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

[Manfred Spraul]

Hello together,

On 8/18/18 3:15 AM, Waiman Long wrote:
> 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
> Solaris.

- I think we should use the same numbers as Solaris.
Otherwise we later have to touch it again.

- What is the performance when using shmget() with already 10M segments 
present?

- I like the new logic for updating the sequence counter.

Is there a reason why you only enable it for extended mode?

You create a rarely used codepath, and I don't understand what speaks 
against switching to the 'deleted' approach for all systems.


--

ÂÂÂ Manfred