in/9044