Re: [APC-DEV] atomaticity/thread safeness of apc shared memory functions+mutexes

Exception e wrote:
> 
>> apc_fetch() grabs a copy of the value in shared memory.  What you do
>> with it locally in the process after the fetch is irrelevant.  It is
>> just a copy.  You can then overwrite the value in shared memory with a
>> new version with an apc_store() call.  Locking shared memory the way you
>> suggest would be a really bad idea.  It would be a performance
>> nightmare.  You need to design your code in a way that does not require
>> that.
>>
> 
> Locking all shared memory would indeed be bad, I was already wondering
> whether you really meant that. But I use php for a multi-user game as
> part of an academic investigation. To speed things up I
> store(=serialize) objects in shared memory. I need to ensure that
> certain objects in shared memory keep a coherent state. That is, when I
> use object1 and object2 they should not be modified during a request. So
> then I need to lock those two entries. Atomic reads and writes won't
> help here.
> But since the apc functions are themselves atomic, I could utilize them
> for implementing mutexes. A simple example would be
> 
> function lock($entry)    {
>     // busy wait
>     while(!apc_add($entry.'.lock'))
>         sleep(1);
> }
> 
> function unlock($entry)    {
>     return apc_delete($entry.'.lock');
> }
> 
> lock('obj1');
> lock('obj2');
> $obj1= apc_fetch('obj1');
> $obj2 = apc_fetch('obj2');
> 
> //process, apc_store val1 and val2 again
> 
> unlock('obj1');
> unlock('obj2');
> 
> 
> Then it would only be nice if lock and unlock would be part of apc_*
> userland functions. I am curious about your ideas.

You could do something like that, but it would be amazingly inefficient.
and can't possibly scale to multiple machines behind a load balancer.
Also, storing objects in shared memory is slow and usually pointless.
Only the properties of the object are stored anyway, so you may as well
store an array directly.  That has the benefit of not needing serialization.

-Rasmus