From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Tom May" Subject: Re: [PATCH 0/8][for -mm] mem_notify v6 Date: Wed, 2 Apr 2008 10:45:05 -0700 Message-ID: References: <2f11576a0802090719i3c08a41aj38504e854edbfeac@mail.gmail.com> <20080402154910.9588.KOSAKI.MOTOHIRO@jp.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20080402154910.9588.KOSAKI.MOTOHIRO@jp.fujitsu.com> Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org To: KOSAKI Motohiro Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org List-Id: linux-mm.kvack.org On Wed, Apr 2, 2008 at 12:31 AM, KOSAKI Motohiro wrote: > Hi Tom, > > Thank you very useful comment. > that is very interesting. > > > > I tried it with a real-world program that, among other things, mmaps > > anonymous pages and touches them at a reasonable speed until it gets > > notified via /dev/mem_notify, releases most of them with > > madvise(MADV_DONTNEED), then loops to start the cycle again. > > > > What tends to happen is that I do indeed get notifications via > > /dev/mem_notify when the kernel would like to be swapping, at which > > point I free memory. But the notifications come at a time when the > > kernel needs memory, and it gets the memory by discarding some Cached > > or Mapped memory (I can see these decreasing in /proc/meminfo with > > each notification). With each mmap/notify/madvise cycle the Cached > > and Mapped memory gets smaller, until eventually while I'm touching > > pages the kernel can't find enough memory and will either invoke the > > OOM killer or return ENOMEM from syscalls. This is precisely the > > situation I'm trying to avoid by using /dev/mem_notify. > > Could you send your test program? Unfortunately, no, it's a Java Virtual Machine (which is a perfect user of /dev/mem_notify since it can garbage collect on notification, among other times). But it should be possible to make a small program with the same behavior; I'll do that. > I can't reproduce that now, sorry. > > > > > The criterion of "notify when the kernel would like to swap" feels > > correct, but in addition I seem to need something like "notify when > > cached+mapped+free memory is getting low". > > Hmmm, > I think this idea is only useful when userland process call > madvise(MADV_DONTNEED) periodically. Do you have a recommendation for freeing memory? I could maybe use munmap/mmap, but that's not atomic and may be "worse" (more overhead, etc.) than madvise(MADV_DONTNEED). > but I hope improve my patch and solve your problem. > if you don' mind, please help my testing ;) It's my pleasure to help in any way I can. .tom