PHP OPCache, Realpath Cache and Preloading (PHPUK Conference 2020)

@coudenysj - jachim.be
PHP OPCache, Realpath
Cache and Preloading
How to tune your PHP installation

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New PHP versions go
faster, but we can
make them go even
faster!

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Some groundwork

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PHP
● Scripting Language
● Fire and forget
● No "manual" compilation

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PHP-FPM
● Only "relevant" SAPI nowadays :)
● Master process
● Dynamic worker child processes
● Shared memory in the master
jachimbe 24546 0.0 0.0 301644 10152 ? Ss 2019 1:04 php-fpm: master process (/conf/jachimbe/php/fpm.conf)
jachimbe 21457 3.6 0.1 304680 35732 ? S 10:57 0:00 \_ php-fpm: pool jachimbe

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Shared memory???
In computer hardware, shared memory refers to
a (typically large) block of random access
memory (RAM) that can be accessed by several
different central processing units (CPUs) in
a multiprocessor computer system.

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All set?

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Jachim Coudenys

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So this talk is about
performance, right?

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Right!

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Why this
presentation?

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80% of performance issues have nothing to do
with your server!
https://www.combell.com/en/performance-team

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Latency Comparison Numbers (~2012)
L1 cache reference 0.5 ns
Branch mispredict 5 ns
L2 cache reference 7 ns 14x L1 cache
Main memory reference 100 ns 20x L2 cache, 200x L1
Send 1K bytes over 1 Gbps network 10,000 ns 10 us
Read 4K randomly from SSD* 150,000 ns 150 us ~1GB/sec SSD
Read 1 MB sequentially from memory 250,000 ns 250 us
Round trip within same datacenter 500,000 ns 500 us
Read 1 MB sequentially from SSD* 1,000,000 ns 1,000 us 1 ms ~1GB/sec SSD, 4X memory
Disk seek 10,000,000 ns 10,000 us 10 ms 20x datacenter roundtrip
Read 1 MB sequentially from disk 20,000,000 ns 20,000 us 20 ms 80x memory, 20X SSD
Send packet CA->Netherlands->CA 150,000,000 ns 150,000 us 150 ms

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https://gist.github.com/jboner/2841832

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How can we ﬁx it?

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Disk IO

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Network File System (NFS)

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Tackle these
challenges in PHP
Realpath Cache
OPCache
Preloading

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Realpath Cache

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Realpath Cache
● Is used to reduce IO
● Caches all possible paths to dest path
● explode('/')
● No shared memory! Mem*Workers
● 4M
○ Prior to PHP 7.0.16 and 7.1.2, the default was "16K"
● Helps a lot with NFS

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php.net/manual/en/ref.ﬁlesystem.php
● realpath_cache_get()
● realpath_cache_size()
● realpath()

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demo
php realpath.php

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$presentation = file_get_contents(
__DIR__ . '/../ffi.php'
);
$presentation2 = file_get_contents(
dirname(__DIR__) . '/ffi.php'
);
$filesize = filesize(
__DIR__ . '/../est.txt'
);
print_r(realpath_cache_get());

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[/home/jachim/demo] => Array
(
[key] => 1.6354972010384E+19
[is_dir] => 1
[realpath] => /home/jachim/demo
[expires] => 1579859105
)
[/home] => Array
(
[key] => 4353355791257440477
[is_dir] => 1
[realpath] => /home
[expires] => 1579859105
)
[/home/jachim] => Array
(
[key] => 5522554812971572568
[is_dir] => 1
[realpath] => /home/jachim
[expires] => 1579859105
)

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[/home/jachim/demo/../ffi.php] => Array
(
[key] => 1.6164035761241E+19
[is_dir] =>
[realpath] => /home/jachim/ffi.php
[expires] => 1579859105
)
[/home/jachim/ffi.php] => Array
(
[key] => 5100116734180765326
[is_dir] =>
[realpath] => /home/jachim/ffi.php
[expires] => 1579859105
)
[/home/jachim/demo/realpath.php] => Array
(
[key] => 1.8190176096283E+19
[is_dir] =>
[realpath] => /home/jachim/demo/realpath.php
[expires] => 1579859105
)

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Realpath Cache: what
can we tune?

