Re: [PATCH v8 3/5] ipc: Allow boot time extension of IPCMNI from 32k to 2M

From: Waiman Long
Date: Fri Aug 17 2018 - 21:15:54 EST

On 08/17/2018 12:45 PM, Davidlohr Bueso wrote:
> Cc'ing Manfred.
> On Mon, 18 Jun 2018, 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. To satisfy
>> the need of those users, a new boot time kernel option "ipcmni_extend"
>> is added to extend the IPCMNI value to 2M. This is a 64X increase which
>> hopefully is big enough for them.
> Could you please provide more info on the need of these users and how
> you came up with this new value (which just seems quite arbitrary)?
> Thanks,
> Davidlohr

Red Hat has a customer that is migrating from Solaris to Linux. Some of
their applications just happen to use more than 32k of shared memory
segments. I think Solaris allows up to 16M unique ID.

Yes, the amount of increase is a bit arbitrary. I was trying to balance
how many bits should be left for sequence number. Maybe I should just
take 8 more bits for ID and leave 8 bits for sequence number to match