linux-kernel - Re: [RFC][PATCH] ipc: Remove IPCMNI

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Date:   Thu, 29 Mar 2018 20:07:44 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Davidlohr Bueso <dave@...olabs.net>,
        Waiman Long <longman@...hat.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Stanislav Kinsbursky <skinsbursky@...allels.com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        linux-api@...r.kernel.org
Subject: Re: [RFC][PATCH] ipc: Remove IPCMNI

Hello Mathew,

On 03/29/2018 12:56 PM, Matthew Wilcox wrote:
> On Thu, Mar 29, 2018 at 10:47:45AM +0200, Manfred Spraul wrote:
>>>>>>> This can be implemented trivially with the current code
>>>>>>> using idr_alloc_cyclic.
>> Is there a performance impact?
>> Right now, the idr tree is only large if there are lots of objects.
>> What happens if we have only 1 object, with id=INT_MAX-1?
> The radix tree uses a branching factor of 64 entries (6 bits) per level.
> The maximum ID is 31 bits (positive signed 32-bit integer).  So the
> worst case for a single object is 6 pointer dereferences to find the
> object anywhere in the range (INT_MAX/2 - INT_MAX].  That will read 12
> cachelines.  If we were to constrain ourselves to a maximum of INT_MAX/2
> (30 bits), we'd reduce that to 5 pointer dereferences and 10 cachelines.
I'm concerned about the up to 6 branches.
But this is just guessing, we need a test with a realistic workload.

--
     Manfred