From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f200.google.com (mail-pf0-f200.google.com [209.85.192.200]) by kanga.kvack.org (Postfix) with ESMTP id 4CAE76B000A for ; Tue, 17 Apr 2018 15:15:54 -0400 (EDT) Received: by mail-pf0-f200.google.com with SMTP id 203so11885045pfz.19 for ; Tue, 17 Apr 2018 12:15:54 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id f10-v6si14964119pln.359.2018.04.17.12.15.52 for (version=TLS1 cipher=AES128-SHA bits=128/128); Tue, 17 Apr 2018 12:15:53 -0700 (PDT) Subject: Re: [PATCH RESEND] slab: introduce the flag SLAB_MINIMIZE_WASTE References: <20c58a03-90a8-7e75-5fc7-856facfb6c8a@suse.cz> <20180413151019.GA5660@redhat.com> <20180416142703.GA22422@redhat.com> <20180416144638.GA22484@redhat.com> From: Vlastimil Babka Message-ID: <8ab1e75f-9cf9-2a99-d071-c8c7a3554b95@suse.cz> Date: Tue, 17 Apr 2018 21:13:53 +0200 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Mikulas Patocka Cc: Christopher Lameter , Mike Snitzer , Matthew Wilcox , Pekka Enberg , linux-mm@kvack.org, dm-devel@redhat.com, David Rientjes , Joonsoo Kim , Andrew Morton , linux-kernel@vger.kernel.org On 04/17/2018 07:26 PM, Mikulas Patocka wrote: > > > On Tue, 17 Apr 2018, Vlastimil Babka wrote: > >> On 04/17/2018 04:45 PM, Christopher Lameter wrote: >>> On Mon, 16 Apr 2018, Mikulas Patocka wrote: >>> >>>> This patch introduces a flag SLAB_MINIMIZE_WASTE for slab and slub. This >>>> flag causes allocation of larger slab caches in order to minimize wasted >>>> space. >>>> >>>> This is needed because we want to use dm-bufio for deduplication index and >>>> there are existing installations with non-power-of-two block sizes (such >>>> as 640KB). The performance of the whole solution depends on efficient >>>> memory use, so we must waste as little memory as possible. >>> >>> Hmmm. Can we come up with a generic solution instead? >> >> Yes please. >> >>> This may mean relaxing the enforcement of the allocation max order a bit >>> so that we can get dense allocation through higher order allocs. >>> >>> But then higher order allocs are generally seen as problematic. >> >> I think in this case they are better than wasting/fragmenting 384kB for >> 640kB object. > > Wasting 37% of memory is still better than the kernel randomly returning > -ENOMEM when higher-order allocation fails. Of course, see below. >>> That >>> means that callers need to be able to tolerate failures. >> >> Is it any different from now? I suppose there would still be >> smallest-order fallback involved in sl*b itself? And if your allocation ^ There: "I suppose there would still be smallest-order fallback involved in sl*b itself?" If SLAB doesn't currently support fallback to different order, it either learns to do that, or keeps wasting memory and more people will migrate to SLUB. Simple.