From mboxrd@z Thu Jan 1 00:00:00 1970 From: Borislav Petkov Subject: Re: fork on processes with lots of memory Date: Tue, 26 Jan 2016 17:38:43 +0100 Message-ID: <20160126163843.GA8471@pd.tnic> References: <20160126160641.GA530@qarx.de> <20160126162853.GA1836@qarx.de> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Return-path: Content-Disposition: inline In-Reply-To: <20160126162853.GA1836@qarx.de> Sender: linux-kernel-owner@vger.kernel.org To: Felix von Leitner Cc: linux-kernel@vger.kernel.org, linux-mm List-Id: linux-mm.kvack.org + linux-mm On Tue, Jan 26, 2016 at 05:28:53PM +0100, Felix von Leitner wrote: > > Dear Linux kernel devs, > > > I talked to someone who uses large Linux based hardware to run a > > process with huge memory requirements (think 4 GB), and he told me that > > if they do a fork() syscall on that process, the whole system comes to > > standstill. And not just for a second or two. He said they measured a 45 > > minute (!) delay before the system became responsive again. > > I'm sorry, I meant 4 TB not 4 GB. > I'm not used to working with that kind of memory sizes. > > > Their working theory is that all the pages need to be marked copy-on-write > > in both processes, and if you touch one page, a copy needs to be made, > > and than just takes a while if you have a billion pages. > > > I was wondering if there is any advice for such situations from the > > memory management people on this list. > > > In this case the fork was for an execve afterwards, but I was going to > > recommend fork to them for something else that can not be tricked around > > with vfork. > > > Can anyone comment on whether the 45 minute number sounds like it could > > be real? When I heard it, I was flabberghasted. But the other person > > swore it was real. Can a fork cause this much of a delay? Is there a way > > to work around it? > > > I was going to recommend the fork to create a boundary between the > > processes, so that you can recover from memory corruption in one > > process. In fact, after the fork I would want to munmap almost all of > > the shared pages anyway, but there is no way to tell fork that. > > > Thanks, > > > Felix > > > PS: Please put me on Cc if you reply, I'm not subscribed to this mailing > > list. > -- Regards/Gruss, Boris. ECO tip #101: Trim your mails when you reply.