From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN Date: Mon, 09 Feb 2015 12:18:04 -0800 Message-ID: <54D915FC.7010003@amacapital.net> References: <68a0ad4a99551ea3bfff89da461bb490d63b0ca8.1423509605.git.jbaron@akamai.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Cc: akpm@linux-foundation.org, normalperson@yhbt.net, davidel@xmailserver.org, mtk.manpages@gmail.com, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org To: Jason Baron , peterz@infradead.org, mingo@redhat.com, viro@zeniv.linux.org.uk Return-path: In-Reply-To: <68a0ad4a99551ea3bfff89da461bb490d63b0ca8.1423509605.git.jbaron@akamai.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On 02/09/2015 12:06 PM, Jason Baron wrote: > Epoll file descriptors that are added to a shared wakeup source are always > added in a non-exclusive manner. That means that when we have multiple epoll > fds attached to a shared wakeup source they are all woken up. This can > lead to excessive cpu usage and uneven load distribution. > > This patch introduces two new 'events' flags that are intended to be used > with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event > source in an exclusive manner such that the minimum number of threads are > woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can > also be added to the 'events' flag, such that we round robin around the set > of waiting threads. > > An implementation note is that in the epoll wakeup routine, > 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful > wakeup, only when there are current waiters. The idea is to use this additional > heuristic in order minimize wakeup latencies. I don't understand what this is intended to do. If an event has EPOLLONESHOT, then this only one thread should be woken regardless, right? If not, isn't that just a bug that should be fixed? If an event has EPOLLET, then the considerations are similar to EPOLLONESHOT, right? If an event is a normal level-triggered non-one-shot event, then I don't understand how a round-robin wakeup makes any sense. It's level-triggered, after all. --Andy