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Wed, 26 Mar 2025 15:06:09 +0000 (GMT) Message-ID: <89bb1d0e-4835-43d4-9e63-0b1717a9921e@linux.ibm.com> Date: Wed, 26 Mar 2025 20:36:08 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v10 20/21] futex: Implement FUTEX2_NUMA To: Peter Zijlstra Cc: Sebastian Andrzej Siewior , =?UTF-8?Q?Andr=C3=A9_Almeida?= , Darren Hart , Davidlohr Bueso , Ingo Molnar , Juri Lelli , Thomas Gleixner , Valentin Schneider , Waiman Long , linux-kernel@vger.kernel.org References: <20250312151634.2183278-1-bigeasy@linutronix.de> <20250312151634.2183278-21-bigeasy@linutronix.de> <20250325225600.GP36322@noisy.programming.kicks-ass.net> <4d0059f1-d834-44d5-bc57-7425980f5a97@linux.ibm.com> <20250326133736.GF25239@noisy.programming.kicks-ass.net> From: Shrikanth Hegde Content-Language: en-US In-Reply-To: <20250326133736.GF25239@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: oqDYTGkS1p-zjuK_461LF_IMRfeai3V_ X-Proofpoint-GUID: oqDYTGkS1p-zjuK_461LF_IMRfeai3V_ X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1095,Hydra:6.0.680,FMLib:17.12.68.34 definitions=2025-03-26_07,2025-03-26_02,2024-11-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 spamscore=0 malwarescore=0 suspectscore=0 phishscore=0 clxscore=1015 adultscore=0 impostorscore=0 mlxscore=0 lowpriorityscore=0 mlxlogscore=666 priorityscore=1501 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2502280000 definitions=main-2503260091 On 3/26/25 19:07, Peter Zijlstra wrote: > On Wed, Mar 26, 2025 at 06:27:20PM +0530, Shrikanth Hegde wrote: >> >> >> On 3/26/25 04:26, Peter Zijlstra wrote: >>> On Wed, Mar 26, 2025 at 01:22:19AM +0530, Shrikanth Hegde wrote: >>> >>>>> + return &futex_queues[node][hash & futex_hashmask]; >>> >>> ^^^^^^^ >>> >>>>> + hashsize = 256 * num_possible_cpus(); >>>>> + hashsize /= num_possible_nodes(); >>>> >>>> Wouldn't it be better to use num_online_nodes? each node may get a bigger >>>> hash bucket which means less collision no? >>> >>> No. There are two problems with num_online_nodes, and both are evident >>> above. >>> >>> Consider the case of a sparse set. >> >> I am sorry, i didn't understand. Could you please explain? > > I was confused; I should've just gone sleep :-) > > The futex_queues[] array is sized MAX_NUMNODES, such that every possible > node_id has a spot. I thought we did dynamic sizing, but not so. > > Anyway, using online here would lead to having to deal with hotplug, > which in turn either leads to more over-all hash buckets in the system, > or having to resize and rehash everything. > > Neither are really attractive options. > Ok. got it. Thanks. Keeping with possible nodes seems simpler.