The optimisation chapter ended up being a lot longer than I planned, which in turn means you can't just flick through it and make the appropriate changes to your script! So, I've summarised the various optimisation hints and tips here; for more information on any of these, please go back to the Optimisation chapter.
First up, here are the PHP optimisations:
Use the Zend OPcache or the Zend Optimizer, depending on your PHP version
Use the comma operator to join arguments when calling echo
Priorities optimisation of tight loops for the most payback
Get into the habit of using pre-increment rather tha post-increment
Don't rely on references without testing; they are rarely as effective as you'd think
Get a good understanding of how garbage collection works
Listen to all errors, big and small - they slow things down
Store a local pointer to an array element to save indexing into the array each time you need it
Compress your output to save network bandwidth
Try to avoid using PHP in CGI mode
Avoid calling the dl() function
Use persistent resource where appropriate
Optimise compilation of your PHP binary if possible
Avoid mod_access if you can
Don't fret about Apache 2.0
As a last resort, try inlining functions
Next, the SQL optimisations:
Prioritise your queries as LOW_PRIORITY, HIGH_PRIORITY, or DELAYED when it matters
Don't use SELECT * unless you must
Use the EXPLAIN keyword to see how MySQL will execute your query - make sure your indexes are being used!
Load your data before adding indexes is faster than adding indexes first
Be wary of locks
Use --log-slow-queries to see where your tables can be optimised
Increase your buffer sizes so that MySQL can cache more
Use SHOW STATUS to make sure your MySQL server is in good condition
Don't create indexes you aren't going to use
Do as much as your filtering in SQL as you can
Indexes are good for reads, but bad for writes
Use OPTIMIZE TABLE and ANALYZE TABLE regularly
Create your tables with a fixed-table format if possible
Use the most efficient table type for each table
Use the best data type, including NOT NULL if appropriate - don't rely on automatic type conversion
Use default values for INSERT when you can
Use temporary tables rather than heavy PHP work
SELECT foo IN (list, of, constants) is very fast
When joining tables, use numbers instead of strings if you can
Finally, here are the server-related tips:
Having a dual-CPU machine (or more) makes an enormous difference
Don't worry about SSE, Hyperthreading, or 64-bit for Apache web servers, but do consider 64-bit for your MySQL server
Try to get the fastest RAM available: 533MHz RAM is highly recommended
Most data should be cached in RAM, but don't skimp on hard disks just in case - get S-ATA at the very least
Resist moving from one machine to two until it becomes financially necessary
When you network, Gigabit Ethernet is essential, preferably with each machine connected directly to a switch
Using a round robin DNS system is a simple load-balancing system, but don't forget to put session data in a database
Blade servers are a cheap and efficient solution for web server systems, but not for SQL servers.
If you are able, set up one machine as a reverse proxy for your site.
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