Posted by Chris Smith on 03/06/05 16:34

Evert - Rooftop Solutions wrote:
> Mark Charette wrote:
>
>> Evert - Rooftop Solutions wrote:
>>
>>> I heard that shared memory is actually slower than writing and
>>> reading a file and it is not available on windows systems.
>>
>>
>>
>> Hmmm ... that's an interesting thing you heard concerning shared
>> memory. Care to share _who_ told you that?
>>
>> I'd like to make sure I don't hire him/her in the future.
>>
> http://www.onlamp.com/pub/a/php/2001/10/11/pearcache.html
>
> Check it out, is in old article, but it says "|shm| -- The |shm|
> container stores the cached data in the shared memory. Benchmarks
> indicate that the current implementation of this container is much
> slower than the |file| container."
>
> It is a bit outdated, but I haven't seen any recent benchmarks.

They both scale differently - shm linearly considering it's IO bandwidth
and latency is considerably higher and lower respectively. The shm
doesn't require a disk head to move around to get and save data to cache
(majority writes if it's caching to disk as reads get done from kernel
cache) which takes time.

shm will hold out a lot longer than files (I know from experience with
SVR4 many eons ago).

Cheers,

Chris Smith
Ninja Labs
http://www.ninjalabs.co.uk/

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