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php.net/manual/en/ini.core.php
● realpath_cache_size
● realpath_cache_ttl

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OPCache
OPcache improves PHP
performance by
storing precompiled
script bytecode in
shared memory,
thereby removing the
need for PHP to load
and parse scripts on
each request.

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https://engineering.facile.it/blog/eng/realpath-cache-is-it-
all-php-opcache-s-fault/

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Throwaway language
● Always performs same steps
○ Read PHP code
○ Lexing + Parsing: Tokens
○ Compiling: Opcodes
○ Executing
● All information is discarded
● Request per request

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opcodes?
In computing, an opcode (abbreviated from
operation code) is the portion of a machine
language instruction that specifies the
operation to be performed.

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Vulcan Logic Dumper
https://3v4l.org/A1B4H/vld

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This can get big quite quickly
● Libraries
● Frameworks
● etc...

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OPCodes (usually) don't change
● Let’s add in some cache!

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OPCache
● Bunch of opcode cachers
○ APC
○ Turck MMCache
○ Zend Optimizer
○ etc...
● Zend Optimizer "donated" by Zend
● Since PHP 5.5

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Shared memory
● Stored in the master process of FPM

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OPCache Optimizer
● Gets better every release
● Optimizes:
○ Branches
○ Dead code
○ ++$a vs $a++

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Optimized opcodes (example)
https://3v4l.org/A1B4H/vld

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php.net/manual/en/ref.opcache.php
● Information
○ opcache_get_configuration()
○ opcache_get_status()
○ opcache_is_script_cached()
● Actions
○ opcache_compile_file()
○ opcache_invalidate()
○ opcache_reset()

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demo
php opcache.php

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// php -d opcache.enable_cli=On opcache.php
print_r(opcache_get_status());

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[opcache_enabled] => 1
[cache_full] =>
[restart_pending] =>
[restart_in_progress] =>
[memory_usage] => Array
(
[used_memory] => 9168648
[free_memory] => 125049080
[wasted_memory] => 0
[current_wasted_percentage] => 0
)
[interned_strings_usage] => Array
(
[buffer_size] => 6291008
[used_memory] => 371880
[free_memory] => 5919128
[number_of_strings] => 7869
)
...

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[opcache_statistics] => Array
(
[num_cached_scripts] => 1
[num_cached_keys] => 2
[max_cached_keys] => 16229
[hits] => 0
[start_time] => 1579859174
[last_restart_time] => 0
[oom_restarts] => 0
[hash_restarts] => 0
[manual_restarts] => 0
[misses] => 1
[blacklist_misses] => 0
[blacklist_miss_ratio] => 0
[opcache_hit_rate] => 0
)
...

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[scripts] => Array
(
[/home/jachim/demo/opcache.php] => Array
(
[full_path] => /home/jachim/demo/opcache.php
[hits] => 0
[memory_consumption] => 880
[last_used] => Fri Jan 24 10:46:14 2020
[last_used_timestamp] => 1579859174
[timestamp] => 1579858691
)
)
...

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Interned strings
● Used in a lot of languages
● Since PHP 5.4
● "Compression" for source code
● In shared memory (FPM master process)

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Keys
● Full path to file
● Relative paths to files

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Wasted memory
● Opcache doesn't do "defragmentation"
● File changes cause recompilation

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Opcache restarts
● OOM Restarts
○ Memory
● Hash Restarts
○ Keys
● Manual Restarts
○ opcache_reset()

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NEVER have a full cache!

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Opcache File Cache
● Read opcodes from disk
● Can help busy sites
● Supports CLI (used to be "fire and forget")
● Cache to "user directory"
opcache.file_cache=/var/tmp/php/opcache
opcache.file_cache_only=1 # Useful for CLI
opcache.file_cache_consistency_checks=1 # Adler checksum

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demo
php -d opcache.enable_cli=On -d
opcache.file_cache="/tmp/opcache"
opcache.php

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// php -d opcache.enable_cli=On -d
opcache.file_cache="/home/jachim/demo/cache" opcache.php
print_r(opcache_get_status());

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└╼ tree cache
cache
└── 26f7903bdd40555497c24242ea455a66
└── home
└── jachim
└── demo
└── opcache.php.bin
4 directories, 1 file

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PHPArch Opcache article
https://www.phparch.com/article/diving-in-the
-opcache/
● Explores the option of using opcache file
caching as pre-compiled PHP programs.

