rfc:opcache.no_cache

PHP RFC: Opcache optimization without any caching

Introduction

Currently, it isn't possible to enable optimizations without enabling caching. They should be orthogonal features - it's already possible to cache without optimization passes by setting opcache.optimization_level=0.

Without the feature being proposed, users would either have to enable shared memory caching, or opcache.file_cache_only. Doing that has the following drawbacks:

  • For opcache.file_cache_only, users would be forced to manage the file cache. The end users of an application using opcache.file_cache_only may be unfamiliar with opcache.
    (Enabling opcache.file_cache_only could lead to issues such as running out of disk space, needing to clear stale entries, concerns about opcode corruption not being fixed after restarting a process or computer, etc)
  • Each shared memory block used for caching (and each additional file being cached) uses up RAM (Random-Access Memory) unnecessarily, when individual PHP processes are known to be long-lived and don't have a common memory address space. It's inefficient to cache opcodes in situations where the cache would never be used.

Proposal

Make the opcode optimizer and JIT available without opcode caching, through a new setting opcache.allow_cache.

Use Cases

opcache.allow_cache=0 is useful when there isn't much available memory and/or there are multiple long-lived php scripts managed by something that is not a php script. Some example use cases are:

  • supervisord managing hundreds of long-lived PHP CLI processes.
  • Tools that run in the background in IDEs for end users of PHP. (Especially if they are long-lived, large applications that don't use pcntl_fork)
  • Long-lived CLI Web Servers where a single CLI process serves many different requests (e.g. ReactPHP's HTTP Server)

Even when barely any files are run, the virtual memory to track the shared memory segment seems to add 2MB extra per independent php process in “shared memory” segments, reducing the free RAM available for other processes.
(starting a large number of php CLI scripts that sleep() in a loop, free (Linux program to report free memory) reports that shared (shared memory) increases by 2MB per process with the default (opcache.allow_cache=1), but barely increases with opcache.allow_cache=0. This will vary on different systems, and will use up more memory if many php files are loaded.

opcache.allow_cache=0 is not intended for running web servers (e.g. apache), where PHP would share a common memory address space (it would almost always be better to cache when optimizations are enabled).

opcache.allow_cache=0 is also not intended for extremely short-lived CLI processes (Opcode optimization may be more time-consuming than the program being run, making opcache.file_cache or opcache.enable_cli=0 a better choice).

Interactions with other opcache features

opcache.allow_cache=0 takes precedence over opcache.file_cache and opcache.file_cache_only. Neither the shared memory cache nor the file cache will be used when opcache.allow_cache=0 is used to disable opcode caching.

It is an error to both set opcache.allow_cache=0 and provide a script to preload (opcache.preload). auto_prepend_file can be used instead of opcache.preload if you want to run a script before your file without caching it.

Opcache's opcode optimizations and JIT are unaffected.

Backward Incompatible Changes

None. Code that does not enable opcache.file_cache should not be affected.

Proposed PHP Version(s)

8.0

RFC Impact

To SAPIs

The APIs those SAPIs use will be unaffected.

Applications using the CLI, Development web server, embedded PHP, and so on will be able to take advantage of opcache.allow_cache=0 if it was useful to optimize without caching.

To Existing Extensions

Extensions that check if opcache is enabled may have to update their checks if they did so by checking if opcache caching settings are enabled.

To Opcache

The code changes to opcache's optimization and caching are minimal - the implementation of this RFC is effectively the same opcache.file_cache_only without reading or writing files.

opcache_get_status() now includes the following new booleans

  • optimizations_enabled, which is true if any optimization passes will get run.
  • allow_cache, which will be true if opcache caching (in shared memory or the file cache) is enabled.

opcache_get_status() already had the undocumented field opcache_enabled. Looking at the implementation, it appears to be true when shared memory caching is successfully enabled, whether or not optimizations are enabled. Similarly to the existing behavior for file_cache_only, when opcache.allow_cache=0, the field opcache_enabled will be false.

php.ini Defaults

opcache.allow_cache=1 (caching is allowed) will be the hardcoded default and the default value in php.ini-development and php.ini-production.

Discussion

Move optimizer and JIT into core instead?

On an unrelated PR, Dmitry Stogov mentioned that

Also, it would be great to move optimizer and JIT into core, to make them available even without opcode caching.

On the PR implementing opcache.allow_cache=0, Nikita Popov wrote:

I like the idea of having optimization without caching, but the way to go about this is definitely moving the optimizer into Zend and making it available completely independently of the opcache extension. This has been “planned” for a long time, but never actually happened.

  • I would prefer to have this than have no way to disable caching. When an RFC/implementation for moving opcode caching into core is created, the authors could re-evaluate whether opcache.allow_cache=0 should continue to be used, and either continue using that setting name, or deprecate it and emit a migration notice/warning on startup.
  • I think that creating a new setting along the lines of opcache.allow_cache=0 would have a use case before and after such a refactoring, providing the benefits I mentioned for the use cases in this RFC. This would continue to be have a use case even if the caching parts of opcache.so moved into PHP's core (e.g. if that was done, and the caching module were loaded in php.ini as zend_extension=opcache.so, there'd still be a use case for a configuration setting to override that default to disable caching for running individual programs)

    Users may strongly prefer for opcache.enable, opcache.enable_cli, and opcache.optimization_level to continue controlling whether optimizations are performed (so setting the combination of opcache.enable=1, opcache.enable_cli=1, and a setting such as opcache.allow_cache=0 to optimize without caching would still make sense even after optimizations were moved into core.)
  • If nobody's currently planning to work on moving the optimizer into Zend (i.e. into PHP's core), then it may be several minor releases before it's possible to have optimization without caching.
  • There may be unforeseen objections to moving the optimizer into Zend from creators/users of profiling tools, debuggers, code coverage tools, alternative optimizers (if any exist), etc. Hopefully not, but that'd depend on the proposed implementation details.
  • There may be objections to any refactoring (increasing the size of a minimal install, requiring changing php.ini settings, making it harder for users to perform system-wide changes to add/remove opcache or the JIT, etc). Hopefully not.

