From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f197.google.com (mail-qk0-f197.google.com [209.85.220.197]) by kanga.kvack.org (Postfix) with ESMTP id EE0286B000C for ; Tue, 17 Apr 2018 13:26:58 -0400 (EDT) Received: by mail-qk0-f197.google.com with SMTP id g138so9268633qke.22 for ; Tue, 17 Apr 2018 10:26:58 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id a20si6755271qth.204.2018.04.17.10.26.57 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 17 Apr 2018 10:26:57 -0700 (PDT) Date: Tue, 17 Apr 2018 13:26:51 -0400 (EDT) From: Mikulas Patocka Subject: Re: [PATCH RESEND] slab: introduce the flag SLAB_MINIMIZE_WASTE In-Reply-To: Message-ID: References: <20c58a03-90a8-7e75-5fc7-856facfb6c8a@suse.cz> <20180413151019.GA5660@redhat.com> <20180416142703.GA22422@redhat.com> <20180416144638.GA22484@redhat.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka 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 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. > > 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 > is so large it can fail even with the fallback (i.e. >= costly order), > you need to tolerate failures anyway? > > One corner case I see is if there is anyone who would rather use their > own fallback instead of the space-wasting smallest-order fallback. > Maybe we could map some GFP flag to indicate that. For example, if you create a cache with 17KB objects, the slab subsystem will pad it up to 32KB. You are wasting almost 1/2 memory, but the allocation is realiable and it won't fail. If you use order higher than 32KB, you get less wasted memory, but you also get random -ENOMEMs (yes, we had a problem in dm-thin that it was randomly failing during initialization due to 64KB allocation). Mikulas