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Date: Sat, 22 Jan 2022 19:22:03 +0100
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To: Rowan Tommins <rowan.collins@gmail.com>, internals@lists.php.net
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Subject: Re: [PHP-DEV] Re: Weak Closures
From: cmbecker69@gmx.de ("Christoph M. Becker")

On 22.01.2022 at 17:18, Rowan Tommins wrote:

> 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.=C2=A0 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 like the constant 10000 mentioned there has been replaced by an adaptive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.

Ah, right.  Thanks Rowan!

Christoph