On 10/02/2018 12:32 PM, Manfred Spraul wrote: > 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. As said in my patch, it is a trade-off between # of uniq identifiers versus the chance of id reuse. I don't have an objection to increase it further, but I don't see the customers to really need such a large value. > > - What is the performance when using shmget() with already 10M > segments present? I am not sure the performance impact as I had not measure it myself. The shmget() function is considered in slowpath. We are generally less concern about its performance than other code paths that are in a performance critical path. > > - I like the new logic for updating the sequence counter. > > Is there a reason why you only enable it for extended mode? I tried not to disturb the existing logic for backward compatibility concern. I don't mind switching it all over to use the new "deleted" approach if other people have no objection. Cheers, Longman > You create a rarely used codepath, and I don't understand what speaks > against switching to the 'deleted' approach for all systems. > > > -- > > Manfred >
