From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:34106 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727449AbeJCA1q (ORCPT ); Tue, 2 Oct 2018 20:27:46 -0400 Subject: Re: [PATCH v8 3/5] ipc: Allow boot time extension of IPCMNI from 32k to 2M To: Manfred Spraul , Davidlohr Bueso Cc: "Luis R. Rodriguez" , Kees Cook , Andrew Morton , Jonathan Corbet , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-doc@vger.kernel.org, Al Viro , Matthew Wilcox , "Eric W. Biederman" , Takashi Iwai , Davidlohr Bueso References: <1529317698-16575-1-git-send-email-longman@redhat.com> <1529317698-16575-4-git-send-email-longman@redhat.com> <20180817164548.GA32382@linux-r8p5> <5ddb67bb-872b-c8c1-7838-2622195ae1fc@redhat.com> <02ebafee-a295-58cf-044e-e2df0e878e0d@colorfullife.com> From: Waiman Long Message-ID: <8c9dda74-c9fb-cc18-5af4-c081dbb8b154@redhat.com> Date: Tue, 2 Oct 2018 13:43:13 -0400 MIME-Version: 1.0 In-Reply-To: <02ebafee-a295-58cf-044e-e2df0e878e0d@colorfullife.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Content-Language: en-US Sender: linux-fsdevel-owner@vger.kernel.org List-ID: 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 >