From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751203AbWAQTSR (ORCPT ); Tue, 17 Jan 2006 14:18:17 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751265AbWAQTSQ (ORCPT ); Tue, 17 Jan 2006 14:18:16 -0500 Received: from odyssey.analogic.com ([204.178.40.5]:21769 "EHLO odyssey.analogic.com") by vger.kernel.org with ESMTP id S1751203AbWAQTSQ convert rfc822-to-8bit (ORCPT ); Tue, 17 Jan 2006 14:18:16 -0500 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT X-MimeOLE: Produced By Microsoft Exchange V6.5.7226.0 In-Reply-To: <43CD3CE4.3090300@comcast.net> X-OriginalArrivalTime: 17 Jan 2006 19:18:14.0134 (UTC) FILETIME=[C3409160:01C61B9A] Content-class: urn:content-classes:message Subject: Re: Huge pages and small pages. . . Date: Tue, 17 Jan 2006 14:18:13 -0500 Message-ID: X-MS-Has-Attach: X-MS-TNEF-Correlator: Thread-Topic: Huge pages and small pages. . . Thread-Index: AcYbmsNHmPSAZ6HmQmCdTfAq0M5stg== References: <43CD3CE4.3090300@comcast.net> From: "linux-os \(Dick Johnson\)" To: "John Richard Moser" Cc: Reply-To: "linux-os \(Dick Johnson\)" Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 17 Jan 2006, John Richard Moser wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > Is there anything in the kernel that shifts the physical pages for 1024 > physically allocated and contiguous virtual pages together physically > and remaps them as one huge page? This would probably work well for the A page is something that is defined by the CPU. Perhaps you mean "order"? When acquiring pages for DMA, they need to be contiguous if you are going to access more than one page at a time. Therefore, one can attempt to get two or more pages, i.e., the order or pages. Since the CPU uses virtual memory always, there is no advantage to having contiguous pages. You just map anything that's free into what looks like contiguous memory and away you go. > low end of the heap, until someone figures out a way to tell the system > to free intermittent pages in a big mapping (if the heap has an > allocation up high, it can have huge, unused areas that are allocated). The actual allocation only occurs when an access happens. You can allocate all the virtual memory in the machine and never use any of it. When you allocate memory, the kernel just marks a promised page 'not present'. When you attempt to access it, a page-fault occurs and the kernel tries to find a free page to map into your address space. > It may possibly work for disk cache as well, albeit I can't say for > sure if it's common to have a 4 meg contiguous section of program data > loaded. > But it __is__ contiguous as far as the program is concerned. The only time you need physically contiguous pages is when a DMA operation occurs that crosses page boundaries. Otherwise, it's a waste of time and CPU resources trying to make something contiguous. Also, modern DMA engines use scatter- lists so one can DMA to pages scattered all over the address- space in one operation. In this case, you just build a list of pages. You don't care where they physically reside, although you do need to tell the DMA engine their correct locations. Now there are some M$Garbage "high-memory" so-called enhancements that, using page-registers, "map" more that 4 GB of memory into the 4 GB address space. This is like the garbage that M$ created to use "high memory" for DOS. Use of this kind of hardware-hack is not relevant to the discussion about virtual memory. > Shifting odd huge allocations around would be neat to, re: > > {2m}[4M ]{2m} -> [4M ][4M ] > > - -- > All content of all messages exchanged herein are left in the > Public Domain, unless otherwise explicitly stated. > > Creative brains are a valuable, limited resource. They shouldn't be > wasted on re-inventing the wheel when there are so many fascinating > new problems waiting out there. > -- Eric Steven Raymond > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v1.4.2 (GNU/Linux) > Comment: Using GnuPG with Thunderbird - http://enigmail.mozdev.org > > iD8DBQFDzTzjhDd4aOud5P8RAud1AJ9MVy90XzvJWmgHmlBUdHcpsYNtWACfVxY6 > f/jYDM1XiM8/09TfrzEDI3w= > =CsLK > -----END PGP SIGNATURE----- > - > 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/ > Cheers, Dick Johnson Penguin : Linux version 2.6.13.4 on an i686 machine (5589.54 BogoMips). Warning : 98.36% of all statistics are fiction. . **************************************************************** The information transmitted in this message is confidential and may be privileged. 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