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Date: Fri, 03 Feb 2006 09:17:14 +1300
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To: Brian Moon <brianm@dealnews.com>
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Subject: Re: [PHP-DEV] PHP, Apache 2, memory and APC
From: rasmus@lerdorf.com (Rasmus Lerdorf)

Brian Moon wrote:
> I have some questions about the Apache 2 SAPI.  We had been using 
> apache_child_terminate to control memory usage with Apache 1.3.  This 
> function is not available with the Apache 2 SAPI.  Is this function just 
> not offered in the Apache 2 API?  If it is offered and there is some 
> gotcha or simply work to be done, I would be happy to roll up my 
> sleeves, dust off my C hat and submit a patch.  I just need a push in 
> the right direction.  I can't really find an Apache 2 SAPI API reference 
> anywhere.
> 
> Because we can not use this function, we decided to try out the worker 
> MPM to help our memory woes.  So far, we have had great success with it. 
>  Our limited extension list appears to have no problems.  The memory 
> usage on our web servers is down about 15% now.

That makes very little sense.  An individual Apache process will use the 
same amount of memory whether it is an Apache1 process or an Apache2 
Worker process.  It probably uses more under Apache2 ZTS actually (and 
it runs slower).  The only difference is that the threads within the 
Apache2 ZTS process will re-use each others' memory while under Apache1 
the memory will only be reused by requests to the same process.  Trying 
to hand it back to the OS via an sbrk or something is usually rather 
redundant since another request will come along and allocate that same 
amount again.  If you are running out of system memory the only 
realistic recourse is to lower the number of concurrent processes.  I 
tend to never run more than about 50 per machine, depending on the size 
of the machine.  Trying to run more doesn't win you anything.  You are 
better off serving less concurrent requests faster on each of your 
machines behind your load balancer.

> However, this meant we can not use APC.  Are there plans to make APC 
> work with ZTS?  I know APC is not developed by the PHP group, but I also 
> know Rasmus is involved and I hoped this would find him the easiest.

I have no plans to work on ZTS stuff.  Work has been done on it in the 
past though and I know it works in ZTS mode at least under Windows.

> Speaking of those memory woes, I know there is a lot of confusion about 
> that problem with PHP.  Can one of the core guys explain why the PHP 
> memory manager can't give the memory back like a simple emalloc can?

emalloc is the PHP memory manager.  Did you mean malloc?  Standard 
malloc()/free() doesn't give memory back to the system.  If your process 
mallocs 100M and then frees that memory, your process still hangs onto 
that 100M until it exits (with some caveats/exceptions on that, but 
generically that is how UNIX works).

-Rasmus