Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932936AbXB0KTF (ORCPT ); Tue, 27 Feb 2007 05:19:05 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932948AbXB0KTF (ORCPT ); Tue, 27 Feb 2007 05:19:05 -0500 Received: from relay.2ka.mipt.ru ([194.85.82.65]:52271 "EHLO 2ka.mipt.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932936AbXB0KTD (ORCPT ); Tue, 27 Feb 2007 05:19:03 -0500 Date: Tue, 27 Feb 2007 13:13:32 +0300 From: Evgeniy Polyakov To: Davide Libenzi Cc: Ingo Molnar , Ulrich Drepper , Linux Kernel Mailing List , Linus Torvalds , Arjan van de Ven , Christoph Hellwig , Andrew Morton , Alan Cox , Zach Brown , "David S. Miller" , Suparna Bhattacharya , Jens Axboe , Thomas Gleixner Subject: Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3 Message-ID: <20070227101332.GB23170@2ka.mipt.ru> References: <20070223122224.GB5392@2ka.mipt.ru> <20070225174505.GA7048@elte.hu> <20070225180910.GA29821@2ka.mipt.ru> <20070225190414.GB6460@elte.hu> <20070225194250.GA1353@2ka.mipt.ru> <20070226123922.GA1370@elte.hu> <20070226140500.GA31629@2ka.mipt.ru> <20070226141518.GA24683@elte.hu> <20070226165513.GB22454@2ka.mipt.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=koi8-r Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.9i X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-3.0 (2ka.mipt.ru [0.0.0.0]); Tue, 27 Feb 2007 13:14:28 +0300 (MSK) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1830 Lines: 45 On Mon, Feb 26, 2007 at 06:18:51PM -0800, Davide Libenzi (davidel@xmailserver.org) wrote: > On Mon, 26 Feb 2007, Evgeniy Polyakov wrote: > > > 2. its notifications do not go through the second loop, i.e. it is O(1), > > not O(ready_num), and notifications happens directly from internals of > > the appropriate subsystem, which does not require special wakeup > > (although it can be done too). > > Sorry if I do not read kevent code correctly, but in kevent_user_wait() > there is a: > > while (num < max_nr && ((k = kevent_dequeue_ready(u)) != NULL)) { > ... > } > > loop, that make it O(ready_num). From a mathematical standpoint, they're > both O(ready_num), but epoll is doing three passes over the ready set. > I always though that if the number of ready events is so big that the more > passes over the ready set becomes relevant, probably the "work" done by > userspace for each fetched event would make the extra cost irrelevant. > But that can be fixed by a patch that will follow on lkml ... No, kevent_dequeue_ready() copies data to userspace, that is it. So it looks roughly following: storage is ready: -> kevent_requee() - ends up in ading event to the end of the queue (list add under spinlock) kevent_wait() -> copy first, second, ... Kevent poll (as long as epoll) model requires _additional_ check in userspace context before it is copied, so we endup with checking the full ready queue again - that what I pointed as O(ready_num), O() implies price for copying to userspace, list_add and so on. > - Davide > -- Evgeniy Polyakov - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/