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PHP RFC: Opcache optimization without any caching
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
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:
opcache.file_cache_only, users would be forced to manage the file cache. The end users of an application using
opcache.file_cache_onlymay be unfamiliar with opcache.
opcache.file_cache_onlycould 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.
Make the opcode optimizer and JIT available without opcode caching, through a new setting
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.
supervisordmanaging 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
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 the 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_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 (
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)
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.
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 a new boolean
optimization_enabled, which is true if any optimization passes will get run. When
allow_cache=0 is used to disable caching, the array will include the field
“no_cache” => true.
opcache.allow_cache=1 (caching is allowed) will be the hardcoded default and the default value in php.ini-development and php.ini-production.
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=0should 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=0would 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.somoved 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.optimization_levelto continue controlling whether optimizations are performed (so setting the combination of
opcache.enable_cli=1, and a setting such as
opcache.allow_cache=0to 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.
- 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=0is 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.
Proposed Voting Choices
opcache.allow_cache ini setting to support opcode optimization without caching. (Yes/No vote, requiring 2/3 majority)
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
https://externals.io/message/109959 opcache.no_cache prototype: Opcode optimization without caching