Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751740AbXA0OQq (ORCPT ); Sat, 27 Jan 2007 09:16:46 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751749AbXA0OQq (ORCPT ); Sat, 27 Jan 2007 09:16:46 -0500 Received: from ug-out-1314.google.com ([66.249.92.171]:14141 "EHLO ug-out-1314.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751740AbXA0OQp (ORCPT ); Sat, 27 Jan 2007 09:16:45 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=googlemail.com; s=beta; h=received:from:to:subject:date:user-agent:cc:references:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:message-id; b=S5Z7nbeIDhP4Rbp9DOpGxHyKMPonvwFDKX8TQZY4JV4JLWfMAONHszupPhPXeyGmfrccNdp9dMnpirHS4sOCPKUAOoSz7F8hbSQg1D7jXX9rGUAZFdgcJ1gckJHsVUtVt5H8qohkEX+vgFNHAt60BMYu+FkXSQNphu0/dnhPDI0= From: Denis Vlasenko To: 7eggert@gmx.de Subject: Re: O_DIRECT question Date: Sat, 27 Jan 2007 15:14:31 +0100 User-Agent: KMail/1.8.2 Cc: Bill Davidsen , Michael Tokarev , Phillip Susi , Linus Torvalds , Viktor , Aubrey , Hua Zhong , Hugh Dickins , linux-kernel@vger.kernel.org, hch@infradead.org, kenneth.w.chen@in References: <7BYkO-5OV-17@gated-at.bofh.it> <7HIPV-8kp-35@gated-at.bofh.it> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200701271514.31203.vda.linux@googlemail.com> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1876 Lines: 48 On Saturday 27 January 2007 15:01, Bodo Eggert wrote: > Denis Vlasenko wrote: > > On Friday 26 January 2007 19:23, Bill Davidsen wrote: > >> Denis Vlasenko wrote: > >> > On Thursday 25 January 2007 21:45, Michael Tokarev wrote: > > >> >> But even single-threaded I/O but in large quantities benefits from > >> >> O_DIRECT significantly, and I pointed this out before. > >> > > >> > Which shouldn't be true. There is no fundamental reason why > >> > ordinary writes should be slower than O_DIRECT. > >> > > >> Other than the copy to buffer taking CPU and memory resources. > > > > It is not required by any standard that I know. Kernel can be smarter > > and avoid that if it can. > > The kernel can also solve the halting problem if it can. > > Do you really think an entropy estamination code on all access patterns in the > system will be free as in beer, Actually I think we need this heuristic: if (opened_with_O_STREAM && buffer_is_aligned && io_size_is_a_multiple_of_sectorsize) do_IO_directly_to_user_buffer_without_memcpy is not *that* compilcated. I think that we can get rid of O_DIRECT peculiar requirements "you *must* not cache me" + "you *must* write me directly to bare metal" by replacing it with O_STREAM ("*advice* to not cache me") + O_SYNC ("write() should return only when data is written to storage, not sooner"). Why? Because these O_DIRECT "musts" are rather unusual and overkill. Apps should not have that much control over what kernel does internally; and also O_DIRECT was mixing shampoo and conditioner on one bottle (no-cache and sync writes) - bad API. -- vda - 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/