VPS Theory
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OOM
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Please note that in a VPS, you can use to the memory alloted to you till the free memory is 0. Once you have reached the guaranteed memory limit, memory from the Burst memory will be alloted for the processes. If the processes are staying long in Burst memory and there are other VPS processes which are waiting, then kernel will kill few processes and free some memory space (OOM). Here your php scripts are taking too much memory which is causing the issue. You have to optimize the scripts or you need to go for a plan up-gradation if this occurs too frequently.