From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx115.postini.com [74.125.245.115]) by kanga.kvack.org (Postfix) with SMTP id 067C36B005A for ; Thu, 26 Jan 2012 19:56:36 -0500 (EST) MIME-Version: 1.0 Message-ID: Date: Thu, 26 Jan 2012 16:56:34 -0800 (PST) From: Dan Magenheimer Subject: RE: [PATCH] mm: implement WasActive page flag (for improving cleancache) References: <4F218D36.2060308@linux.vnet.ibm.com> <9fcd06f5-360e-4542-9fbb-f8c7efb28cb6@default> <20120126163150.31a8688f.akpm@linux-foundation.org> In-Reply-To: <20120126163150.31a8688f.akpm@linux-foundation.org> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Dave Hansen , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Konrad Wilk , Seth Jennings , Nitin Gupta , Nebojsa Trpkovic , minchan@kernel.org, KAMEZAWA Hiroyuki , riel@redhat.com, Chris Mason > From: Andrew Morton [mailto:akpm@linux-foundation.org] > Subject: Re: [PATCH] mm: implement WasActive page flag (for improving cle= ancache) Thanks for the reply! =20 > On Thu, 26 Jan 2012 13:28:02 -0800 (PST) > Dan Magenheimer wrote: >=20 > > > I do think it also needs to get cleared on the way in to the page > > > allocator. Otherwise: > > > > > > =09PageSetWasActive(page); > > > =09free_page(page); > > > =09... > > > =09another_user_page =3D get_free_page() > > > =09// now cleancache sees the active bit for the prev user > > > > > > Or am I missing somewhere it gets cleared non-explicitly somewhere? > > > > True, it is not getting cleared and it should be, good catch! >=20 > It should be added to PAGE_FLAGS_CHECK_AT_FREE. I was thinking of clearing it in free_pages_prepare() before the call to free_pages_check(). Does that make sense? If so, then it could also be added to PAGE_FLAGS_CHECK_AT_FREE, though it might be a bit redundant. > > I'll find the place to add the call to ClearPageWasActive() for v2. >=20 > AFAICT this patch consumes our second-last page flag, or close to it. > We'll all be breaking out in hysterics when the final one is gone. I'd be OK with only using this on 64-bit systems, though there are ARM folks playing with zcache that might disagree. Am I correct in assuming that your "second-last page flag" concern applies only to 32-bit systems? > This does appear to be a make or break thing for cleancache - if we > can't fix https://lkml.org/lkml/2012/1/22/61 then cleancache is pretty > much a dead duck. Hmmm... is that URL correct? If so, there is some subtlety in that thread that I am missing as I don't understand the relationship to cleancache at all? > But I'm going to ask for great effort to avoid > consuming another page flag. Either fix cleancache via other means or, > much less desirably, find an existing page flag and overload it. Cleancache isn't broken. The fix is not a requirement for other cleancache users (Xen and RAMster), though it is definitely useful. It's not a _requirement_ for zcache either but definitely helps on certain workloads and systems, see below. > And I'm afraid that neither I nor other MM developers are likely to > help you with "fix cleancache via other means" because we weren't > provided with any description of what the problem is within cleancache, > nor how it will be fixed. All we are given is the assertion "cleancache > needs this". The patch comment says: The patch resolves issues reported with cleancache which occur especially during streaming workloads on older processors, see https://lkml.org/lkml/2011/8/17/351 I can see that may not be sufficient, so let me expand on it. First, just as page replacement worked prior to the active/inactive redesign at 2.6.27, cleancache works without the WasActive page flag. However, just as pre-2.6.27 page replacement had problems on streaming workloads, so does cleancache. The WasActive page flag is an attempt to pass the same active/inactive info gathered by the post-2.6.27 kernel into cleancache, with the same objectives and presumably the same result: improving the "quality" of pages preserved in memory thus reducing refaults. Is that clearer? If so, I'll do better on the description at v2. Thanks, Dan -- 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