Trying to get to grips with the different types of cache engines File, APC, Xcache, Memcache. Anybody know of any good resources/links?
Note I am using Linux, PHP and mysql
There are 2 types of caching terminology thrown around in PHP.
First is an optcode cache:
http://en.wikipedia.org/wiki/PHP_accelerator
Second is a data cache:
http://simas.posterous.com/php-data-caching-techniques
A few of the technologies can cross boundaries into both realms, but the basics behind them are simple. The idea is: Keep as much data in ram and precompiled because compiling and HD seeks are very expensive processes. HD Seeks can be done to find a file to compile / query the DB to get data / looking for a temp file, and every time that happens it slows down the user experience.
Memcached is generally the way to go, but it has some "features" such as once you save some data to t cache, it doesn't necessarily guarantee that it will be available later as it dynamically removes old caches to make way for new ones. It's also fairly basic, you'll need to roll your own system for handling timeouts and preventing cascading but it's all fairly simple. There's tons of info in the Memcached FAQ, or feel free to ask and I'll post some code examples. Memcached can also act as a session handler which is great if you have lots of users or more than one server.
Otherwise disc caching is good if you only have one server or don't mind generating separate caches of each server. Generally faster than memcached as it doesn't have the network overhead (unless you have memcached on the same server). There are plenty of good disc caching frameworks but probably the best are Pear Cache_Lite and APC.
APC also has the added advantage that it can cache your compiled PHP code which may help on high-performance websites.
Related
I'm using Cache_Lite for html and array Cache in my project. I found Cache_Lite may lead to high system IO problem. Maybe because the performance of Cache_Lite is not good
I'm asking is there any stable php html/page cache to use?
I already have APC installed for opcode cache, Memcached installed for common data/array cache.
I've had exact problem with Cache Lite, as library doesn't properly implement file locks.
Solved it with new library and drop in replacement for Cache Lite.
https://github.com/mpapec/simple-cache/blob/master/example_clite1.php
https://github.com/mpapec/simple-cache/blob/master/example_clite2.php
https://github.com/mpapec/simple-cache/blob/master/example_clite3.php
Just to mention that library lacks some features that I didn't found useful like cache cleaning and caching in memory (_memoryCaching property which is false by default and marked as "beta quality" in original library).
Algorithm which is used for file locking follows this diagram,
Without more information it is hard to know if you are currently experiencing an IO problem or are likely to experience an IO problem in the future. (If your site is not getting much traffic or you are using a SSD you are unlikely to have a problem)
Cache Lite appears to be a file based caching system. This may lead to IO problems if your site experiences heavy load / lots of concurrent users / is hosted on a shared server / has other programs heavily using the filesystem.
An alternative to Cache Lite is memcache which is a key/value store that stores data in memory. This may not be suitable if you are storing large amounts of data or you server does not have any spare RAM as it stores all of its information in memory. Another benefit of memory is that it is much faster than accessing files from the disk. If you are only accessing a small amount of data or the same data repeatedly this is not likely to be an issue though because of disk/OS caching.
I would suggest checking to see if your system is currently experiencing any issues with IO before worrying about IO performance (unless you plan on getting slashdotted or something)
You could install a tool like Munin http://munin-monitoring.org/ and monitor your system to see if IO is a problem or is becoming a problem. Once installed check the cpu graph and look at the iowait data.
EDIT: Just saw the comment above, depending on your needs reverse proxys are another great tool checkout https://www.varnish-cache.org/ . At work we use a combination of the two ( memcache and varnish) We have 1 machine serving over 900,000 page views per month, this site includes static and dynamic content.
If you're talking about https://pear.php.net/package/Cache_Lite then i could tell you a story. We used it once, but it proved to be unreliable for websites with lots of request.
We then switched to Zend_Cache (ZF1) in combination with memcached. I can be used as standalone component.
