From mboxrd@z Thu Jan 1 00:00:00 1970 From: Xiao Guangrong Subject: Re: [PATCH 07/12] migration: hold the lock only if it is really needed Date: Thu, 28 Jun 2018 17:33:58 +0800 Message-ID: <5745f752-50b5-0645-21a7-3336ea0dd5c2@gmail.com> References: <20180604095520.8563-1-xiaoguangrong@tencent.com> <20180604095520.8563-8-xiaoguangrong@tencent.com> <20180619073650.GB14814@xz-mi> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, mst@redhat.com, mtosatti@redhat.com, Xiao Guangrong , dgilbert@redhat.com, qemu-devel@nongnu.org, wei.w.wang@intel.com, jiang.biao2@zte.com.cn, pbonzini@redhat.com To: Peter Xu Return-path: In-Reply-To: <20180619073650.GB14814@xz-mi> Content-Language: en-US List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+gceq-qemu-devel2=m.gmane.org@nongnu.org Sender: "Qemu-devel" List-Id: kvm.vger.kernel.org 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): 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? :)