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

Exception e wrote:
> Rasmus Lerdorf schreef:
>> Exception e wrote:
>>> Rasmus Lerdorf schreef:
>>>
>>>> Yes, they are atomic and they are locked appropriately.  You don't need
>>>> any sort of mutex.  This is shared memory.  Generally we don't suggest
>>>> running PHP in any sort of threaded environment though.
>>> To respond to your last remark: php processes can be exectuted in
>>> parallel by Apache—at least this is my understanding. So if I need to
>>> work with multiple shared memory entries per request i still need
>>> mutexes in order to lock concurrent access to these pieces.
>>
>> That's not multi-threading, that is multi-processing, and of course APC
>> handles that correctly and locks appropriately.  Multi-threading is when
>> you have multiple threads of execution within the same process.
>>
> 
> 
> Yes, but do you mean that all shared memory entries are blocked for the
> duration of a process? Let's give me an example
> 
> <?php
> 
> $val1 = apc_fetch('val1');
> 
> // (#1) different process does: apc_store('val1', 34);
> 
> $val2 = apc_fetch('val2');
> 
> echo $val1 + $val2;
> 
> ?>
> 
> 
> From what I understand is that when the same code runs in a different
> process, the shared memory segment 'val1' can be changed on (#1) by that
> other process. So in fact I need two mutexes.
> 
> But if all shared memory is locked for each process, than such a
> situation cannot happen. Did you really mean that? I hope you could
> clarify this.

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.

APC 3.1 has functions that let you atomically increment and decrement a
value in shared memory.  See the apc_inc() and apc_dec() functions.
That is the only way that makes sense to manipulate shared memory
entries in a way resembling what you are suggesting.

-Rasmus