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List:       php-qa
Subject:    Bug #77879 [Nab]: Usable memory
From:       nikic () php ! net
Date:       2019-04-15 8:14:45
Message-ID: E1hFwlJ-0003wx-AX () bugs ! php ! net
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Edit report at https://bugs.php.net/bug.php?id=77879&edit=1

 ID:                 77879
 Updated by:         nikic@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:
------------------------------------------------------------------------
[2019-04-14 03:20:53] xxalfa at gmail dot com

Reply to requinix@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 nikic@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] nikic@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] requinix@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] requinix@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?

------------------------------------------------------------------------


The remainder of the comments for this report are too long. To view
the rest of the comments, please view the bug report online at

    https://bugs.php.net/bug.php?id=77879


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Edit this bug report at https://bugs.php.net/bug.php?id=77879&edit=1


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