Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752028AbaBXI24 (ORCPT ); Mon, 24 Feb 2014 03:28:56 -0500 Received: from mail-pa0-f48.google.com ([209.85.220.48]:44789 "EHLO mail-pa0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751514AbaBXI2z (ORCPT ); Mon, 24 Feb 2014 03:28:55 -0500 Date: Mon, 24 Feb 2014 00:28:01 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Weijie Yang cc: Andrew Morton , Mel Gorman , Michal Hocko , Christian Ehrhardt , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: swap: Use swapfiles in priority order In-Reply-To: Message-ID: References: <20140213104231.GX6732@suse.de>

User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 16 Feb 2014, Weijie Yang wrote: > On Fri, Feb 14, 2014 at 9:10 PM, Christian Ehrhardt > wrote: > > Weijie Yang gmail.com> writes: > > > >> > >> On Thu, Feb 13, 2014 at 6:42 PM, Mel Gorman suse.de> wrote: > > [...] > >> > - for (type = swap_list.next; type >= 0 && wrapped < 2; type = next) { > >> > + for (type = swap_list.head; type >= 0 && wrapped < 2; type = next) { > >> > > [...] > >> Does it lead to a "schlemiel the painter's algorithm"? > >> (please forgive my rude words, but I can't find a precise word to describe it > >> > >> How about modify it like this? > >> > > [...] > >> - next = swap_list.head; > >> + next = type; > > [...] > > > > Hi, > > unfortunately withou studying the code more thoroughly I'm not even sure if > > you meant you code to extend or replace Mels patch. > > > > To be sure about your intention. You refered to algorithm scaling because > > you were afraid the new code would scan the full list all the time right ? > > > > But simply letting the machines give a try for both options I can now > > qualify both. > > > > Just your patch creates a behaviour of jumping over priorities (see the > > following example), so I hope you meant combining both patches. > > With that in mind the patch I eventually tested the combined patch looking > > like this: > > Hi Christian, > > My patch is not appropriate, so there is no need to combine it with Mel's patch. > > What I worried about Mel's patch is not only the search efficiency, > actually it has > negligible impact on system, but also the following scenario: > > If two swapfiles have the same priority, in ordinary semantic, they > should be used > in balance. But with Mel's patch, it will always get the free > swap_entry from the > swap_list.head in priority order, I worry it could break the balance. > > I think you can test this scenario if you have available test machines. > > Appreciate for your done. Weijie, I agree with you on both points: Schlemiel effect of repeatedly restarting from head (already an unintended defect before Mel's patch), and more importantly the breakage of swapfiles at the same priority. Sorry, it has to be a Nak to Mel's patch, which fixes one behavior at the expense of another. And if we were to go that way, better just to rip out all of swap_list.next and highest_priority_index. I had hoped to respond today with a better patch; but I just haven't got it right yet either. I think we don't need to rush to fix it, but fix it we certainly should. Christian, congratulations on discovering this wrong behavior: at first I assumed it came from Shaohua's 3.9 highest_priority_index changes, but no; then I assumed it came from my 2.6.14 swap_lock changes; but now I think it goes back even before 2.4.0, probably ever since there have been swap priorities. Hugh -- 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/