However, you have to tune it a bit in order to use tags. There are a few implementations out there to get the job done: https://github.com/bigwhoop/taggable-zend-memcached-backend
I am going to develop a social + professional networking website using Php (Zend or Yii framework). We are targeting over 5000 requests per minute. I have experience in developing advanced websites, using MVC frameworks.
But, this is the first time, I am going to develop something keeping scalability in mind. So, I will really appreciate, if someone can tell me about the technologies, I should be looking for.
I have read about memcache and APC. Which one should I look for? Also, should I use a single Mysql server or a master/slave combination (if its later, then why and how?)
Thanks !
You'll probably want to architect your site to use, at minimum, a master/slave replication system. You don't necessarily need to set up replicating mysql boxes to begin with, but you want design your application so that database reads use a different connection than writes (even if in the beginning both connections connect to the same db server).
You'll also want to think very carefully about what your caching strategy is going to be. I'd be looking at memcache, though with Zend_Cache you could use a file-based cache early on, and swap in memcache if/when you need it. In addition to record caching, you also want to think about (partial) page-level caching, and what kind of strategies you want to plan/implement there.
You'll also want to plan carefully how you'll handle the storage and retrieval of user-generated media. You'll want to be able to easily move that stuff off the main server onto a dedicated box to serve static content, or some kind of CDN (content distribution network).
Also, think about how you're going to handle session management, and make sure you don't do anything that will prevent you from using a non-file-based session storage ((dedicated) database, or memcache) in the future.
If you think carefully, and abstract data storage/retrieval, you'll be heading in a good direction.
Memcached is a distributed caching system, whereas APC is non-distributed and mainly an opcode cache.
If (and only if) your website has to live on different webservers (loadbalancing), you have to use memcache for distributed caching. If not, just stick to APC and its cache.
About MySQL database, I would advise a gridhosting which can autoscale according to requirements.
Depending on the requirements of your site it's more likely the database will be your bottle neck.
MVC frameworks tend to sacrifice performance for easy of coding, especially in the case of ORM. Don't rely on the ORM, instead benchmark different ways of querying the database and see which suits. You want to minimise the number of database queries, fetch a chunk of data at once instead of doing multiple small queries.
If you find that your php code is a bottle neck(profile it before optimizing) you might find facebook's hiphop useful.
I'm running a php/mysql-driven website with a lot of visits and I'm considering the possibility of caching result-sets in shared memory in order to reduce database load.
However, right now MySQL's query cache is enabled and it seems to be doing a pretty good job since if I disable query caching, the use of CPU jumps to 100% immediately.
Given that situation, I dont know if caching result-sets (or even the generated HTML code) locally in shared memory with PHP will result in any noticeable performace improvement.
Does anyone out there have any experience on this matter?
PS: Please avoid suggesting heavy-artillery solutions like memcached. Right now I'm looking for simple solutions that dont require too much time to implement, deploy and maintain.
Edit:
I see my comment about memcached deviated answers from the actual point, which is whether caching DB queries in the application layer would result in a noticeable performace impact considering that the result of those queries are already being cached at the DB level.
I know you didn't want to hear about memcached, but it is one of the best solutions for what you're trying to do. Depending on your site usage, there can be massive improvements in performance. By simply using memcached's session handler over my database session handler, I was able to cut the load in half and cut back on request serving times by over 30%.
Realistically, memcached is a simple solution. It's already integrated with PHP (if you have the extension loaded), and it requires virtually no configuration (I simply had to add memcached as a service on my linux box, which is done in one or two shell commands).
I would suggest storing session data (and anything that lends itself to caching) in memcache. For dynamic pages (such as stack overflow homepage), I would recommend caching output for a couple of seconds to prevent flooding.
A decent single box solution is file-based caching, but you have to sweep them out manually. Other than that, you could use APC, which is very fast and in-memory (still have to expire them yourself though).
