From mboxrd@z Thu Jan 1 00:00:00 1970 From: Niraj Tolia Subject: Re: Seamlessly sharing identical memory pages among domains Date: Fri, 21 May 2004 01:52:47 -0400 Sender: xen-devel-admin@lists.sourceforge.net Message-ID: <40AD992F.3070606@hotmail.com> References: <8D41D1F60264314591FA7C0B012EB7BB58615B@spbexc01.americas.cpqcorp.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Errors-To: xen-devel-admin@lists.sourceforge.net List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , List-Archive: To: Ian Pratt Cc: xen-devel@lists.sourceforge.net List-Id: xen-devel@lists.xenproject.org While not exactly related, this paper (Section 4.7) presented at OSDI'02 showed that there was significant overhead when trying to detect dirty pages using SHA-1 hash digests. It would be interesting to see if this applied to Xen. Niraj -- http://www.andrew.cmu.edu/~ntolia Ian Pratt wrote: >>I'd like to know if XEN is or will be implementing any mechanism for >>transparently sharing pages with identical content among domains, to >>minimize memory consumption, similar to what is implemented in VMWare's >>ESX server (chapter 4 of the paper whose link is below). > > > I'm not convinced there's that much sharing to be had between > VMs: One side effect of implementing some test code for live > migration was that I collected some traces containing crc > fingerprints of the memory pages of several domains. There > weren't many hashes in common. Perhaps this shouldn't be > surprising -- application text is typically small compared to the > heap data they operate on. The actual datasets operated on by > domains are typically distinct. > > However, there might be some value in having a shared read-only > buffer cache, if only for the saved IO ops. Implementing a cache > that replicates pages in memory (rather than sharing) is trivial. > > Doing a proper shared cache is slightly trickier given the > paravirtualised memory interface -- we'd have to introduce guests > to a new kind of write fault: "A write fault has occurred, and > you'll have to copy the page because this machine page is > immutable as it is already shared with other domains". Modifying > Linux to handle this wouldn't be hard. > > Of course, if you're running a domain on Xen's shadow page tables > then all of this manipulation is easy and can be done > transparently to the domain. Of course, you pay a performance > penalty... > > Ian > > > ------------------------------------------------------- > This SF.Net email is sponsored by: Oracle 10g > Get certified on the hottest thing ever to hit the market... Oracle 10g. > Take an Oracle 10g class now, and we'll give you the exam FREE. > http://ads.osdn.com/?ad_id=3149&alloc_id=8166&op=click ------------------------------------------------------- This SF.Net email is sponsored by: Oracle 10g Get certified on the hottest thing ever to hit the market... Oracle 10g. Take an Oracle 10g class now, and we'll give you the exam FREE. http://ads.osdn.com/?ad_id=3149&alloc_id=8166&op=click