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OPCache: what can
we tune?

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“Disaster Recovery” Scenarios
● Memory Full?
● Interned Strings Full?
● Key store full?

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php.net/manual/en/opcache.conﬁguration.php
● Memory
○ opcache.memory_consumption
○ opcache.interned_strings_buffer
○ opcache.max_accelerated_files (keys)
● Invalidation
○ opcache.validate_timestamps
○ opcache.revalidate_freq
● Thresholds
○ opcache.max_wasted_percentage

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Getting stats
● opcache_get_status()

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https://github.com/rlerdorf/opcache-status

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https://gist.github.com/ck-on/

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Opcache Priming / Deployment Strategies
● opcache_compile_file()
● FPM Pools
● or…

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Preloading
Preload PHP
functions and
classes once and
use them in the
context of any
future request
without
overhead.

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Preload PHP functions and
classes once and use them
in the context of any
future request without
overhead.

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OPCache on steroids

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Preloading
● PHP 7.4
● Part of opcache
● Starts in master process before anything
else
● Loads code in memory "permanently"
● No need to copy from shared memory to
process memory

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Preloading
● Opcache only works per file
● Preloading helps with class libraries
● Code will perform as internal entities
(e.g. strlen, etc...)
● Simple file with autoloading magic

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php.net/manual/en/opcache.conﬁguration.php
● opcache.preload=/path/to/preload.php
● opcache_compile_file()

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Unlinked classes aren’t preloaded

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Preloading in the wild
● github.com/brendt/laravel-preload/blob/master/preload.php
● symfony.com/blog/new-in-symfony-4-4-preloading-symfony-applications-in-php
-7-4
// var/cache/dev/srcApp_KernelDevDebugContainer.preload.php
// This file has been auto-generated by the Symfony Dependency Injection Component
// You can reference it in the "opcache.preload" php.ini setting on PHP >= 7.4 when
preloading is desired
use Symfony\Component\DependencyInjection\Dumper\Preloader;
require dirname(__DIR__, 3).'/vendor/autoload.php';
require __DIR__.'/ContainerZxvZ783/srcApp_KernelDevDebugContainer.php';
$classes = [];
$classes[] = 'App\Kernel';
Preloader::preload($classes);

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Caveats
● “Full server” (fpm)
○ php.ini setting
● Server restart

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Composer
● Sounds like a job for composer, right?
● [RFC] Preloading support
(https://github.com/composer/composer/issue
s/7777)

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Composer benchmarks
https://github.com/composer/composer/issues/7
777#issuecomment-440268416
● No preloading
● Preloading only "hot" classes
○ $files = opcache_get_status(true)['scripts'];
● Preloading all the classes
○ $files = require 'vendor/composer/autoload_classmap.php';

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Composer benchmarks
https://github.com/composer/composer/issues/7777#issuecomment-440268416

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Composer at the moment
https://github.com/composer/composer/issues/7777#issuecomment-559725760

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Preloading issues
● There were some problems with using
require() instead of opcache_compile_file()
○ Should be fixed in latest release
● Some issues in bugtracker, but not that bad
○ https://bugs.php.net/search.php?cmd=display&search_for
=preload

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Performance
● Is it worth the effort?
● Depends on the situation
● We mostly see an improvement
● Stick with the hot classes
● Currently looking at how we can apply it at scale for our
customers at Combell
● Some reports:
○ https://stitcher.io/blog/php-preload-benchmarks
○ https://ezplatform.com/blog/php-7.4-opcache-preloading-benchmark
○ https://developer.happyr.com/php-74-preload

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Preload resources
● https://externals.io/message/103333
● https://wiki.php.net/rfc/preload
● https://github.com/php/php-src/pull/3538

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In conclusion

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PHP Performance
● Know enough of PHP internals
● Know your application (use information
functions)
● Finetune
● Repeat

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Elastic Stack @ Combell
● Gather realpath/opcache/etc... data from
all accounts
● Act upon that data (which is changing
constantly)
● Autotuning & autoconfigure preload

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Thank you!
https://joind.in/talk/d1cfa

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PHP OPCache, Realpath Cache and Preloading (PHPUK Conference 2020)

PHP OPCache, Realpath Cache and Preloading (PHPUK Conference 2020)

Jachim Coudenys

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