Future Scope

  • Normally, opcache optimizes a file based only on that one file's contents (this makes it safe to read from cache even when loading a different combination of files). When opcache.allow_cache=0 is used, it may be possible to use all of the class, function, constant, etc. definitions parsed from previously parsed files (to eliminate dead code, inline function calls, etc). https://wiki.php.net/rfc/preload mentioned something similar in the Future Scope.

Vote

Voting started on May 30th and ends on June 13th

Add opcache.allow_cache ini setting to support opcode optimization without caching
Real name Yes No
alec (alec)  
ashnazg (ashnazg)  
bmajdak (bmajdak)  
bwoebi (bwoebi)  
derick (derick)  
duncan3dc (duncan3dc)  
galvao (galvao)  
jasny (jasny)  
jbnahan (jbnahan)  
kalle (kalle)  
kguest (kguest)  
mike (mike)  
nikic (nikic)  
ocramius (ocramius)  
patrickallaert (patrickallaert)  
pollita (pollita)  
reywob (reywob)  
ruudboon (ruudboon)  
sebastian (sebastian)  
sergey (sergey)  
stas (stas)  
tandre (tandre)  
zimt (zimt)  
Final result: 10 13
This poll has been closed.

If you voted no, why?

The Discussion section mentioned alternative approaches to this RFC. This feedback is being gathered if it may be useful for other work on Opcache such as moving optimizations into PHP's core.

  1. I would only vote for optimizations without caching if Opcache's opcode optimizations were moved into core first.
  2. I don't want any form of optimization without caching / I think opcache.file_cache should be used instead
  3. I think different ini options/values should be used to do this
  4. Other
If you voted no on opcache.allow_cache, why?
Real name 1 2 3 4
ashnazg (ashnazg)    
bwoebi (bwoebi)    
kguest (kguest)    
nikic (nikic)    
patrickallaert (patrickallaert)    
ruudboon (ruudboon)    
sergey (sergey)    
zimt (zimt)    
Final result: 8 0 0 0
This poll has been closed.

Also, would you be interested in moving opcode optimizations and the JIT out of the zend_extension opcache into PHP's core?

I would be interested in moving opcode optimizations into core
Real name Yes No
ashnazg (ashnazg)  
bwoebi (bwoebi)  
dmitry (dmitry)  
galvao (galvao)  
jasny (jasny)  
kguest (kguest)  
nikic (nikic)  
ocramius (ocramius)  
patrickallaert (patrickallaert)  
pollita (pollita)  
ruudboon (ruudboon)  
sebastian (sebastian)  
sergey (sergey)  
zimt (zimt)  
Final result: 14 0
This poll has been closed.

Changelog

0.2: Previously, the ini setting override to disable caching was opcache.no_cache=1. This was changed to opcache.allow_cache=0 to avoid double negatives and to be consistent with naming of other ini settings such as allow_url_fopen and allow_url_include.

0.3: Fix documentation of changes to opcache_get_status()

0.4: Improve documentation of ini settings, add another example use case.

Ideas on moving the optimizer into core instead

There are various ways the suggestion in Discussion could be implemented. My ideas on a way that could be implemented are below (I'm not familiar enough with opcache to implement that or to be aware of any problems it would cause):

  • Move the optimizer into core (e.g. move code related to opcode optimizations from ext/opcache to a new folder ext/optimizer such as Zend/optimizer). Keep all of the functionality related to caching in the zend_extension Zend Opcache (some build environments may not support or have a use case for any forms of shared memory caching).
  • Continue optimizing according to opcache.enable and opcache.enable_cli when the opcode caching is enabled.
    Add a new flag such as optimizer.always_optimize=1 or opcache.always_optimize=1 which will ignore that and unconditionally optimize using the optimization passes in opcache.optimization_level. This would ensure that existing use cases work without modifying php.ini and won't suffer from high startup time for short-lived processes which don't have opcodes cached.
  • Provide C function pointers to lock shared memory and acquire pointer locations so that the JIT can emit executable code without conflicting with other processes/threads, and make opcache override those function pointers. I'm not sure how protecting memory with opcache jit without the opcache extension managing shared memory would work with threaded php and no shared cache, but should be possible - I assume the JIT already works with opcache.file_cache_only. I'm also unfamiliar with how those pointers/mutexes would get released if php crashed.
  • Limit opcache.preload to only be used when the opcode caching zend_extension is used.

Nikita Popov mentions that

To be clear, “move into core” means moving optimizations into Zend/ and making them part of the compilation process (optionally). They shouldn't be in a separate ext/optimizer extension – that would be not much better than having them in ext/opcache :)

References

https://externals.io/message/109959 “opcache.no_cache prototype: Opcode optimization without caching”

https://externals.io/message/110187 “[RFC] opcache.no_cache: Opcache optimization without any caching”

rfc/opcache.no_cache.txt · Last modified: 2020/06/13 17:45 by tandre