Edit report at https://bugs.php.net/bug.php?id=77879&edit=1
ID: 77879
Updated by: ni...@php.net
Reported by: xxalfa at gmail dot com
Summary: Usable memory
Status: Not a bug
Type: Bug
Package: Testing related
Operating System: Windows
PHP Version: 7.3.4
Block user comment: N
Private report: N
New Comment:
@xxalfa With your script, this is the last output I get:
memory_allocated -- 10 MByte -- memory_used -- 6740856 -- data_length -- 6290432
Fatal error: Allowed memory size of 16777216 bytes exhausted at
/home/nikic/php-7.4/Zend/zend_string.h:205 (tried to allocate 6291488 bytes) in
/home/nikic/php-7.4/t022.php on line 65
As you can see, the attempt is to allocate 6MB additional memory, which ends up
going above the memory limit. Even though the string doesn't take up the full
16MB, in order to reallocate it (if it's not possible to do so in-place) we
first have to create a new larger allocation, copy the string and then release
the old one. So you may need about 2x more memory to handle the reallocation.
Previous Comments:
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[2019-04-14 03:20:53] xxalfa at gmail dot com
Reply to requi...@php.net
I am glad of every answer, only we both do not speak the same language. PHP can
allocate as much memory as it needs. The amount of data I want to process
requires knowing the amount of memory consumed to initiate paging operations.
Any write access to the hard drive takes time, so I want to use as much memory
as possible.
Reply to ni...@php.net
I thank you for your answer, which I now understand the way PHP works. I hope
you can briefly answer my second question? When I open the task manager, I can
keep track of how much memory a script consumes. Although PHP reserves full
RAM, I can only fill it up to 40% at 16 MB. If I set the limit to 41% and fill
the memory, I get a fatal error. In the Task Manager, a value of about 12 MB is
given, why does not "memory_get_usage" provide the true consumption of 12 MB?
------------------------------------------------------------------------
[2019-04-12 07:48:07] ni...@php.net
Memory allocations are restricted to certain sizes for performance reasons.
Small allocations are rounded up the next largest bin size. When the memory is
reallocated with a size that still fits within the same bin, the reallocation
is performed in-place and the reported memory usage will not change.
------------------------------------------------------------------------
[2019-04-12 03:06:55] requi...@php.net
No, the memory does not increase by 1024 each time. It increases by whatever
amount PHP allocates in order to expand the string by another 1024 bytes. As
your output shows, the last step it went up by 4096 - enough for approximately
4 increases.
Should it have only allocated 1k? Is it bad that PHP is reserving more memory
than the precise amount it needs at that given moment?
------------------------------------------------------------------------
[2019-04-12 02:58:35] xxalfa at gmail dot com
The script executes successfully because it has detected because the level of
memory has not changed. However, the level increases by 1024 per cycle, if the
function were to work properly, the script would continue to work. That's why
it's a bug.
------------------------------------------------------------------------
[2019-04-12 02:26:32] requi...@php.net
Your script is quitting because PHP didn't need to allocate more memory to
handle another 1k characters. Why is that a bug?
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