[PATCH] Disable the Zend memory cache

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

Let's hold on this patch until we finish benchmarking. We might just "do
the right thing" automagically. I prefer the user not to have to be smart
about such internals.

Andi

At 12:46 PM 9/2/2003 +0300, 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

diff -ruN php5-200309020130.orig/Zend/Zend.m4 php5-200309020130/Zend/Zend.m4
--- php5-200309020130.orig/Zend/Zend.m4 2003-08-11 06:07:39.000000000 +0000
+++ php5-200309020130/Zend/Zend.m4 2003-09-02 09:28:46.000000000 +0000
@@ -122,6 +122,13 @@
ZEND_INLINE_OPTIMIZATION=yes
])

+AC_ARG_ENABLE(memory-cache,
+[ --disable-memory-cache Disable the Zend memory cache. ], [

• ZEND_MEMORY_CACHE=$enableval
  +],[
• ZEND_MEMORY_CACHE=yes
  +])

AC_ARG_ENABLE(memory-limit,
[ --enable-memory-limit Compile with memory limit support. ], [
ZEND_MEMORY_LIMIT=$enableval
@@ -142,6 +149,9 @@
AC_MSG_CHECKING(whether to enable inline optimization for GCC)
AC_MSG_RESULT($ZEND_INLINE_OPTIMIZATION)

+AC_MSG_CHECKING(whether to enable Zend memory cache)
+AC_MSG_RESULT($ZEND_MEMORY_CACHE)

AC_MSG_CHECKING(whether to enable a memory limit)
AC_MSG_RESULT($ZEND_MEMORY_LIMIT)

@@ -172,6 +182,12 @@
LIBZEND_CPLUSPLUS_CHECKS
fi

+if test "$ZEND_MEMORY_CACHE" = "yes"; then

• AC_DEFINE(ZEND_DISABLE_MEMORY_CACHE, 0, [Memory cache])
  +else
• AC_DEFINE(ZEND_DISABLE_MEMORY_CACHE, 1, [Memory cache])
  +fi

if test "$ZEND_MEMORY_LIMIT" = "yes"; then
AC_DEFINE(MEMORY_LIMIT, 1, [Memory limit])
else
diff -ruN php5-200309020130.orig/Zend/zend_alloc.c
php5-200309020130/Zend/zend_alloc.c
--- php5-200309020130.orig/Zend/zend_alloc.c 2003-08-28
17:06:54.000000000 +0000
+++ php5-200309020130/Zend/zend_alloc.c 2003-09-02 09:28:46.000000000 +0000
@@ -38,12 +38,6 @@

#ifdef ZEND_MM
-#define ZEND_DISABLE_MEMORY_CACHE 0
-#else
-#define ZEND_DISABLE_MEMORY_CACHE 0
-#endif

-#ifdef ZEND_MM
#define ZEND_DO_MALLOC(size) zend_mm_alloc(&AG(mm_heap), size)
#define
ZEND_DO_FREE(ptr) zend_mm_free(&AG(mm_heap), ptr)
#define ZEND_DO_REALLOC(ptr, size) zend_mm_realloc(&AG(mm_heap),
ptr, size)