From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt1-f197.google.com (mail-qt1-f197.google.com [209.85.160.197]) by kanga.kvack.org (Postfix) with ESMTP id 967946B026B for ; Mon, 15 Oct 2018 18:41:04 -0400 (EDT) Received: by mail-qt1-f197.google.com with SMTP id l6-v6so22316474qtc.12 for ; Mon, 15 Oct 2018 15:41:04 -0700 (PDT) Received: from a9-46.smtp-out.amazonses.com (a9-46.smtp-out.amazonses.com. [54.240.9.46]) by mx.google.com with ESMTPS id n30-v6si1277463qtl.92.2018.10.15.15.41.03 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Mon, 15 Oct 2018 15:41:03 -0700 (PDT) Date: Mon, 15 Oct 2018 22:41:03 +0000 From: Christopher Lameter Subject: Re: [patch] mm, slab: avoid high-order slab pages when it does not reduce waste In-Reply-To: <20181012151341.286cd91321cdda9b6bde4de9@linux-foundation.org> Message-ID: <0100016679e3c96f-c78df4e2-9ab8-48db-8796-271c4b439f16-000000@email.amazonses.com> References: <20181012151341.286cd91321cdda9b6bde4de9@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: David Rientjes , Pekka Enberg , Joonsoo Kim , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Fri, 12 Oct 2018, Andrew Morton wrote: > > If the amount of waste is the same at higher cachep->gfporder values, > > there is no significant benefit to allocating higher order memory. There > > will be fewer calls to the page allocator, but each call will require > > zone->lock and finding the page of best fit from the per-zone free areas. There is a benefit because the management overhead is halved. > > Instead, it is better to allocate order-0 memory if possible so that pages > > can be returned from the per-cpu pagesets (pcp). Have a benchmark that shows this? > > > There are two reasons to prefer this over allocating high order memory: > > > > - allocating from the pcp lists does not require a per-zone lock, and > > > > - this reduces stranding of MIGRATE_UNMOVABLE pageblocks on pcp lists > > that increases slab fragmentation across a zone. The slab allocators generally buffer pages from the page allocator to avoid this effect given the slowness of page allocator operations anyways. > Confused. Higher-order slab pages never go through the pcp lists, do > they? I'd have thought that by tending to increase the amount of > order-0 pages which are used by slab, such stranding would be > *increased*? Potentially. > > We are particularly interested in the second point to eliminate cases > > where all other pages on a pageblock are movable (or free) and fallback to > > pageblocks of other migratetypes from the per-zone free areas causes > > high-order slab memory to be allocated from them rather than from free > > MIGRATE_UNMOVABLE pages on the pcp. Well does this actually do some good?