From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx111.postini.com [74.125.245.111]) by kanga.kvack.org (Postfix) with SMTP id 2AD786B002C for ; Wed, 8 Feb 2012 12:15:47 -0500 (EST) MIME-Version: 1.0 Message-ID: <409797c4-a6e7-493d-9681-4166a9473ab8@default> Date: Wed, 8 Feb 2012 09:15:36 -0800 (PST) From: Dan Magenheimer Subject: RE: [PATCH 1/5] staging: zsmalloc: zsmalloc memory allocation library References: <1326149520-31720-1-git-send-email-sjenning@linux.vnet.ibm.com> <1326149520-31720-2-git-send-email-sjenning@linux.vnet.ibm.com> <4F21A5AF.6010605@linux.vnet.ibm.com> <4F300D41.5050105@linux.vnet.ibm.com> <4F32A55E.8010401@linux.vnet.ibm.com> In-Reply-To: <4F32A55E.8010401@linux.vnet.ibm.com> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Dave Hansen , Seth Jennings Cc: Greg Kroah-Hartman , Nitin Gupta , Brian King , Konrad Wilk , linux-mm@kvack.org, devel@driverdev.osuosl.org, linux-kernel@vger.kernel.org > From: Dave Hansen [mailto:dave@linux.vnet.ibm.com] > Subject: Re: [PATCH 1/5] staging: zsmalloc: zsmalloc memory allocation li= brary >=20 > On 02/06/2012 09:26 AM, Seth Jennings wrote: > > On 01/26/2012 01:12 PM, Dave Hansen wrote: > >> void *kmap_atomic_prot(struct page *page, pgprot_t prot) > >> { > >> ... > >> type =3D kmap_atomic_idx_push(); > >> idx =3D type + KM_TYPE_NR*smp_processor_id(); > >> vaddr =3D __fix_to_virt(FIX_KMAP_BEGIN + idx); > >> > >> I think if you do a get_cpu()/put_cpu() or just a preempt_disable() > >> across the operations you'll be guaranteed to get two contiguous addre= sses. > > > > I'm not quite following here. kmap_atomic() only does this for highmem= pages. > > For normal pages (all pages for 64-bit), it doesn't do any mapping at a= ll. It > > just returns the virtual address of the page since it is in the kernel'= s address > > space. > > > > For this design, the pages _must_ be mapped, even if the pages are dire= ctly > > reachable in the address space, because they must be virtually contiguo= us. >=20 > I guess you could use vmap() for that. It's just going to be slower > than kmap_atomic(). I'm really not sure it's worth all the trouble to > avoid order-1 allocations, though. Seth, Nitin, please correct me if I am wrong, but... Dave, your comment makes me wonder if maybe you might be missing the key value of the new allocator. The zsmalloc allocator can grab any random* page "A" with X unused bytes at the END of the page, and any random page "B" with Y unused bytes at the BEGINNING of the page and "coalesce" them to store any byte sequence with a length** Z not exceeding X+Y. Presumably this markedly increases the density of compressed-pages-stored-per-physical-page***. I don't=20 see how allowing order-1 allocations helps here but if I am missing something clever, please explain further. (If anyone missed Jonathan Corbet's nice lwn.net article, see: https://lwn.net/Articles/477067/ ) * Not really ANY random page, just any random page that has been previously get_free_page'd by the allocator and hasn't been free'd yet. ** X, Y and Z are all rounded to a multiple of 16 so there is still some internal fragmentation cost. *** Would be interesting to see some random and real workload data comparing density for zsmalloc and xvmalloc. And also zbud too as a goal is to replace zbud with zsmalloc too. -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org