From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with SMTP id CAD1C6B005A for ; Tue, 24 Feb 2009 00:22:02 -0500 (EST) Received: from mt1.gw.fujitsu.co.jp ([10.0.50.74]) by fgwmail5.fujitsu.co.jp (Fujitsu Gateway) with ESMTP id n1O5M0BF004127 for (envelope-from kamezawa.hiroyu@jp.fujitsu.com); Tue, 24 Feb 2009 14:22:00 +0900 Received: from smail (m4 [127.0.0.1]) by outgoing.m4.gw.fujitsu.co.jp (Postfix) with ESMTP id D0C0345DE52 for ; Tue, 24 Feb 2009 14:21:59 +0900 (JST) Received: from s4.gw.fujitsu.co.jp (s4.gw.fujitsu.co.jp [10.0.50.94]) by m4.gw.fujitsu.co.jp (Postfix) with ESMTP id A6DF345DE4F for ; Tue, 24 Feb 2009 14:21:59 +0900 (JST) Received: from s4.gw.fujitsu.co.jp (localhost.localdomain [127.0.0.1]) by s4.gw.fujitsu.co.jp (Postfix) with ESMTP id 8C7001DB8041 for ; Tue, 24 Feb 2009 14:21:59 +0900 (JST) Received: from ml14.s.css.fujitsu.com (ml14.s.css.fujitsu.com [10.249.87.104]) by s4.gw.fujitsu.co.jp (Postfix) with ESMTP id 3F4691DB8037 for ; Tue, 24 Feb 2009 14:21:56 +0900 (JST) Date: Tue, 24 Feb 2009 14:20:40 +0900 From: KAMEZAWA Hiroyuki Subject: Re: [PATCH 04/20] Convert gfp_zone() to use a table of precalculated value Message-Id: <20090224142040.5db39fbd.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <200902241459.35435.nickpiggin@yahoo.com.au> References: <1235344649-18265-1-git-send-email-mel@csn.ul.ie> <20090223164047.GO6740@csn.ul.ie> <20090224103226.e9e2766f.kamezawa.hiroyu@jp.fujitsu.com> <200902241459.35435.nickpiggin@yahoo.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Nick Piggin Cc: Mel Gorman , Christoph Lameter , Linux Memory Management List , Pekka Enberg , Rik van Riel , KOSAKI Motohiro , Johannes Weiner , Nick Piggin , Linux Kernel Mailing List , Lin Ming , Zhang Yanmin List-ID: On Tue, 24 Feb 2009 14:59:34 +1100 Nick Piggin wrote: > On Tuesday 24 February 2009 12:32:26 KAMEZAWA Hiroyuki wrote: > > On Mon, 23 Feb 2009 16:40:47 +0000 > > > > Mel Gorman wrote: > > > On Mon, Feb 23, 2009 at 10:43:20AM -0500, Christoph Lameter wrote: > > > > On Tue, 24 Feb 2009, Nick Piggin wrote: > > > > > > Are you sure that this is a benefit? Jumps are forward and pretty > > > > > > short and the compiler is optimizing a branch away in the current > > > > > > code. > > > > > > > > > > Pretty easy to mispredict there, though, especially as you can tend > > > > > to get allocations interleaved between kernel and movable (or simply > > > > > if the branch predictor is cold there are a lot of branches on > > > > > x86-64). > > > > > > > > > > I would be interested to know if there is a measured improvement. > > > > > > Not in kernbench at least, but that is no surprise. It's a small > > > percentage of the overall cost. It'll appear in the noise for anything > > > other than micro-benchmarks. > > > > > > > > It > > > > > adds an extra dcache line to the footprint, but OTOH the instructions > > > > > you quote is more than one icache line, and presumably Mel's code > > > > > will be a lot shorter. > > > > > > Yes, it's an index lookup of a shared read-only cache line versus a lot > > > of code with branches to mispredict. I wasn't happy with the cache line > > > consumption but it was the first obvious alternative. > > > > > > > Maybe we can come up with a version of gfp_zone that has no branches > > > > and no lookup? > > > > > > Ideally, yes, but I didn't spot any obvious way of figuring it out at > > > compile time then or now. Suggestions? > > > > Assume > > ZONE_DMA=0 > > ZONE_DMA32=1 > > ZONE_NORMAL=2 > > ZONE_HIGHMEM=3 > > ZONE_MOVABLE=4 > > > > #define __GFP_DMA ((__force gfp_t)0x01u) > > #define __GFP_DMA32 ((__force gfp_t)0x02u) > > #define __GFP_HIGHMEM ((__force gfp_t)0x04u) > > #define __GFP_MOVABLE ((__force gfp_t)0x08u) > > > > #define GFP_MAGIC (0400030102) ) #depends on config. > > > > gfp_zone(mask) = ((GFP_MAGIC >> ((mask & 0xf)*3) & 0x7) > > Clever! > > But I wonder if it is even valid to perform bitwise operations on > the zone bits of the gfp mask? Hmm, I see a few places doing it, > but if we stamped that out, we could just have a simple zone mask > that takes the zone idx out of the gfp, which would be slightly > simpler again and more extendible. > IIRC, __GFP_MOVALE works as flag. And, one troube is that there is no __GFP_NORMAL flag. I wrote follwoing in old days(before ZONE_MOVABLE). Assume ZONE_NORMAL=2. //trasnslate gfp_mask to zone_idx. #define __GFP_DMA (2) #define __GFP_DMA32 (3) #define __GFP_HIGHMEM (1) #define __GFP_MOVABLE (6) gfp_zone(mask) = (mask & 0x7) ^ 0x2 //ZONE_NORMAL=2) But, this doesn't work. ZONE_NORMAL can be 0,1,2. (and ppc doesn't have ZONE_NORMAL) Thanks, -Kame -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org