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What is Intimate Shared Memory or ISM?
Intimate Shared Memory or ISM is a specific feature of Sun Solaris. The feature was developed so that when multiple processes (at OS level) try to access a shared memory region, they do not use multiple TLBs (Transalation Lookaside Buffers) at OS kernel level. This saves lot of kernel memory space.
I don't think that does a whole lot for Sybase, more for Oracle I suppose. However, there is a side effect that is useful. If there is enough memory available on the machine, typically Solaris will not swap out process memory marked as ISM if it can possibly help it.
Swapping in Solaris is done in three phases, reserved, allocated and used. Locking the shared memory has the advantage of increasing performance. Of course, if there are lot's of processes on the machine and if new processes starve for memory, there is a potential that ISM will get swapped.
For performance reasons, it is worth ensuring that Sybase can allocated its shared memory segment using ISM. ASE tries by default to use ISM and will display an error message during start up if this is not possible. It is probably worth starting Sybase soon after a machine is rebooted to give it the best possible chance of using ISM.
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