Re: [PATCH v11 1/3] ipc: Allow boot time extension of IPCMNI from 32k to 16M

[Manfred Spraul]

On 11/9/18 9:11 PM, Waiman Long wrote:
> The maximum number of unique System V IPC identifiers was limited to
> 32k.  That limit should be big enough for most use cases.
>
> However, there are some users out there requesting for more, especially
> those that are migrating from Solaris which uses 24 bits for unique
> identifiers. To satisfy the need of those users, a new boot time kernel
> option "ipcmni_extend" is added to extend the IPCMNI value to 16M. This
> is a 512X increase which should be big enough for users out there that
> need a large number of unique IPC identifier.
>
> The use of this new option will change the pattern of the IPC identifiers
> returned by functions like shmget(2). An application that depends on
> such pattern may not work properly.  So it should only be used if the
> users really need more than 32k of unique IPC numbers.
>
> This new option does have the side effect of reducing the maximum number
> of unique sequence numbers from 64k down to 128. So it is a trade-off.
>
> The computation of a new IPC id is not done in the performance critical
> path.  So a little bit of additional overhead shouldn't have any real
> performance impact.
>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
Acked-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>