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Date: Tue, 02 Sep 2003 14:33:54 +0300
To: "Vesselin Atanasov" <vma1@abv.bg>
Cc: "PHP-DEV" <internals@lists.php.net>
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Subject: Re: [PHP-DEV] [PATCH] Disable the Zend memory cache
From: zeev@zend.com (Zeev Suraski)
References: <JJEIJKBBKGFHEDGHDANNAEHFCAAA.vma1@abv.bg>

At 12:46 02/09/2003, Vesselin Atanasov wrote:
>Hello.
>The memory cache code in zend_alloc.c has serious problems with memory 
>fragmentation.
>When a big script is run which allocates both lots of small objects and
>a few big objects, the memory cache grows until the allowed limits
>(11 buckets x 256 blocks each) keeping all of the cached memory occupied 
>although
>the script uses much less memory. This is caused by the cache code which 
>caches small
>objects and does not allow the low level memory manager (either the 
>zend_mm.c or the
>C runtime malloc) to merge small free blocks into bigger free blocks. From 
>the point
>of view of the low level memory manager there are lots of occupied memory 
>blocks
>interspersed among the allocated memory segment. the occupied memory 
>blocks are
>actually cached by the high level cache code. So when the script is run a 
>new segment
>is allocated although there is free memory. With each call new segment(s) 
>are allocated
>until the memory cache is full. This scenario happens when a script first 
>creates lots
>of small objects and then a few large objects. Even if the small objects 
>are freed,
>the memory is fragmented and a new block is occupied for the big 
>object(s). So with
>each request the lost memory increases, effectively this is like a memory 
>leak.
>I attach a patch which adds --disable-memory-cache option which disables 
>the memory
>cache.

Vesselin,

The memory fragmentation issues were reported by Sterling a few weeks ago - 
we know they're there.  The simplest solution appears to be decreasing the 
zend_mm block size to 16KB instead of 256KB.  Due to bugs elsewhere 
(libxml2), making this change appeared to cause problems, but with these 
bugs now fixed - it appears to work fine.  After we perform some more 
benchmarks we'll commit this change.
Disabling memory caching altogether is probably not a good idea since PHP 
allocates and frees small blocks VERY often.

Zeev