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 B0A956B005C for ; Thu, 31 May 2012 15:09:05 -0400 (EDT) Received: by qcsd16 with SMTP id d16so906233qcs.14 for ; Thu, 31 May 2012 12:09:04 -0700 (PDT) Message-ID: <4FC7C1CD.7020701@gmail.com> Date: Thu, 31 May 2012 15:09:01 -0400 From: KOSAKI Motohiro MIME-Version: 1.0 Subject: Re: [RFC Patch] fs: implement per-file drop caches References: <1338385120-14519-1-git-send-email-amwang@redhat.com> <4FC6393B.7090105@draigBrady.com> <1338445233.19369.21.camel@cr0> <4FC70FFE.50809@gmail.com> <1338466281.19369.44.camel@cr0> In-Reply-To: <1338466281.19369.44.camel@cr0> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Cong Wang Cc: KOSAKI Motohiro , =?UTF-8?B?UMOhZHJhaWcgQnI=?= =?UTF-8?B?YWR5?= , linux-kernel@vger.kernel.org, Andrew Morton , Cong Wang , Alexander Viro , Matthew Wilcox , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org (5/31/12 8:11 AM), Cong Wang wrote: > On Thu, 2012-05-31 at 02:30 -0400, KOSAKI Motohiro wrote: >> (5/31/12 2:20 AM), Cong Wang wrote: >>> On Wed, 2012-05-30 at 16:14 +0100, PA!draig Brady wrote: >>>> On 05/30/2012 02:38 PM, Cong Wang wrote: >>>>> This is a draft patch of implementing per-file drop caches. >>>>> >>>>> It introduces a new fcntl command F_DROP_CACHES to drop >>>>> file caches of a specific file. The reason is that currently >>>>> we only have a system-wide drop caches interface, it could >>>>> cause system-wide performance down if we drop all page caches >>>>> when we actually want to drop the caches of some huge file. >>>> >>>> This is useful functionality. >>>> Though isn't it already provided with POSIX_FADV_DONTNEED? >>> >>> Thanks for teaching this! >>> >>> However, from the source code of madvise_dontneed() it looks like it is >>> using a totally different way to drop page caches, that is to invalidate >>> the page mapping, and trigger a re-mapping of the file pages after a >>> page fault. So, yeah, this could probably drop the page caches too (I am >>> not so sure, haven't checked the code in details), but with my patch, it >>> flushes the page caches directly, what's more, it can also prune >>> dcache/icache of the file. >> >> madvise should work. I don't think we need duplicate interface. Moreomover >> madvise(2) is cleaner than fcntl(2). >> > > I think madvise(DONTNEED) attacks the problem in a different approach, > it munmaps the file mapping and by the way drops the page caches, my > approach is to drop the page caches directly similar to what sysctl > drop_caches. > > What about private file mapping? Could madvise(DONTNEED) drop the page > caches too even when the other process is doing the same private file > mapping? At least my patch could do this. Right. But a process can makes another mappings if a process have enough permission. and if it doesn't, a process shouldn't be able to drop a shared cache. > I am not sure if fcntl() is a good interface either, this is why the > patch is marked as RFC. :-D But, if you can find certain usecase, I'm not against anymore. -- 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