As soon as you scale past one web server, though, you're going to need a shared cache, which is memcached. Why are you so adamant about not deploying this? It's not hard, and it's just going to save you time down the road. You can either start using memcache now and be done with it, or you could use one of the above methods for now and then end up switching to memcache later anyways, resulting in even more work. Plus too, you don't have to deal with running a cronjob or some other ugly hack to get cache expiration features: it does that for you.
The mysql query cache is nice, but it's not without issues. One of the big ones is it expires automatically every time the source data is changed, which you probably don't want.
I am creating a new PHP framework depending on Zend Framework.
It will be a general purpose MVC framework for web development.
I am worried about 2 aspects:
Logging:
Should I use logging? Is there any substantial performance problems when using logging?
Caching database queries:
I am caching some queries from database.
I am concerned about caching user related information. Suppose there are some information related to users. Like their personal info, etc.
If I cache such data, for every user a cache file will be generated in my data folder. Now suppose there are 10,000 - 20,000 users online in 2 hours span of time. These means that there will be 20000 files on my folder.
My question is that, will it affect the performance of my server. Is there any upper limit on how many files a folder can have on server.
Do not use a file based cache. File system operations are exceptionally slow: http://imgur.com/X1Hi1.gif . Use memcached, you don't need a lot of memory contrary to what the above post says, the amount of memory you need for it is totally proportional to how much stuff you want to store, plus memcached can cull data based on access frequency.
1) You definitely want logging, I'd recommend xdebug available at http://www.xdebug.org/. You can read further about the performance overheads at their site. (plus it integrates nicely with Eclipse's PHP version.)
2) I'm not really sure I'd want to cache much user information, but memcache is probably one of the better choices for caching in php (http://se2.php.net/memcache). And yeah, there's no limit on file number, and you'll probably not be going over the 32-bit filesize limit either =)
Caching is a real problem it's almost impossible to get it right from a user/programmer perspective. I wouldn't cache things as simple as user data. This is already cached in the database. Focus more on complex queries and complete webpages (or parts of it).
Unless you have a page like stackoverflow where i see really few ways to cache anything you have to search hard and check your logfiles about what users do on your site and you will see some hotspots soon.
Memcache is not recommended by me unless you have a lot of memory (> 8GB) on your machine. Memcache works best if you throw in Memcache servers with 16 GB doing nothing else them caching things.
For smaller sites, hardware and requirements you should consider APC as this is a very low overhead cache for data and it speeds up the execution of php at the same time (you don't want to run a production server without a bytecode cache).
I'm currently implementing memcached into my service but what keeps cropping up is the suggestion that I should also implement APC for caching of the actual code.
I have looked through the few tutorials there are, and the PHP documentation as well, but my main question is, how do I implement it on a large scale? PHP documentation talks about storing variables, but it isn't that detailed.
Forgive me for being uneducated in this area but I would like to know where in real sites this is implemented. Do I literally cache everything or only the parts that are used often, such as functions?
Thanks!
As you know PHP is an interpreted language, so everytime a request arrives to the server it need to open all required and included files, parse them and execute them. What APC offers is to skip the require/include and parsing steps (The files still have to be required, but are stored in memory so access is much much faster), so the scripts just have to be executed. On our website, we use a combination of APC and memcached. APC to speed up the above mentioned steps, and memcached to enable fast and distributed storing and accessing of both global variables (precomputed expensive function calls etc that can be shared by multiple clients for a certain amount of time) as well as session variables. This enables us to have multiple front end servers without losing any client state such as login status etc.
When it comes to what you should cache... well, that really depends on your application. If you have a need for multiple frontends somewhere down the line, I would try to go with memcached for such caching and storing, and use APC as an opcode cache.
APC is both an opcode cache and a general data cache. The latter works pretty much like memcached, whereas the opcode cache works by caching the parsed php-files, so that they won't have to be parsed on each request. That can generally speed up execution time up quite a bit.
You don't have to implement the opcode caching features of APC, you just enable them as a php module.
APC cache size and other configuration information is here.