Newsgroups: php.internals
Path: news.php.net
Xref: news.php.net php.internals:116908
Message-ID: <0bcb3e32-9afe-8957-7aa8-ea70f4308407@gmail.com>
Date: Sat, 22 Jan 2022 16:18:16 +0000
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101
 Thunderbird/91.5.0
Content-Language: en-GB
To: internals@lists.php.net
References: <CAFKaYMpquAowfpjbvS91fR5CjCwRVAj_QKMtJgF+AH_AXVEn9g@mail.gmail.com>
 <7ea94d3b-7f1f-b604-b94a-781e0b992c69@gmx.de>
In-Reply-To: <7ea94d3b-7f1f-b604-b94a-781e0b992c69@gmx.de>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Subject: Re: [PHP-DEV] Re: Weak Closures
From: rowan.collins@gmail.com (Rowan Tommins)

On 22/01/2022 15:30, Christoph M. Becker wrote:
> If you trigger the garbage collector manually (i.e. call
> gc_collect_cycles() after unset($callback)), the loop terminates right
> away.  I'm not sure why it doesn't without manually triggering the GC.


Most values are freed as soon as their refcount reaches zero, which 
obviously won't happen for circular references, so an additional 
algorithm is needed for those. This "cycle collection" algorithm is 
relatively expensive, so doesn't run every time a possible candidate is 
found, but only when a list of candidates reaches a particular 
threshold. The algorithm is outlined in the manual, although it looks 
like the constant 10000 mentioned there has been replaced by an adaptive 
threshold (which can be inspected with gc_status()): 
https://www.php.net/manual/en/features.gc.collecting-cycles.php

If you measure memory usage while running the example code in a loop, 
you should see it slowly growing and then periodically dropping each 
time a cycle collection is run. That's why gc_collect_cycles() exists - 
if you _know_ you've created circular references, you can tell the 
engine to find and free them immediately, rather than waiting for the 
next pass.

Regards,

-- 
Rowan Tommins
[IMSoP]