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In PHP xCache and APC, there are options that allows to set maximum memory. Do xCache and APC actually RESERVE the memory, or they just use it whenever they need to?

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I'm not sure what you mean by the RESERVE. If you use malloc on Linux to allocate some memory, you get pointer to some virtual memory, which is not yet mapped to physical RAM. Physical memory pages are allocated only when you start putting data there.

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