From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:38178) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fYrU3-0004nB-1o for qemu-devel@nongnu.org; Fri, 29 Jun 2018 07:22:36 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fYrTy-0000Fw-JH for qemu-devel@nongnu.org; Fri, 29 Jun 2018 07:22:35 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:53998 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fYrTy-0000EZ-DC for qemu-devel@nongnu.org; Fri, 29 Jun 2018 07:22:30 -0400 Date: Fri, 29 Jun 2018 12:22:25 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20180629112224.GG2568@work-vm> References: <20180604095520.8563-1-xiaoguangrong@tencent.com> <20180604095520.8563-8-xiaoguangrong@tencent.com> <20180619073650.GB14814@xz-mi> <5745f752-50b5-0645-21a7-3336ea0dd5c2@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5745f752-50b5-0645-21a7-3336ea0dd5c2@gmail.com> Subject: Re: [Qemu-devel] [PATCH 07/12] migration: hold the lock only if it is really needed List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Xiao Guangrong Cc: Peter Xu , pbonzini@redhat.com, mst@redhat.com, mtosatti@redhat.com, qemu-devel@nongnu.org, kvm@vger.kernel.org, jiang.biao2@zte.com.cn, wei.w.wang@intel.com, Xiao Guangrong * Xiao Guangrong (guangrong.xiao@gmail.com) wrote: > > > On 06/19/2018 03:36 PM, Peter Xu wrote: > > On Mon, Jun 04, 2018 at 05:55:15PM +0800, guangrong.xiao@gmail.com wrote: > > > From: Xiao Guangrong > > > > > > Try to hold src_page_req_mutex only if the queue is not > > > empty > > > > Pure question: how much this patch would help? Basically if you are > > running compression tests then I think it means you are with precopy > > (since postcopy cannot work with compression yet), then here the lock > > has no contention at all. > > Yes, you are right, however we can observe it is in the top functions > (after revert this patch): Can you show the matching trace with the patch in? Dave > Samples: 29K of event 'cycles', Event count (approx.): 22263412260 > + 7.99% kqemu qemu-system-x86_64 [.] ram_bytes_total > + 6.95% kqemu [kernel.kallsyms] [k] copy_user_enhanced_fast_string > + 6.23% kqemu qemu-system-x86_64 [.] qemu_put_qemu_file > + 6.20% kqemu qemu-system-x86_64 [.] qemu_event_set > + 5.80% kqemu qemu-system-x86_64 [.] __ring_put > + 4.82% kqemu qemu-system-x86_64 [.] compress_thread_data_done > + 4.11% kqemu qemu-system-x86_64 [.] ring_is_full > + 3.07% kqemu qemu-system-x86_64 [.] threads_submit_request_prepare > + 2.83% kqemu qemu-system-x86_64 [.] ring_mp_get > + 2.71% kqemu qemu-system-x86_64 [.] __ring_is_full > + 2.46% kqemu qemu-system-x86_64 [.] buffer_zero_sse2 > + 2.40% kqemu qemu-system-x86_64 [.] add_to_iovec > + 2.21% kqemu qemu-system-x86_64 [.] ring_get > + 1.96% kqemu [kernel.kallsyms] [k] __lock_acquire > + 1.90% kqemu libc-2.12.so [.] memcpy > + 1.55% kqemu qemu-system-x86_64 [.] ring_len > + 1.12% kqemu libpthread-2.12.so [.] pthread_mutex_unlock > + 1.11% kqemu qemu-system-x86_64 [.] ram_find_and_save_block > + 1.07% kqemu qemu-system-x86_64 [.] ram_save_host_page > + 1.04% kqemu qemu-system-x86_64 [.] qemu_put_buffer > + 0.97% kqemu qemu-system-x86_64 [.] compress_page_with_multi_thread > + 0.96% kqemu qemu-system-x86_64 [.] ram_save_target_page > + 0.93% kqemu libpthread-2.12.so [.] pthread_mutex_lock > > I guess its atomic operations cost CPU resource and check-before-lock is > a common tech, i think it shouldn't have side effect, right? :) > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK