From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-yb1-f200.google.com (mail-yb1-f200.google.com [209.85.219.200]) by kanga.kvack.org (Postfix) with ESMTP id C63356B038E for ; Tue, 6 Nov 2018 14:00:57 -0500 (EST) Received: by mail-yb1-f200.google.com with SMTP id w17-v6so2233736ybe.13 for ; Tue, 06 Nov 2018 11:00:57 -0800 (PST) Received: from aserp2120.oracle.com (aserp2120.oracle.com. [141.146.126.78]) by mx.google.com with ESMTPS id 1-v6si27759928ywz.277.2018.11.06.11.00.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 06 Nov 2018 11:00:56 -0800 (PST) Date: Tue, 6 Nov 2018 11:00:29 -0800 From: Daniel Jordan Subject: Re: [RFC PATCH v4 00/13] ktask: multithread CPU-intensive kernel work Message-ID: <20181106190029.epktpxhimrca4f4a@ca-dmjordan1.us.oracle.com> References: <20181105165558.11698-1-daniel.m.jordan@oracle.com> <20181106022024.ndn377ze6xljsxkb@ca-dmjordan1.us.oracle.com> <7E53DD63-4955-480D-8C0D-EB07E4FF011B@cs.rutgers.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7E53DD63-4955-480D-8C0D-EB07E4FF011B@cs.rutgers.edu> Sender: owner-linux-mm@kvack.org List-ID: To: Zi Yan Cc: Daniel Jordan , linux-mm@kvack.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, aarcange@redhat.com, aaron.lu@intel.com, akpm@linux-foundation.org, alex.williamson@redhat.com, bsd@redhat.com, darrick.wong@oracle.com, dave.hansen@linux.intel.com, jgg@mellanox.com, jwadams@google.com, jiangshanlai@gmail.com, mhocko@kernel.org, mike.kravetz@oracle.com, Pavel.Tatashin@microsoft.com, prasad.singamsetty@oracle.com, rdunlap@infradead.org, steven.sistare@oracle.com, tim.c.chen@intel.com, tj@kernel.org, vbabka@suse.cz On Mon, Nov 05, 2018 at 09:48:56PM -0500, Zi Yan wrote: > On 5 Nov 2018, at 21:20, Daniel Jordan wrote: > > > Hi Zi, > > > > On Mon, Nov 05, 2018 at 01:49:14PM -0500, Zi Yan wrote: > >> On 5 Nov 2018, at 11:55, Daniel Jordan wrote: > >> > >> Do you think if it makes sense to use ktask for huge page migration (the data > >> copy part)? > > > > It certainly could. > > > >> I did some experiments back in 2016[1], which showed that migrating one 2MB page > >> with 8 threads could achieve 2.8x throughput of the existing single-threaded method. > >> The problem with my parallel page migration patchset at that time was that it > >> has no CPU-utilization awareness, which is solved by your patches now. > > > > Did you run with fewer than 8 threads? I'd want a bigger speedup than 2.8x for > > 8, and a smaller thread count might improve thread utilization. > > Yes. When migrating one 2MB THP with migrate_pages() system call on a two-socket server > with 2 E5-2650 v3 CPUs (10 cores per socket) across two sockets, here are the page migration > throughput numbers: > > throughput factor > 1 thread 2.15 GB/s 1x > 2 threads 3.05 GB/s 1.42x > 4 threads 4.50 GB/s 2.09x > 8 threads 5.98 GB/s 2.78x Thanks. Looks like in your patches you start a worker for every piece of the huge page copy and have the main thread wait. I'm curious what the workqueue overhead is like on your machine. On a newer Xeon it's ~50usec from queueing a work to starting to execute it and another ~20usec to flush a work (barrier_func), which could happen after the work is already done. A pretty significant piece of the copy time for part of a THP. bash 60728 [087] 155865.157116: probe:ktask_run: (ffffffffb7ee7a80) bash 60728 [087] 155865.157119: workqueue:workqueue_queue_work: work struct=0xffff95fb73276000 bash 60728 [087] 155865.157119: workqueue:workqueue_activate_work: work struct 0xffff95fb73276000 kworker/u194:3- 86730 [095] 155865.157168: workqueue:workqueue_execute_start: work struct 0xffff95fb73276000: function ktask_thread kworker/u194:3- 86730 [095] 155865.157170: workqueue:workqueue_execute_end: work struct 0xffff95fb73276000 kworker/u194:3- 86730 [095] 155865.157171: workqueue:workqueue_execute_start: work struct 0xffffa676995bfb90: function wq_barrier_func kworker/u194:3- 86730 [095] 155865.157190: workqueue:workqueue_execute_end: work struct 0xffffa676995bfb90 bash 60728 [087] 155865.157207: probe:ktask_run_ret__return: (ffffffffb7ee7a80 <- ffffffffb7ee7b7b) > > > > It would be nice to multithread at a higher granularity than 2M, too: a range > > of THPs might also perform better than a single page. > > Sure. But the kernel currently does not copy multiple pages altogether even if a range > of THPs is migrated. Page copy function is interleaved with page table operations > for every single page. > > I also did some study and modified the kernel to improve this, which I called > concurrent page migration in https://lwn.net/Articles/714991/. It further > improves page migration throughput. Ok, over 4x with 8 threads for 16 THPs. Is 16 a typical number for migration, or does it get larger? What workloads do you have in mind with this change?