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Biederman" , "Luis R. Rodriguez" , Kees Cook , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , Al Viro , Stanislav Kinsbursky , Linux Containers , linux-api@vger.kernel.org References: <87woyfyh57.fsf@xmission.com> <5d4a858a-3136-5ef4-76fe-a61e7f2aed56@redhat.com> <87o9jru3bf.fsf@xmission.com> <935a7c50-50cc-2dc0-33bb-92c000d039bc@redhat.com> <87woyego2u.fsf_-_@xmission.com> <047c6ed6-6581-b543-ba3d-cadc543d3d25@redhat.com> <87h8ph6u67.fsf@xmission.com> <7d3a1f93-f8e5-5325-f9a7-0079f7777b6f@redhat.com> <20180329021409.gcjjrmviw2lckbfk@linux-n805> <3e201de2-bed2-6f7d-0783-700d095142e0@colorfullife.com> <20180329105601.GA597@bombadil.infradead.org> From: Manfred Spraul Message-ID: <05772f83-d680-aea1-b222-cef2430dcc83@colorfullife.com> Date: Thu, 29 Mar 2018 20:07:44 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: <20180329105601.GA597@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-api-owner@vger.kernel.org X-Mailing-List: linux-api@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: 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