Zeev Suraski wrote:
> At 12:55 AM 6/9/2002, Brian Pane wrote:
>
>> I just looked through zend_alloc.c. It looks like the HeapCreate only
>> happens once, at startup--did I get that right?
>
>
> It's called on the per-thread startup (start_memory_manager(), which
> is called from alloc_globals_ctor(), which is the per-thread
> allocation 'constructor').
>
>> For PHP processing, do
>> you need a memory heap that is created at startup and used by all
>> requests,
>> or is it sufficient to create and destroy a private heap for each
>> request?
>
>
> We can go either way, but won't allocating and freeing these heaps
> increase fragmentation? Remember that PHP functions will also
> allocate memory outside these heaps (in 3rd party libraries).
In the httpd, we've done two things to minimize the fragmentation:
* Memory for these heaps is almost always allocated in chunks of
a fixed size, 8KB.
* There's a two-layer structure to the heaps:
- apr_pool objects are what the application uses. Each pool
provides a fast alloc interface, no free function, and a
"destructor" that returns all the allocated space when the
pool is destroyed.
- apr_allocator objects provide blocks of memory for the
apr_pools to use. Each apr_allocator maintains a free
list of blocks. When a pool is destroyed, its space is
returned to the apr_allocator from which it was created.
The next time we create a pool from that apr_allocator,
we can take an 8KB block from that allocator's free list
rather than interacting with the system heap.
--Brian
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